Weird Life: Must Life Be Based on Carbon and Water?1
When we look up into the sky, it is hard not to think about how vast the universe is. Our universe is estimated to contain around 100 billion galaxies with each galaxy having around 100 billion stars. And most of those stars are expected to harbor planets. That is a mind-boggling number of stars and possible planets. This leads inevitably to questions about whether there is life on planets orbiting those distant specks of light. If so, what might it look like? Are there alien civilizations out there with intelligence and technology similar to or even more advanced than our own? If so, can we communicate with them?
Why Even Consider "Weird Life"?
The Search for Extra-Terrestrial Intelligence (SETI) was inaugurated in the early 1960s to scan the heavens for any signals that might be coming from extraterrestrial civilizations. One thing is certain; SETI proponents are firmly convinced that we will eventually find extraterrestrial life. For example, astronomer Frank Drake who helped found SETI stated, "At this very minute, with almost absolute certainty, radio waves sent forth by other intelligent civilizations are falling on the Earth" (our italics).4 Astronomer Carl Sagan was just as confident about the certainty of extraterrestrial life. He wrote, "Given sufficient time and an environment which is not entirely static, the evolution of complex organisms is, in this view, inevitable. The finding of even relatively simple life forms on Mars or other planets in our solar system would tend to confirm this hypothesis" (our italics).5 Together with Frank Drake he also wrote, "There can be little doubt that civilizations more advanced than the earth's exist elsewhere in the universe" (our italics).6
SETI: A Twenty-First Century Perspective
We have come a long way since SETI was inaugurated 50 years ago. What have we found so far?
- No confirmed detections of extraterrestrial civilizations. While this does not strictly rule out the existence of extraterrestrials, it does place some strong limits on how prevalent advanced intelligent life might be. For example, we can effectively rule out extraterrestrial civilizations with a level of technology equal to or greater than our own out to a distance of 50 light-years.7 Partial searches extend that conclusion out to 4,000 light-years and for advanced civilizations (type I on the Kardashev scale) out to 40,000 light-years.8
- No evidence of extraterrestrial visitation. A belief that some reports of UFOs (unidentified flying objects) represent sightings of alien spacecraft is widely popular, but we do not have credible evidence of extraterrestrial visitation.9
- Other solar systems are not similar to our own. When SETI was starting in the 60s, it was widely believed that not only would most stars have planets, but that these alien solar systems would be very similar to our own. If this were the case, habitable Earth-like planets should be very common. As of August 2014, we now have the confirmed detection of close to two thousand extra-solar planets with a large number of additional planetary candidates. This gives us the ability to make some preliminary conclusions, although the detection techniques are not sufficiently robust and have not been carried out for long enough to make decisive claims.10 One thing that has been discovered is that there are numerous ways that a solar system can differ from our own in ways that are hostile to life.11 For example, a number of Jupiter-like gas giant planets have been found to be orbiting close to their stars or with highly eccentric (non-circular) orbits.12 Either scenario is disastrous for rocky planets like Venus, Earth, and Mars, and therefore makes life in those solar systems highly unlikely.
- Requirements for habitability increased greatly. Originally, SETI enthusiasts assumed that only a few conditions were required for an extra-solar planet to be a reasonable candidate for harboring life. However, astronomers today know that hundreds of conditions are required for any kind of advanced life—and this list continues to grow as new discoveries are made.13 This has led some astronomers to adopt the "rare earth" hypothesis—that complex life is exceptionally rare in the universe.14
Our failure to find any evidence for extraterrestrial life, combined with mounting evidence that habitable planets are exceptionally rare, provides a powerful one-two punch against the whole idea of intelligent life on other planets.15
"Weird Life"—Reassessing the Case for SETI
So, is that the end of the story? Not by a long shot according to many SETI enthusiasts. Despite setbacks, some SETI proponents are even more optimistic than ever that they will find evidence for extraterrestrial life. As an example, Andrei Finkelstein, director of the Russian Academy of Sciences' Applied Astronomy Institute, recently declared that extraterrestrials definitely exist, and that we're likely to find them within two decades.16
One reason for this optimism is that scientists are increasingly recognizing that life can survive under a much broader range of conditions than previously thought possible. For example, the discovery of extremophilic organisms has shown us that some microorganisms can exist under exceptionally harsh conditions that were once thought too inhospitable for life.17 Equally impressive is the discovery of complex communities of life forms around deep sea hydrothermal vents.18 These amazing creatures live under extreme temperatures and pressures with no sunlight for photosynthesis. For energy, microbes process the noxious chemical brew coming from the vents and they in turn serve as the base for the local food chain.
If even conventional life can thrive under such hostile conditions, then what about life that might be different from anything currently known? Early attempts to answer this question ranged from fanciful to highly speculative; but the emerging field of astrobiology is seeking to address the issues of unconventional life based on credible scientific principles. A large number of ideas have emerged about just what kinds of life might exist, where it might arise, and where it might be found. For the purposes of this paper, we will divide all proposed alternative life forms into two basic categories: "exotic life" and "weird life."
The first kind—"exotic life"—refers to organisms that are similar to known life but utilize a different set of biomolecules. For example, scientists are experimenting with microbes that incorporate non-standard amino acids, use different molecules to form the backbone of DNA/RNA, or use non-universal genetic codes.19 Other recent research has focused on incorporating non-natural nucleic acids into DNA.20 Since "exotic life" still fundamentally relies on carbon for its basic makeup and needs water as its internal solvent, it would likely require the same basic environmental conditions that known life requires. So while this research is of great scientific interest, it does little to expand the number of places life might exist and consequently will not be discussed.
The holy grail of astrobiology is "weird life"—life that is radically different from anything currently known. The challenge in investigating such possibilities is that it requires us to completely rethink even the most fundamental aspects of life chemistry. Most speculation about alternative biochemistries focuses on the possibility of life utilizing a biochemistry that doesn't rely on carbon and/or one that utilizes an internal solvent other than water. (A few scientists have proposed some more extreme ideas than these, but they are beyond the scope of this paper.)21 If "weird life" can exist, it would have some important implications for SETI:
- Non-carbon-based life. The big attraction of non-carbon-based life is that such life might be able to thrive under conditions too extreme for known life. For example, many silicon compounds can withstand significantly higher temperatures than similar carbon compounds, thus hypothetical silicon-based life might be able to reside on planets considered too hot for conventional carbon-based life.22
- Non-water-based life. Water is abundant in the universe, but paradoxically liquid water is actually rather rare. This is because a large number of things must be "just right" (finely tuned) for a planet to retain large quantities of water and keep it in the correct temperature range for extremely long periods of time.23 On the other hand, liquid environments other than water may be common in the universe.24 For example in our own solar system, pools of liquid methane/ethane have been discovered on Saturn's moon Titan, ammonia is known to exist within the clouds of Saturn and Jupiter, and Neptune's moon Triton appears to have geysers of liquid nitrogen.25 It has been hypothesized that some of these liquids could serve as a medium for life.
Taken together, this suggests many new possibilities about where life might arise. So is "weird life" a real game changer for SETI? This remains to be seen. Weird life claims remain highly controversial and very speculative. So what can we do to assess whether or not some form of weird life might exist? Since any hypothetical alternative life must be chemically possible, the universal rules of chemistry can provide us many useful insights. In this paper, we will examine the most prominent weird life ideas and evaluate them using our knowledge of chemistry. And we will start by looking at carbon and examining if another element might take its place.
Carbon as the Universal Building Block of Life Chemistry
All known life utilizes carbon as a critical component in almost every facet of the cell. The question to consider is whether this is true because carbon is uniquely capable of serving as a basis for biomolecules. Must life be based on carbon? Or is the reliance of Earth-life on carbon just the result of the particular conditions found on Earth? The first step in addressing that question is to examine the features of carbon chemistry that are so pivotal in Earth life. This in turn, will serve as a benchmark against which we can judge the merits of silicon and boron, which we will cover a little later.
Why is Known Life Carbon-based?
Even the simplest known microorganism is staggeringly complex.26 All life must be able to perform a wide variety of tasks (absorb nutrients and food, convert food to energy, remove waste, repair/replace body parts, reproduce, etc.). And all of that complexity is concentrated into an astonishingly small package.27 (The complexity per unit volume of even the simplest cell exceeds that of a computer circuit board.) This necessitates the existence of a large and diverse collection of complex molecular machines capable of carrying out all of these operations. We infer from this that any element that might serve as a basis for life chemistry must be able to (1) support large polymers and (2) generate a vast array of different types of chemical structures. We can use these criteria to help identify which elements have the greatest likelihood of supporting life chemistry.
There are two major reasons why the ability to form large polymers is so critical to life. First, this allows for the creation of complex macromolecules—large molecules composed of simpler sub-units. (Polymers are a special case of macromolecules where all of the sub-units are of the same type.) Macromolecules are a major theme in life chemistry. Using just a few different building blocks one can generate an endless array of stable, but variable and interchangeable molecular forms.28 In terrestrial organisms, proteins (the workhorse molecules of the cell) are polymers made from about twenty different amino acids. Proteins have to be enormous in order to carry out their precise catalytic functions while simultaneously being specific enough to not react with other molecules. Second, long polymer chains are critical for encoding genetic information. In Earth-life, this role is handled by DNA, which is a polymer constructed from just four nucleobases (plus sugars and phosphate). Organisms require an enormous amount of genetic information for reproduction, so the ability to form strands of nearly unlimited length is absolutely vital. In summary, alien life—even weird life—most probably requires the ability to form long polymers even though the specific building blocks would be different from those of Earth life.29
Since most people are unfamiliar with just how enormous some of these molecules are, it may be instructive to provide a few examples to help establish a sense of scale. The average size of a yeast protein is 466 amino acids corresponding to around 8,000 atoms and the largest known proteins can contain up to 27,000 amino acids or about 500,000 atoms.30 For DNA, biochemists have estimated that the absolute minimum number of genes needed for a functional organism is around 200-500 gene products with an average gene product being about 1,000 nucleotide bases in length.31 That corresponds to about 8-20 million atoms. And, of course, most organisms require far more gene products. For example, the minimum for an independent (non-parasitic) microorganism is estimated to be 1,300-2,300 gene produces or about 52-92 million atoms.32 While these figures are drawn from Earth life, it is reasonable to expect that life anywhere regardless of its biochemical makeup would require molecules of similar sizes.
Carbon and the Periodic Table
To best understand carbon and how it differs from all of the other elements, we begin by looking at the periodic table. The periodic table arranges elements according to specific patterns so as to reveal important relationships between them. Starting with this as the big picture and using our knowledge of chemistry, we can eliminate those elements that are clearly unsuitable for life chemistry. This allows us to quickly filter out the majority of elements before focusing closely on the remaining ones.
|Figure 1: Periodic Table illustrating the division of elements into three primary groups: main group elements (red), transitional metals (green), and inner transition metals (blue). Image credit: John Millam.|
|Figure 2: Periodic table illustrating the division of the main group elements into four sub-groups: non-metals (green), metalloids (orange), metals (light blue), and noble gases (pink). Image credit: John Millam.|
Let us begin by dividing the elements of the periodic table into three primary groups: main group (or representative) elements, transition metals, and inner transition metals. Main group elements (red, figure 1) include the most common and familiar elements. Just six of these elements (carbon, oxygen, hydrogen, nitrogen, calcium, and phosphorus) make up 99 percent of the mass of the human body. In contrast, the transition metals (green, figure 1) and inner transition metals (blue, figure 1) are highly valued in industry, but only play a minuscule role in living organisms.33 This is primarily because these elements rarely join together to form larger and more complex structures. As such, none of these metals could possibly serve as a basis for life chemistry. That leaves only the main group elements for further consideration.
Main group elements can be further divided into four subgroups: non-metals, metals, metalloids, and noble gases. Non-metals (green, figure 2)—including carbon, nitrogen, oxygen, hydrogen, sulfur, and phosphorus—are the heroes of our story because they are the principal components of all known life. Main group metals (light blue, figure 2) are not utilized in the construction of complex macromolecules (hence they could not serve as a basis for an alternative biochemistry), although some of them are utilized for specific roles in living systems. For example, sodium and potassium ions are used to build up charge for nerve conduction and calcium is a key component of teeth and bones. Metalloids (orange, figure 2) fall on the border between metals and non-metals, sharing some characteristics of each. In Earth-life, some metalloids play a very small but necessary role in life.34 Two metalloids, boron and silicon, located next to carbon on the periodic table, have been proposed as possible replacements for carbon so we will discuss them later in this paper. The fourth and last group, the noble gases (pink, figure 2), rarely form even the simplest of compounds and need not be considered further. In summary, only main group non-metals (green, figure 2) and some metalloids show any real promise as a basis for life chemistry.
Important Trends in the Periodic Table
Having narrowed down the field of possible candidates, we can focus our discussion on just the upper-right corner of the periodic table. From here, we can effectively compare carbon to all of its neighboring elements by exploiting our knowledge of two primary periodic table trends. The first trend is that elements in the periodic table are arranged into columns known as "families" representing similar chemical behavior. For example, members of the same family prefer to bond to the same number of hydrogen atoms—the carbon family to four, nitrogen family to three, the oxygen family to two, etc. The second critical trend is the row (or "period") in which the element is located.35 While many chemical properties are associated with an element's family, other important behaviors are tied to its period, such as the ability to form multiple (i.e., double and triple) bonds. Taken together, these two trends help us understand the most important chemical behaviors of each element.
The Life-Essential Properties of Carbon
With that as our foundation, we now turn to the question of what makes carbon so special. Chemists have identified at least five major features of carbon that explain why it is so uniquely qualified to serve as a basis for life chemistry. To help visualize this, three of these properties are shown in the context of the periodic table (see figure 3).
- Forms up to four single bonds. This is the general rule for members of the carbon family, whereas the neighboring boron and nitrogen families are usually limited to only three and the other main group families are even more limited. With the exception of hypervalent molecules36 (which will be discussed more later) this represents the effective maximum in bonding, which means that carbon can form an exceptionally wide range of molecules.
- Stable double and triple bonds. Carbon can form strong multiple bonds with carbon, oxygen, nitrogen, sulfur, and phosphorus,37 which greatly increases the number of possible molecules that carbon can form. In contrast, main group elements in the rows below carbon on the periodic table, such as silicon, generally do not form multiple bonds.38
- Forms aromatic compounds. Aromatic molecules (in chemistry, "aromatic" does not refer to the aroma or odor of a molecule) are a special case of multiple bonds in ring systems that display exceptional chemical stability. (Benzene is the most well-known example of this class of molecules.) Because of their unique chemical properties, aromatic molecules play an important role in many biological molecules—including four of the twenty main amino acids, all five nucleic acids, as well as hemoglobin and chlorophyll.
- Strong carbon-carbon bonds. The carbon-carbon single bond is the second strongest same-element single bond among non-metals (after H2).39 This has two important consequences for life. First, carbon-based biomolecules are very stable and can persist over long periods of time.40 Second, stable self-linking (carbon-carbon bonding) allows for rings, long chains, and branched chain structures that can serve as the structural backbone of a dizzying array of different compounds.
- Can form indefinitely long chains. One of the defining characteristics of life—any conceivable life—is the ability to reproduce itself. That capacity requires the presence of complex information storing molecules (which for Earth-life means DNA and RNA). The longer the chains, the greater the amount of information that can be stored. Of all the elements, only carbon and to a lesser degree, silicon, have this ability to form long complex molecules.41
Taken together, these properties allow carbon to form a wider array of possible chemical compounds than any other element, without exception. For perspective, carbon is known to form close to 10 million different compounds with an almost indefinitely larger number being theoretically possible. In fact, the field of organic chemistry which focuses exclusively on the chemistry of carbon is far richer and more diverse than the chemistry of all other elements combined.
|Figure 3: Upper-right corner of the periodic table showing some important trends in bonding. Image credit: John Millam.|
Stability of Carbon-based Molecules
Previously, we noted that while main group elements are typically limited to forming no more than four bonds, there was an exception to that rule. Elements like silicon, in the third row of the periodic table and below (see figure 3), under certain circumstances form hypervalent molecules containing five or even six bonds. Carbon and other elements of the second row do not possess that ability and so are strictly limited to no more than four bonds. This is noteworthy because it seems to contradict our thesis of carbon's specialness. But does it?
It turns out that carbon's inability to form hypervalent molecules is actually an enormous benefit in disguise. While this inability slightly reduces the number of possible compounds carbon can form, it greatly enhances the long-term stability of carbon-based compounds.42 The reason is that if a carbon atom already has four bonds, it must first break one or more of them before forming new bonds. This means that the reaction cannot proceed unless enough energy is present to first break a bond. The net result is that compounds in which the carbon atoms have four bonds are slow to react even when it would be energetically favorable to do so. In contrast, the analogous reaction for silicon would occur freely because it can directly add a new bond (forming a hypervalent molecule).43 As an example, methane (CH4) is stable in air (except in the presence of a spark or flame), whereas silane (SiH4) reacts spontaneously with oxygen (see figure 4).
|Figure 4: Molecular structures of (a) methane and (b) silane. Image credit: John Millam.|
Since carbon is the primary component of all biomolecules, if carbon behaved like silicon, these life-essential molecules would fall apart too easily or react spontaneously (at standard Earth temperatures) rendering carbon-based life impossible. Chemists Michael Dewar and Eamonn Healy contend that this stability (due to carbon's lack of hypervalency) is what makes life possible.44
The Verdict is in—Carbon is Special
In 1961, physicist Robert Dicke said it best when he declared, "It is well known that carbon is required to make physicists."45 Clearly no other element can even come close to matching carbon's chemical virtues as we have outlined above. Based on this, biochemist Norman Pace went so far as to suggest that wherever life might be found, it will be subject to the universal nature of biochemistry.46 Or to put it more simply: life we might find elsewhere will most likely have to be carbon-based, and therefore chemically similar (but not identical) to Earth-life.
Not everyone is convinced that carbon is indispensible for life. While carbon may be ideal for terrestrial conditions, alternative biochemistries might be favored on planets with much more exotic habitats. In 1973, Carl Sagan introduced the term "chauvinism" to describe a conceit that life elsewhere must on some level be similar to known life.47 For example, he applied "temperature chauvinism" to those who hold that all life requires moderate temperatures (close to those found on Earth), "oxygen chauvinism" described those who believe that life requires breathable oxygen, and so forth. The case that concerns us here is "carbon chauvinism "—the parochial belief that life must be carbon-based because this is all that we currently know. Is carbon really the only basis for life or do we just think it is because we have not sufficiently investigated other possibilities?
The crux of Sagan's argument is that we only have one kind of life to study—Earth life. It is therefore premature to rule out other possibilities before having fully investigated them. A second issue is that chemists have put far more effort into studying carbon chemistry than silicon chemistry, a form of observer selection bias.48 In other words, it is possible that chemists have incorrectly ruled out silicon-based life based on an incomplete understanding of what kind of molecules silicon can form. Sagan's concern is a valid one.
Some authors, however, go much further than Sagan by categorically dismissing any claims that all life found anywhere in the universe must be carbon-based.49 Are we (the authors) guilty of carbon chauvinism because we concluded that carbon is uniquely qualified to serve as the basis for life chemistry and that no other element even comes close? We don't think so. While our conclusion could be wrong, it is clearly established based on a wealth of chemical evidence and so cannot be dismissed as mere ignorance or bias. Curiously, even Sagan reluctantly admitted to being a carbon chauvinist based on his recognition of carbon's superior chemical abilities.50 Nevertheless, it is essential to examine the case for hypothetical alternative biochemistries in detail. We will begin by considering silicon, which is the most commonly cited alternative to carbon. And after that, we will consider the only other serious contender—boron.
Could Life Be Based on Silicon?
Silicon was first proposed as an alternative to carbon by astrophysicist Julius Scheiner in 1891. Part of his reasoning was that many silicon compounds are stable at very high temperatures; so hypothetical silicon-based life might therefore be ideally suited for planets that are much hotter than Earth. By 1909, speculation about silicon-based life was sufficiently commonplace that it was considered "no new matter" when chemist J.E. Reynolds speculated that silicon might substitute for carbon in living systems.51 Much later Sir Harold Spenser Jones stated, "The only other element [than carbon] that possesses the power of building up complex molecules to any great extent is silicon ."52 Today, silicon remains the most popular candidate and is advocated by William Bains,53 Peter Molton,54 Stephen Benner,55 and V. Axel Firsoff,56 among others.
Silicon's main virtue is that it belongs to the same family (column of the periodic table) as carbon and so shares many of its strengths, including the ability to form up to four single bonds.57 Silicon is also good at self-linking (bonding directly to other silicon atoms), albeit to a considerably lesser degree than carbon. It can even form chains, branched chains, and ring structures similar to carbon.58 A further advantage of silicon is its abundance—it makes up 28 percent of Earth's crust (second only to oxygen)—making it about 1,000 times more abundant than carbon. This means that, unlike carbon, large amounts of silicon will likely be present on most small rocky planets.
Three Classes of Silicon Compounds
Because silicon has a natural affinity for bonding to oxygen, it is important to divide our discussion of silicon chemistry into three main groups.59
- Silanes. Silanes are the silicon analog of hydrocarbons with silicon directly bonded to other silicon atoms (see figure 5a). We include in this group all molecules in which other elements or side groups are attached to a silane backbone. (This is analogous to hydrocarbons serving as the backbone for carbon-based biochemistry.) The one caveat is that we exclude from consideration molecules where silicon is directly bonded to oxygen, since they are covered in the next two groups.
- Silicates. A silicate unit formally consists of a silicon atom bonded to four oxygen atoms (see figure 5b). These silicate units often connect to similar units by sharing oxygen atoms. For the purposes of this paper, silicates (or generically silicon oxides) refer more broadly to compounds where the silicon atoms are primarily bonded to oxygen atoms.
- Silicone. Silicones represent a half-way compromise between silanes and silicates. The backbone consists of silicon bonding alternately with oxygen. The silicon atom's remaining two bonds are connected to organic (carbon-containing) functional groups (see figure 5c).
We study these three classes separately because each behaves in characteristically distinct ways. So there are three distinct types of silicon-based life to consider: silane-, silicate-, and silicone-based life. Could any (or all) of these support a viable life form and if so what conditions would it require? We will now examine each one in turn.
|Figure 5: Simple examples of the three classes of silicon compounds: (a) a silane, specifically disilane, (b) a silicate unit (SiO4-2), and (c) the simplest silicone, polydimethylsiloxane (PDMS). (The n represents the number of times the repeating unit appears in the polymer and is an integer.) Image credit: John Millam.|
Silanes as the Basis for Life?
Silanes are a natural candidate for an alternative biochemistry, because they are direct analogs of hydrocarbons that form the basis for terrestrial biochemistry. This includes a limited ability to support large polymeric molecules. That is of vital importance for creating complex biomolecules and carbon is the only other element capable of doing this. Despite silicon's resemblance to carbon, silanes fall way short of carbon's ability to form complex molecules for the reasons described below.
- Silicon-silicon bonds are weak. Single bonds between silicon atoms are twenty percent weaker than carbon-carbon single bonds (see table 1). Therefore, silanes are less stable than their carbon counterparts.
- Lack of strong multiple bonds.60 Since silicon very rarely forms multiple bonds, it is far more limited than carbon in terms of the number and diversity of compounds it can form.
- More reactive than carbon. Silicon compounds are generally more reactive than their carbon counterparts.61 For example, methane (CH4) is stable in air and will only react with oxygen in the presence of a spark or flame, whereas its silicon-analog silane (SiH4) reacts spontaneously in ambient air (see figure 4). Silicon's heightened reactivity would be catastrophic to the maintenance of information-storing molecules (except possibly at very low temperatures as we will discuss later).62
- Susceptible to oxidation. Silicon has a strong affinity for oxygen, since it forms a much stronger bond to oxygen than to silicon (see table 1). This means that compounds containing silicon-silicon bonds would generally be damaged by reactions with any oxygen gas (O2) in the environment. Therefore, hypothetical silane-based life would likely require anoxic (oxygen-free) conditions.
- Incompatible with water and ammonia. Silanes generally react with both water and ammonia.63 This is because silicon forms much stronger bonds to both oxygen (in water) and nitrogen (in ammonia) than it does to silicon (see table 1). Silicon biochemistry would therefore likely require an environment that is devoid of both of these compounds in the liquid form. Extremely cold or extremely hot conditions could potentially ensure a liquid water- and ammonia-free environment, but in these scenarios another liquid would be needed to serve as a life solvent in their place.64
- Silicon-hydrogen bonds are not very stable. The silicon-hydrogen bond poses several problems for silanes. First, such bonds are very reactive and are particularly vulnerable to reacting with water and other oxygen-containing compounds. Second, the presence of these bonds tends to destabilize the whole molecule. For example, simple silanes (containing just silicon and hydrogen) are unstable beyond a paltry six consecutive silicon atoms!65 In contrast, there is no known limit on the length of hydrocarbons. One key reason for this difference is that silicon is slightly less electronegative than hydrogen, whereas carbon is slightly more electronegative.66 Because of this, hydrogens bonded to silicon take on a partial negative charge which makes them extremely reactive. Molecules in which a silicon atom is bonded to just one hydrogen are weakly stable but highly reactive, and adding a second or third hydrogen makes them progressively more unstable.67 This effectively limits the number of hydrogen atoms that can be incorporated in a molecule, which seriously limits the number of useful structures that silicon can form.
- Constraints on long chain structures. As we established earlier, the ability to form long complex chain structures is a fundamental requirement for life chemistry. Silicon is the only element (other than carbon) than can form chains of notable length. For this discussion, we will subdivide silanes into two distinct groups. The first group consists of simple (non-polymeric) silanes. Chemists have been able to synthesize a wide variety of these types of molecules, but the largest documented examples are only 26 consecutive silicon atoms in length.68 The second group is polysilanes—long polymers made from simpler silanes. These molecules can reach lengths of around 40,000 consecutive silicon atoms.69 Polysilanes, however, are severely limited. First, they are currently limited to incorporating just a few simple organic (carbon-containing) side groups due to the severity of the reaction mechanism. Second and even more problematic is that these polymers are monotonous with each silicon atom having the same side groups, whereas biologically functional polymers (akin to proteins and DNA) need to support variable and non-repeating sequences.70 In summary, simple silanes are chemically diverse but of limited size, while polysilanes can be very long but lack the diversity needed to be biologically useful.71
Given these issues, silane chemistry would be best suited for low temperature, highly reducing (i.e., hydrogen-rich, oxygen-poor) environments. One special case—ultra cold conditions—will be considered in more detail a bit later.
Note: Actual bond strengths are highly variable depending on which compounds are involved. The values reported here represent the average bond energy over a large representative sample of molecules. Information taken from Plaxco and Gross, Astrobiology, Table 1.1, p. 9.
Silicates as the Basis for Life?
If the weak silicon-silicon bond is the Achilles heel of silicon chemistry, then the silicon-oxygen bond is its greatest strength. The silicon-oxygen bond is sixty percent stronger than the silicon-silicon bond and in fact is even stronger than the carbon-carbon single bond (see table 1). This makes silicate structures very strong and stable, which has three critical consequences. First, silicates are abundant on Earth and other small rocky planets. In fact, 90 percent of Earth's crust consists of silicate rocks (consisting of silicon, oxygen, and other elements). Second, silicates are very stable chemically—they (unlike silanes) generally don't react with oxygen, water, or ammonia. Third, they are very stable thermally—able to withstand very high temperatures. For example, silicates generally require temperatures in excess of 1000°C/1832°F to melt (although a few melt as low as 500°C/932°F).72 Thus, hypothetical silicate-based life73 could be favored on extra-solar planets like Kepler 78b that orbit so close to their parent star that parts of the surface would be molten lava. Such life could even thrive in magma deep below the Earth's crust.
While silicates avoid many of the pitfalls of silanes, they too have some serious drawbacks when it comes to life chemistry.
- Silicates form crystalline structures rather than polymers. Most silicates are found as rocks, such as those that make up the Earth's crust. For example, the simplest silicate, silicon dioxide or silica (SiO2), is typically found in nature as common sand. Such crystalline structures are poorly suited for constructing complex biomolecules.
- Chemical stability as a limitation. While chemical instability is certainly a detriment to possible life chemistry, so is being too stable (chemically inert). Silicon oxides are so stable that they make life-essential reactions unfeasible and therefore render silicon oxides an exceptionally poor choice for life chemistry.
- Inaccessibility of silicon in the environment. Most of our planet's silicon is tightly held in the form of silicate rocks.74 Silicate-based minerals are generally very stable; so that once formed, they will typically remain unchanged over long periods of time. These minerals are largely inert and insoluble in water. In contrast to silicon dioxide, carbon dioxide (CO2) is a gas75 and dissolves readily in water. So even though silicon is far more abundant that carbon in the Earth's crust, very little of it would be available for reactions in either terrestrial or aquatic environments.
- Difficulty in elimination. Silicon dioxide would likely be a byproduct of silicate-based metabolism, just as carbon dioxide is for carbon-based creatures on Earth.76 Since silicon dioxide is a crystalline solid, this would pose a considerable disadvantage for advanced silicate-based life to be able to efficiently excrete from its body. While we cannot predict how difficult eliminating solid silicon dioxide might be, it would almost certainly be far more challenging than exhaling a gas like carbon dioxide.77
- Reactions would be too rapid. Reactions occur faster at higher temperatures. Given the high temperatures required for silicate chemistry (above 1000°C/1800°F), critical life reactions would occur so fast that it would be extremely difficult for organisms to control them.78
- Lack of an adequate solvent. Life requires a liquid medium in which the key chemical reactions can take place. It is very difficult to imagine a substance that would be a liquid at these extreme temperatures and still be able to support the necessary biochemical reactions.
Clearly, silicate chemistry is an extremely poor candidate for hosting life. But if silicate-life could exist, it would be best suited for small rocky (silicate-rich) planets at very high temperatures. In fact, molten magma deep below the Earth's crust would provide an ideal habitat for such life. We have no evidence that such life has ever existed on Earth, so if silicate-based life cannot arise even under such optimal conditions, then we should not expect it to arise anywhere.79 This is the final nail in the coffin for silicate-based life.
Silicones as the Basis for Life?
Silicones represent a novel alternative to more traditional ideas about silicon-based life.80 The backbone for this class of molecules consists of silicon alternating with oxygen, thus taking advantage of the great strength and stability of the silicon-oxygen bond. For each silicon atom, the two remaining bonds attach to organic (carbon-containing) side groups (see figure 5c). Different choices of the organic groups control the properties of the polymers allowing silicones to be tailored for many different applications. In this way, silicones have the best of both worlds—the stability of silicates with the flexibility of hydrocarbons.
Silicones are not naturally occurring, but were first studied by chemist Frederick Kipping in 1901.81 Since then a large variety of different silicone compounds have been studied. They are frequently utilized in industry because of their many desirable properties, such as low chemical reactivity, low toxicity, high thermal stability, resistance to ultraviolet light, resistance to atmospheric oxidation, and water repellency (does not wet). Some common examples of their application include high-temperature lubricants, waterproof caulk, electrical insulation, and cookware. Many silicones are useful in high-temperature applications, such as oven mitts that can withstand temperatures up to 260°C (500°F)—allowing the user to even reach into boiling water.
The main virtue of silicones as a possible basis for life chemistry is their ability to form long and complex polymers. This opens the possibility of forming stable complex molecules that could serve to store genetic information or act as biomolecules (fulfilling the roles that DNA, RNA, and proteins perform in terrestrial life). The second most intriguing feature is their high thermal stability. This means that hypothetical silicone-based life might be able to thrive in environments that are too hot for carbon-based life.
While silicones show a lot of promise, there are at least four critical challenges to the idea of silicone-based life. First, silicone compounds break down around 400 °C/752°F, which is only modestly higher than the upper limit for carbon chemistry (200°C/392°F).82 Therefore, the temperature window within which silicones have the edge over carbon is relatively small. Second, silicones do not occur naturally on Earth but are man-made; therefore it is unlikely the precursors to silicone-based life would occur naturally under prebiotic conditions on other planets or their moons. Third, the carbon side-chains needed to build potential silicone-based life would likely preferentially bond to fellow carbon atoms rather than silicon where it can form stronger bonds (see table 1). These unwanted competing reactions would pose a serious problem for silicone-based life. Fourth, silicones are water repellant, which means that they could not utilize water as the liquid solvent in which their reactions could take place. Even more problematic is that at the higher temperatures (200-400°C) where silicone would be favored over carbon, there is no good choice of solvent. Taken together, these factors present a major—possibly fatal—objection to the notion of silicone-based life.
One Final Scenario—Silane-based Life in Ultra-cold Conditions
Under warm Earth-like conditions, carbon chemistry is clearly superior to that of silicon; however, it has been proposed that silane-based life may be favored under extremely cold conditions, namely temperatures near or even below the point where nitrogen becomes a liquid (196°C/321°F).83 Such environmental conditions exist on the outermost planets (Neptune and Uranus) as well as some of their moons. These extremely harsh conditions result in two unique challenges. First, the solubility of molecules is temperature dependent; therefore, under ultra-cold conditions solvents are typically only able to dissolve small amounts of very simple molecules. Second, reaction rates also decrease with decreasing temperature—rendering traditional carbon-based chemistry completely ineffective under such cryogenic temperatures. If carbon will not work under these circumstances, perhaps something else can.
Silicon has several properties that might prove useful under these extreme conditions. First, certain silanols (silicon analogs of carbon-based alcohols) maintain their solubility even under such cold temperatures and may possibly even condense together to form more complex structures.84 The other critical property is that silicon's greater reactivity (a detriment at standard Earth temperatures) could allow some reactions to proceed even under cryogenic conditions. And as an added bonus, water and ammonia would be frozen solid and so would not be present as liquids to interfere with the silicon-based chemistry. Research in this area is still very preliminary, and many significant problems remain that make silicon-based life, even in ultra-cold conditions, highly improbable.
Silicon is Too Limited to Support Life!
What conclusion can we draw from all of this? First, silicon is far less versatile than carbon. For perspective, chemists currently recognize only about 20,000 silicon compounds. In contrast, carbon is known to form some 10 million compounds and is virtually unlimited in the number it could theoretically form. Therefore, silicon is at least 500 times less versatile than carbon, which for life chemistry is a huge deficiency, because life requires an incredibly rich diversity of chemical reactions. Second, each class of silicon compounds has critical problems of its own. Silanes are too reactive to form a stable basis for life; silicates form crystalline structures instead of useful polymeric structures; and silicones do not occur naturally. Lastly and most significantly, silicon is far more limited than carbon in its ability to form the long chains needed for complex information-storing molecules required by any conceivable form of life. Taken together, this suggests that silicon is unlikely to be able to fill all of the roles needed for life, even single-celled life.
The evidence is now in—silicon is simply too limited. Even the esteemed Carl Sagan (who coined the term "carbon chauvinism") reluctantly recognized that silicon was inadequate to support life and so focusing on just carbon-based life is "not nearly so parochial and chauvinistic as it might seem."85 (He rejected silicon because of its inability to form large information-bearing molecules and the problem of excreting solid silicon dioxide.) And we (the authors) wholeheartedly agree. So bidding adieu to silicon, we will now consider the case for boron chemistry.
Could Life Be Based on Boron?
Boron is the only other significant challenger to carbon. It is located just to the left of carbon on the periodic table (whereas silicon is just below carbon). One of the reasons that chemists are interested in boron is that its chemical behavior is amazingly versatile and highly unusual.86 Moreover, it shares silicon's ability to form many compounds that are stable at high temperatures. While boron chemistry has not yet been fully explored, it does have several known deficiencies. First, boron does not form hydrocarbon analogs or similar chain-like structures that could serve as a backbone for complex biomolecules.87 A second and far more serious objection is that boron is cosmically rare. In Earth's crust, it is about 100,000 times less abundant than silicon and even 100 times rarer than carbon. The net result is that wherever boron is found, carbon will also be found, so even if boron-based life were to arise, it would likely be beaten out by carbon-based or even silicon-based life.88 Third, boron oxide (BO) is a solid, so if it was the byproduct of a boron-based metabolism, it would pose a challenge to eliminate from the body. (This is, essentially the same problem we discussed earlier resulting from silicon dioxide being a solid.)
Special Case: Boron-Nitrogen Chemistry
Given the problems with elemental boron, some have suggested a modified scenario—boron alternating with nitrogen. Boron is located just to the left of carbon on the periodic table and nitrogen just to the right, so it is not surprising that when they pair up they behave similar to a pair of carbon atoms. These boron-nitrogen pairs are sometimes referred to as "pseudocarbons" and allow for the construction of many analogs of carbon compounds. For example, the pseudocarbon analog of benzene is borazine (see figure 6). There are also known analogs of diamond, graphite, and carbon nanotubes.
|Figure 6: Structure of (a) benzene and (b) its boron-nitrogen analog borazine. Image credit: John Millam.|
Despite some promising capabilities, boron-nitrogen chemistry is a very poor choice for alternative life chemistry. First, these types of compounds are rarely found in nature, so it is difficult to imagine a prebiotic pathway leading to boron-nitrogen life. Second, many of these compounds are less thermally stable than their carbon-based equivalents. Third, they can only mimic a small subset of carbon compounds and so lack the flexibility and chemical diversity required for life chemistry. Fourth, these compounds are more reactive than their carbon counterparts, so they would likely require lower temperatures. Fifth, many of these compounds decompose in water (including the above mentioned borazine), so they could not utilize water as their internal solvent.89 Based on these major disadvantages, we can rule out life based on boron-nitrogen chemistry.
There is no Alternative to Carbon for Life
Astrobiologists have been eagerly pursuing the possibility of non-carbon-based life. But as intriguing as these ideas are, they all have very serious problems as we have pointed out. Therefore, while the carbon-only view of life is frequently lambasted by those who promote the concept of extraterrestrial life, there are, to date, no working alternatives that have been presented in any detail.90 And there is little reason to expect this to change in the future.
Now, we will turn our attention to the possibility of life based on solvents other than water.
The Elixir of Life—Water as the Solvent of Life
Our focus so far has been on the biochemical makeup of living things. Given how important this is, it is easy to overlook the fact that these molecules do not operate in a vacuum. On Earth, all of the biochemical activity inside of cells occurs in the context of liquid water. While carbon is the star of the show in terms of the composition of life, water works quietly behind the scenes like the army of stage hands who make the performance possible but are never seen by the audience.
Consideration of the liquid medium needed for life processes may not be very glamorous, but it is at least as important as the choice of elemental composition. Perhaps, it is even more important. First, the choice of liquid determines what kind of biochemistry could possibly occur in it. For example, silane chemistry is incompatible with water and ammonia. Second, compounds are only a liquid over a specific range of temperatures (at a given pressure). That means that a given solvent would be restricted to planets with temperatures and pressures that could keep it in the liquid state. For example, water-based life is likely restricted to planets orbiting in the "habitable zone" of their star where there is a chance of liquid water on their surface.91 And all of these considerations apply to any type of life—even weird life.
The Necessity of a Liquid Medium for Life
Water serves as a host environment for all of the biochemical reactions that take place within the cell. Before investigating if another liquid might be able to fill this critical role in some other form of life somewhere else in the universe, we should first consider whether the biochemical environment even needs to be a liquid at all. Could a gas, or maybe a solid host life chemistry? Scientists who have seriously considered these possibilities recognize several reasons why a liquid environment is clearly superior to either alternative.92
- Liquids provide a common environment. A liquid can act as a solvent that allows solids (e.g., proteins and DNA), liquids, and gases (e.g., oxygen and carbon dioxide) to be suspended together in a common environment in which they can efficiently interact with each other. Both solids and gases are far more limited in their abilities to interact with the other two common states of matter—neither gases nor solids dissolve both of the other two states of matter.
- Liquids allow components to be highly mobile. Efficient biochemical reactions necessitate that the reactants are sufficiently mobile within cells so as to be able to reach each other quickly. Moreover, fresh nutrients must regularly be brought in and waste products eliminated. All of this would be very difficult to achieve in a solid medium because diffusion of material through a solid happens very slowly.93 Liquids and gases, on the other hand, are ideal for facilitating these interactions.
- Liquids allow for encapsulation. Life chemistry also requires that all of its key components remain in close proximity so they can interact rather than being scattered throughout the environment. In particular, there needs to be a way to form a barrier that will isolate the biological system from outside interference or contamination. Such a boundary also serves to concentrate vital ingredients inside which helps promote reactions. But at the same time, this boundary needs to be semi-permeable, so that nutrients can enter and waste products can escape. There is no simple natural way to achieve such organization within a gaseous environment, but this is readily achievable for liquids and solids.
- Reactions occur faster in liquids. Another reason liquids are preferred is that reactions taking place within them generally occur faster than they would in either a gaseous or solid environment.94 Specifically, reactions in solids are limited by the poor mobility of the reactants, while reactions in gases are slower due to lower concentrations compared to liquids. Additionally, liquid solvents generally allow many salts and certain compounds to separate into distinct ions, which allows for much faster reactions than if they stayed together as molecules.95
Given these considerations, scientists generally agree that only a liquid environment is capable of supporting life chemistry—even for "weird life." Solids and gases by comparison are plainly poor candidates and so will not be considered further. (Nevertheless, some have seriously proposed non-liquid-based life.)96
Qualities of a Good Life Solvent
Having eliminated gases and solids as serious candidates, this still leaves a large number of liquids for us to consider. And of course when talking about liquids, we must not restrict ourselves to only those compounds that are liquids at ambient Earth temperatures and pressures. Other planets (and their moons) will have environmental conditions quite different from what we find on Earth, so we need to take this into account when we consider possible alternative life solvents. For example, methane (CH4), which is normally considered a gas, is a liquid under very cold conditions, such as found on the surface of Saturn's moon, Titan.
Given the number of potential liquids as candidates, we need to narrow the field a bit. We can do this by considering which properties are most important for a life solvent.97
- Naturally available. While chemists can draw upon a wide range of solvents in the laboratory, hypothetical emerging life would be completely dependent upon what liquids are naturally available in their local environment. Moreover, the solvent would need to be present in sufficient quantities.
- Good solvent. The main criterion for an effective solvent is that it dissolves a wide range of substances (including both organic and inorganic molecules), and dissolves them in sufficiently large quantities. Of particular importance to life chemistry is the ability to dissolve large macromolecules. This is not a trivial issue because the most important life molecules on Earth are very large, e.g., proteins generally have at least thousands of atoms each.
- Large range of liquidity. This property refers to the span of temperatures in which a compound remains in the liquid state, i.e. the difference between the melting and boiling points. The larger the range, the lower the risk of environmental temperature fluctuations causing the life solvent to either freeze or boil—either of which would be deleterious to the organism, possibly even fatal. In comparing the range of liquidity among possible solvents, we must take into account that it depends on two environmental factors. First, the presence of impurities can lower the melting point and therefore extend the range of liquidity. A common example of this phenomenon is adding antifreeze to the car radiator water to prevent it from freezing during the winter. The second factor is the external pressure—increasing the pressure increases the liquid's boiling point. We see this at deep-sea hydrothermal vents where water is vented at very high temperatures yet remains in the liquid state due to the higher pressures at the bottom of the ocean.
- Ability to encapsulate. One simple mechanism by which liquids can spontaneously develop boundaries or partitions is called the hydrophobic effect. That means that "oils" (hydrocarbons and non-polar molecules) in a polar solvent such as water or ammonia will separate out from the surrounding solvent. In fact, a special class of fat molecules called lipids will spontaneously self-assemble in water (under certain conditions) into cell membranes that serve to define an outer boundary for cells. These types of structures can also be used to create compartments within the cell allowing for internal organization.
- Large dielectric constant. A liquid's dielectric constant determines its ability to allow ions (charged atoms or molecules) in solution. Having a large dielectric constant is important for several reasons. First, it facilitates the dissolution of salts (by allowing them to separate into their component ions). Second, it allows large quantities of ions to be present in solution, such as the sodium (Na+) and potassium (K+) ions used in nerve message conduction. Third, it enables efficient acid-base chemistry (which involves the presence of ions, such as H+ and OH-). Fourth, solvents with large dielectric constants are good at dissolving large molecules that have electrically charged centers.98 (Typically, the larger the molecule, the harder it is to dissolve and keep in solution, and a large dielectric constant provides one important mechanism for dealing with this issue.)99
- Good thermal moderator. All organisms must deal with temperature variations in their environment as well as the heat generated by their own internal biochemical processes. A solvent's thermal properties—if they are high enough—can help moderate these temperature changes to protect life from either overheating or freezing. There are three thermal properties that we will focus on in this paper. First, heat capacity is the amount of heat energy required to raise the temperature of a substance by one degree Celsius. For a given gain/loss of heat, a larger heat capacity would result in a smaller change in temperature for the liquid. Second, the heat of fusion (or heat of melting) represents the amount of heat that must be removed in order to freeze a liquid. A large heat of fusion means that a liquid is less likely to freeze relative to a liquid with a smaller value. Third, the heat of vaporization operates in an analogous way to protect a liquid from boiling off. A liquid with a large heat of vaporization would also support efficient evaporative cooling, which can be exploited by organisms to cool themselves. Together, these three properties gauge how well a solvent protects the system from temperature changes.
- Low viscosity. Viscosity is a measure of how easily a liquid will flow. Molasses is a well-known example of a thick (high viscosity) liquid and water is a good example of a low-viscosity liquid. A life solvent with a low viscosity would allow a high degree of mobility, allowing biomolecules to interact efficiently (e.g., cellular enzymes are able to quickly reach the molecules that they need to act upon).
- Surface tension. A high surface tension is the driving force in the phenomenon of adsorption, in which certain dissolved substances will naturally stick to a surface rather than float freely in the solvent. This occurs for substances that lower the surface tension of the solvent, making it energetically more favorable to be at the solvent surface rather than in the middle. Most proteins and similar biomolecules have this surface tension-lowering property, which causes them to aggregate onto cell membranes. This phenomenon is critical for the formation of organized cell membranes. A high surface tension is important for another critical phenomenon—capillary action. Capillary action means that a liquid will be drawn into narrow spaces without requiring an external force. (On Earth, capillary action helps soil retain moisture and allows trees and vascular plants to transport fluids from their roots to their leaves.)
Of the eight properties above, the first three are viewed as being the most critical and the remaining five as of lesser importance.
Water as the solvent of life
Currently, the only liquid known to serve as a life solvent is water. As such, it is useful to consider how optimally it fulfills all of the criteria for a good life solvent. This will then serve as a benchmark against which we can compare other possible solvents. A full review of water's exceptional properties is beyond the scope of this paper. Instead, we refer our readers to our seven-part series titled "Water: Designed For Life."100 Those articles define important terms, provide helpful background information, and explain the roles of water's key properties. Here we will simply summarize only the most relevant details:
- Water is ubiquitous. Water is one of the most abundant molecule in the entire universe.101 As such, it can be expected to be found in large quantities on most planets—although paradoxically, it is rather rare in the liquid state (Water, part 7). On Earth, it is the only naturally-occurring, inorganic liquid found in abundance.102
- Water is the "universal solvent." Water is considered the most effective known solvent, which has earned it the title "universal solvent." A major reason for its success as a solvent is that it's a highly polarized molecule, which helps it dissolve other polar molecules as well as salts (Water, part 1). Non-polar organic molecules (e.g., oils) are one of the few classes of molecules that generally do not dissolve in water. But even this limitation has a positive benefit—the hydrophobic effect (which will be discussed further below).
- Water has a large range of liquidity. Under terrestrial conditions, pure water is a liquid over a rather impressive range: 0-100°C/32-212°F. Adding common salts can reduce water's melting point as low as 23°C/10°F. Increasing the external pressure to 215 times atmospheric pressure can increase the boiling point to 374°C/706°F.103 This means that water has a potential range of liquidity of 397°C/716°F.
- Water has a strong hydrophobic effect. Water is the champion for having the strongest hydrophobic effect, which is essential for the formation and maintenance of strong cell membranes in Earth life. The hydrophobic effect also plays a critical role in protein folding, whereby proteins adopt and maintain the precise three-dimensional shape required to function correctly. (Hydrophobic, i.e. "oily," amino acids in the protein naturally fold toward the center to avoid contact with water while the remaining amino acids fold toward the outside.)
- Water has many exceptional properties. A major virtue of water is that so many of its properties are outside the normal ranges in ways that are beneficial for life. Water has an exceptionally high dielectric constant (Water, part 1), which we know is very important for life solvents; and few liquids can even come close to rivaling water's thermal properties (i.e., those dealing with heat and temperature). Specifically, water has an exceptionally high heat capacity, heat of vaporization, heat of fusion, and thermal conductivity (Water, part 4). Moreover, it has many other useful properties, such as high surface tension, fairly low viscosity, and high rate of diffusion (Water, part 6).
- Water is ideal for carbon chemistry. Given that carbon is the only element capable of supporting life chemistry, it is noteworthy that water is exceptionally well suited for supporting carbon chemistry. This is true for at least two reasons. First, carbon forms very strong bonds to both hydrogen and oxygen, which are the constituents of water (see table 1). In fact, hydrogen's bond to carbon is the strongest it makes to any of the common elements.104 Second, the temperature range at which water is a liquid (0-100°C or 32-212°F at standard pressure) corresponds to the upper range at which carbon chemistry is viable. While carbon chemistry could operate in lower temperature solvents, that would result in slower chemical reactions and retard the development of life. But it would not be helpful to have a much higher boiling point, since most carbon compounds degrade at temperatures above 200°C/392°F. So water is a liquid at the optimal temperature range for carbon chemistry.
In summary, water is an amazing liquid.105 As a life solvent, it is without equal. In the opinion of the authors, these numerous exceptional properties are a clear indication that water is designed for life.
Additional Life Benefits of Water
So far, we have focused exclusively on the ways that water's suite of properties is exquisitely able to support life chemistry. But these are not the only things that water does for life. Water plays a pivotal role throughout our planet's environment in making life possible here. A few key highlights will be presented here, but our readers can go to our "Water: Designed for Life" series for details.
- Helps keep lakes, rivers, and arctic oceans from freezing solid
- Moderates global temperatures and reduces climate fluctuations
- Assists geological transformation through erosion, carving channels, and transporting soil
- Provides global transportation and concentration of salts and minerals
- Facilitates plate tectonics
- Hosts aquatic life
- Regulates body temperatures of animals and humans by cooling through sweating or panting
These factors are important because living organisms are highly dependent upon their environment. Yet discussions of alternative life solvents rarely take these types of issues into account in their evaluations. On Earth, water unquestionably plays an essential role in maintaining a climate in which life can exist, which is particularly important for advanced life.106 No other liquid can even come close to doing everything water does.
Criticism of Water
Despite water's many beneficial properties, some argue that water may not be ideal—even for Earth life.107 Three specific issues will be addressed here:
- Water can damage cells when it freezes. Water is truly exceptional in that it expands when it freezes. While this helps protect lakes, rivers, and arctic oceans from freezing solid (Water, part 3), it also means that when water freezes inside of cells it can rupture the cell membrane, thus killing the cells. This would not occur for life based on other solvents which contract upon freezing rather than expanding. In practice, however, freeze damage is not a significant limitation. Many plants and animals have developed a variety of strategies to deal with water's expanding when it freezes.108 For example, some creatures produce a natural antifreeze that keeps the water in their bodies a liquid even at temperatures below the freezing point of (pure) water.
- Water is reactive—can damage key biomolecules. Water is typically thought of as being non-reactive, thus making it safe to use in a wide variety of household, commercial and industrial applications. While it is relatively inert, it is more reactive than many familiar compounds, such as hydrocarbons ("oils").109 For perspective, organic chemists use solvents other than water for their work about 80 percent of the time in order to avoid its tendency to interfere with their reactions.110 The main issue to consider here is that water will slowly degrade certain key biological molecules, such as the nuclear bases of DNA, requiring the presence of complex repair mechanisms to fix the damage.111 But this is not the whole story. It is actually advantageous that water is not too inert, since it is a vital participant in many essential biochemical reactions. Chemist Felix Franks went so far as to declare that "it is hardly an exaggeration to claim that biochemistry is primarily the chemistry of water."112
- Water is incompatible with carbon dioxide (CO2). Carbon dioxide is a vital carbon source for photosynthesis (or carbon fixation) in plants. (For perspective, plants consume about 258 billion tons of CO2 each year.) As a gas, CO2 quickly diffuses throughout the atmosphere making it readily available to terrestrial plants. In water, however, very little carbon dioxide directly dissolves; instead it reacts with water to form highly soluble bicarbonate ions (HCO3).This is critical for making CO2 available in aquatic environments (e.g., lakes, oceans) as well as inside cells. The conundrum, however, is that bicarbonate ions are not very reactive (hence difficult for cells to metabolize), whereas carbon dioxide is reactive but rather insoluble.113 This poses a significant and costly challenge for plants, both aquatic and terrestrial. One way plants handle this is through an enzyme called biotin. The main problem is that biotin is metabolically expensive and cannot handle large quantities of CO2. The majority of carbon fixation is handled instead through an enzyme called rubisco (or ribulose-1,5-bisphosphate carboxylase oxygenase). Unfortunately, rubisco has a reputation for being wasteful and inefficient. The reaction that it catalyzes is slow, but even worse it has a hard time distinguishing between CO2 and oxygen (O2). When oxygen is taken up by mistake, this results in unwanted compounds and means that even more rubisco is needed to compensate. Ultimately, however, the problem is the intrinsic chemical similarity between CO2 and O2, rather than the fault of water or the design of rubisco.114
In summary, these detractions are all rather minor. Truly, water is exquisitely designed for our planet and all life on it.
Nothing else even comes close to matching water's ability to support life. For this reason, NASA's search for life on other planets has largely embraced a "follow the water" strategy of looking for liquid water. But not everyone agrees that water is the only possible life solvent. Astrobiologists have been seriously studying ammonia and a wide variety of other compounds that could potentially serve as a host for alien biochemistry.
Could Planets Support Liquids Other than Water?
Given that the properties of water are so ideally suited for hosting life, why should we even consider other liquids? The main reason is that with the exception of Earth, water in the liquid state is rather rare in our solar system. For perspective, planets in our solar system exhibit a huge span of temperatures—ranging from hellish Venus 462°C/863°F to frigid Neptune (218°C/360 °F). Pure water (at atmospheric pressure) is a liquid between 0-100°C/32-212 °F, which represents only a small fraction of the temperature range found within our solar system. So if life requires liquid water, there are likely very few places where life might exist in the universe. On the other hand, life based on other liquids could potentially thrive on planets either too hot or too cold for Earth life.
Cosmic Abundance of Liquids
To begin, we need to consider which liquids might actually be present on other planets (or their moons). Theoretical studies by William Bains suggest that many planets should be able to support at least one liquid on or beneath their surfaces.115 Planets vary dramatically in composition, size, orbit, temperature, and atmospheric pressure, yet there is a large selection of potential liquid solvents to choose from. To illustrate this point, just twelve proposed life solvents considered in this paper are sufficient to span the temperature range from 200°C/328°F to 325°C/617°F (see figure 7). This is sufficiently broad to cover most of the temperatures observed on planets within our solar system. We conclude that for most planetary conditions at least a few naturally-occurring compounds might be found in the liquid state. Therefore, liquid environments should be relatively common in the universe.
|Figure 7: Temperature ranges for thirteen pure solvents at standard atmospheric pressure (1 atm). Data taken from table 2. Image credit: John Millam.|
|Figure 8: William Bains's predictions for the probability of finding selected naturally-occurring solvents in a liquid state on a planet in a hypothetical "average" solar system as a function of distance.116 Distances are given in astronomical units (AU), where 1 AU equals the distance from the sun to the Earth. The locations of the planets in our solar system are shown for reference. This demonstrates that multiple liquid environments may be present in many solar systems. Image credit: John Millam.|
Bains's model attempts to describe conditions in an "average" solar system. Based on this, he estimates the probability of finding specific liquids as a function of the planet's distance from its parent star (see figure 8). For scale, our own solar system will be used as a point of reference. Planets located close to their stars (like Mercury and Venus in our solar system) are very hot, but sulfuric acid (H2SO4) is a possibility since it has a much higher boiling point (337°C/639°F) than water. Planets located a bit farther out (similar to Earth and Mars) would be cooler, and might support liquid water (H2O) and/or ammonia (NH3). Moving even farther out into the solar system (corresponding to Jupiter, Saturn, and their moons), it gets cold enough that methane (CH4) will liquefy and could form pools or lakes. And finally at the outer edges of the solar system (near Uranus and Neptune), conditions are likely to be so cold that only liquid nitrogen (N2) might be present. No planet in our solar system is cold enough to allow for liquid hydrogen (H2), but hypothetically it could be present on a planet extremely distant from its parent star.
Observations of various naturally occurring liquids present in our own solar system lend general support to Bains's model. In addition to the obvious water on Earth, astronomers have observed that (1) Venus has droplets of sulfuric acid high in its atmosphere;117 (2) Jupiter and Saturn may be able to support small amounts of liquid ammonia in their cloud layers; (3) Saturn's moon, Titan, is known to have significant lakes of liquid methane (and probably ethane) on its surface; and (4) probes have detected geysers of nitrogen on Neptune's moon, Triton.118 In addition, there is some evidence for subsurface oceans consisting of water or a water/ammonia mix on Jupiter's moons, Europa and Callisto, and Saturn's moon, Titan.119
A few important caveats. First, observations of extra-solar planets reveal that "hot Jupiters" (gas-giant planets orbiting very close to their parent stars) are relatively common. This indicates that other solar systems differ from our own—sometimes very significantly—so the presence of surface liquids is probably less common than is suggested by our own solar system.120 Second, Mercury and Mars are completely dry—a poignant reminder that surface liquids on planets are certainly not guaranteed. (Mars originally had surface water, but that was lost over time, although temporary water flow may still have occurred relatively recently.)121
A Survey of Possible Life Solvents
While Bains may be correct in predicting the commonality of liquids in other solar systems, that only accounts for the first of the eight qualities needed to effectively support life chemistry. As a reminder, these qualities are (1) naturally abundant, (2) a good solvent, (3) large range of liquidity, (4) ability to encapsulate, (5) large dielectric constant, (6) good thermal moderator, (7) low viscosity, and (8) high surface tension. Starting with all known solvent-like compounds and applying these criteria, astrobiologists have been able to quickly screen out the vast majority of possibilities. For this study, we have selected sixteen of the most commonly cited candidates given in the scientific literature.
To help understand these sixteen solvents, we provide a listing of some of their most important chemical properties (see table 2). Water is also shown since it serves as a benchmark against which we can judge its competitors. But before moving on, it might be helpful to explain the significance of each of the chemical properties given in the table and how they relate to the eight qualities of a good solvent.
- Melting Point, Boiling Point, and Range of Liquidity. The range of liquidity (criteria 3) is just the difference between the melting and boiling points. A large range means that creatures utilizing the solvent inside their cells would be less vulnerable to climatic temperature changes. The melting and boiling points also establish the conditions under which it will be a liquid with low values requiring a cold environment and high values requiring a hot environment. This is of immediate importance in considering which planets or moons could support it as a liquid. This temperature range also influences the rates of reactions taking place within it. If the temperature and therefore reaction rates are too low, biochemistry would be impeded or perhaps halted altogether.
- Critical Temperature and Pressure. The reported melting and boiling points are for the pure liquid at standard pressure. (Standard pressure—denoted 1 atmosphere or 1 atm—corresponds to atmospheric pressure at sea-level on Earth.) Higher atmospheric pressures can raise the boiling point increasing the range of liquidity. There is, however, a fundamental limit to how far this can be pushed. When the temperature and pressure exceed the critical temperature and pressure listed, it will behave as a supercritical fluid, rather than a liquid. Thus the critical temperature represents a liquid's maximum boiling point and therefore establishes an upper limit on its range of liquidity (criteria 3).
- Dipole Moment. The dipole moment is a good measure of the polarity of the solvent, which in turn relates to the solvent's ability to dissolve polar molecules (criteria 2). A solvent with a large dipole moment (hence highly polar) would be good at dissolving polar molecules and salts while a low dipole moment (hence non-polar) would favor dissolving non-polar molecules.
- Dielectric Constant. This measures a solvent's ability to dissolve charged ions and molecules (criteria 5). A large dielectric constant is important for dissolving salts, supporting acid-base chemistry, and dissolving large charged molecules.
- Heat Capacity, Heat of Fusion, and Heat of Vaporization. These three properties measure how well a solvent can moderate temperature changes (criteria 6). Larger values here indicate a superior ability to protect life from internal and external temperature changes.
- Viscosity. Viscosity indicates to how easily molecules dissolved in the solvent can move around (criteria 7). Having a low viscosity is important for facilitating cellular interactions.
- Surface Tension. A high surface tension (criteria 8) improves the efficiency of adsorption (which helps concentrate biomolecules on a cell's surface) and capillary action (where liquids are drawn into narrow spaces). (On Earth, capillary action helps soil retain moisture and allows trees and vascular plants to transport fluids from their roots to their leaves.)
A solvent's cosmic abundance (criteria 1) and the strength of its hydrophobic effect (criteria 4) cannot be reported as a simple value and so are not included in the table. We will, however, comment on these two factors in the next section when it is relevant to our discussion.
|Melting Point (°C at 1 atm)||0||77.73||83.4||10||13.3||3|
|Boiling Point (°C at 1 atm)||100||33.33||20.0||337||26.0||211|
|Range of Liquidity (°C at 1 atm)||100||44.4||103.4||327||39.3||208|
|Critical Temperature (°C)||374||132||188||NA||184||NA|
|Critical Pressure (atm)||215||111||64.8||NA||54||NA|
|Dipole Moment (Debye)||1.85||1.47||1.83||2.7||2.99||3.4|
|Dielectric Constant (ε0)||80.1||16.6||83.6||101||115||110|
|Heat Capacity (J/mol K at 25°C)||75.3||80.8||NA||138.9||70.6||NA|
|Heat of Fusion (kJ/mol)||6.0||5.7||4.6||10.7||8.4||8.44|
|Heat of Vaporization (kJ/mol)||40.7||23.3||30.3||NA||25.2||60.1 at
|Viscosity (10-3 P)||9.6||2.7 (at
|Surface Tension (10-3 J/m2)||71.99||19.8||NA||NA||18.1||57.03|
|Melting Point (°C at 1 atm)||85.5||0.4||1.6||94||182||172|
|Boiling Point (°C at 1 atm)||59.0||150.2||111||65||161.5||89|
|Range of Liquidity (at 1 atm)||25.9||151||111||159||18||65|
|Critical Temperature (°C)||100||455||380||240||82.6||32.3|
|Critical Pressure (atm)||88||215||14.2||78||45.4||47.8|
|Dipole Moment (Debye)||0.98||2.01||1.9||1.6||0.0||0.0|
|Dielectric Constant (ε0)||5.9||89(?)||51.7||35.4 at
|Heat Capacity (J/mol K at 25°C)||34.2||98.1||98.9||81.6||NA||NA|
|Heat of Fusion (kJ/mol)||2.4||12.50||12.7||2.2||0.94||2.7|
|Heat of Vaporization (kJ/mol)||18.7||51.6||41.8||40.5||8.2||14.7|
|Viscosity (10-3 P)||4.3||11.4||9.8||5.9||0.009 at
|Surface Tension (10-3 J/m2)||NA||79.3||66.39||22.1||NA||NA|
|Melting Point (°C at 1 atm)||185||−68.74||−90||210||−259.16|
|Boiling Point (°C at 1 atm)||122||57.65||−86||196||−252.88|
|Range of Liquidity (at 1 atm)||63||126.39||4||14||6.28|
|Critical Temperature (°C)||NA||235.0||NA||147||−240|
|Critical Pressure (atm)||NA||35.46||NA||33.3||12.69|
|Dipole Moment (Debye)||0||0||0||0||0|
|Dielectric Constant (ε0)||NA||NA||NA||1.45||NA|
|Heat Capacity (J/mol K at 25°C)||NA||145||NA||NA||28.836|
|Heat of Fusion (kJ/mol)||NA||7.60||NA||0.71||0.117|
|Heat of Vaporization (kJ/mol)||NA||28.7||NA||5.56||0.904|
|Viscosity (10-3 P)||NA||994 at
|Surface Tension (10-3 J/m2)||NA||NA||NA||10.53||NA|
NA = not available. Sources: Schulze-Makuch and Irwin, Life in the Universe, Table 7.1, p. 111 and 7.2, p. 114; Bailar et al., Chemistry, A-14-21; and Wikipedia "silane," "silane tetrachloride," "silicon tetrafluoride," and "hydrogen."
Evaluating Possible Life Solvents
Tabulated chemical properties provide a powerful overview, but they don't tell the whole story. For that, we need to shift gears and focus on solvents individually. In this section, we will highlight specific strengths and weaknesses of each solvent. For this, we will rely on material drawn largely from Schulze-Makuch and Irwin122 and Steven Benner.123
For the purposes of this paper, we will divide our prospective solvents into three main groups: water-like, hydrocarbon-like, and non-traditional solvents. The main dividing line between the first two groups is that water-like solvents are generally polar (indicated by a large dipole moment) while hydrocarbon-like solvents are non-polar (with a small dipole moments). A molecule or solvent is considered polar if its electrons are not distributed evenly resulting in some of its atoms having a partial positive charge and others having a partial negative charge.124 For solvents, polarity or lack of polarity has important implications for what substances it is likely to dissolve (criteria 2). Polar solvents tend to be good at dissolving other polar molecules as well as salts, but they only poorly dissolve non-polar molecules. Non-polar solvents are the opposite, typically dissolving other non-polar molecules but not polar molecules or salts.
Another important property associated with many (but not all) of the water-like solvents is hydrogen bonding. Solvents with a significant amount of hydrogen bonding generally have higher melting and boiling points (hence a larger range of liquidity), higher dielectric constants, stronger thermal properties, and large surface tensions (criteria 3, 5, 6, and 8) compared to those without it. Hydrogen bonding is also a key factor in the hydrophobic effect, which is the simplest and most natural way to form boundaries and achieve encapsulation (criterion 4).
Case 1: Water-like (polar) solvents
Because water (H2O) is currently the only known liquid that serves as a life solvent, much research focuses on solvents similar to it. Ammonia (NH3) and hydrogen fluoride (HF) are the two closest water analogs (with nitrogen and fluorine located just to the left and right of oxygen on the periodic table). Both are polar with strong hydrogen bonding (albeit not quite to the same degree as water) and not surprisingly share many of water's strengths. And we don't need to restrict ourselves to strict water analogs. There are several other solvents that display hydrogen bonding or are polar that deserve consideration.
- Ammonia (NH3). The most commonly suggested alternative life solvent is ammonia because it is so chemically similar to water. It is the fourth most abundant molecule in the universe and so would be naturally available on many planets.125 For example, we find it in the clouds of Jupiter and Saturn and probably in the subsurface ocean on Titan. It is typically assumed to be a low-temperature alternative to water, since at standard pressure its boiling point is 33.4°C/28 °F. However, at higher pressures (about 60 times standard pressure) it would be a liquid (boiling point 98°C/208°F) even at Earth temperatures. Ammonia is highly toxic to Earth-life, but a carefully modified version of terrestrial (carbon-based) metabolism might be able to operate successfully in ammonia.126
- Hydrogen Fluoride (HF) Hydrogen fluoride, like ammonia, is a close analog of water. Its boiling point is much higher than ammonia's, although still lower than water's. Moreover, its range of liquidity and dielectric constant are just slightly higher than water's while its heat capacity is slightly smaller.127 These features make HF an attractive alternative. The major problem, however, is that HF is very rare (because the element fluorine is cosmically at least 1,000 times less abundant than carbon, nitrogen, or oxygen).
- Sulfuric Acid (H2SO4) Sulfuric acid is the best solvent candidate for hot planets like Venus, because of its high boiling point (337°C/639°F; although it slowly decomposes at temperatures above 300°C/572°F). Many of its properties match or exceed those of water, but its unusually high viscosity would be a limitation. The biggest challenge, however, is that it is extremely acidic, which would pose severe problems for constructing and maintaining biomolecules. Since sulfuric acid is very hygroscopic, it naturally absorbs nearby water turning it into the equivalent of battery acid.128 (Some terrestrial extremophilic microbes can live in highly acidic environments, but only by keeping their internal pH close to neutral. Of course, that strategy wouldn't apply here since the acid would have to be inside the cell in order to serve as the life solvent.) Terrestrial biochemistry would not survive in sulfuric acid, but chemists have proposed a modified version that might be able succeed.129
- Hydrogen Cyanide (HCN) Hydrogen cyanide is an excellent solvent (although it is highly toxic to Earth life). However, some very useful compounds and metals are poorly soluble in HCN.130 HCN supports hydrogen bonding, so its boiling point (26°C/79°F) and range of liquidity are relatively high although still lower than water's. In at least two areas (dielectric constant and heat of fusion) it does manage to significantly exceed water. The main interest to astrobiologists, however, is that it can react to form a range of biologically useful molecules (purines, pyrimidines, urea, and amino acids).131 A major limitation is of HCN is that reacts readily with water and also tends to polymerize, so it is unlikely to be found in significant quantities on planets. A second major issue is that it is difficult to imagine a system of biomolecules that would be compatible with HCN.132
- Formamide (HCONH2) Formamide is another intriguing solvent, having a large range of liquidity and is likely to be relatively common. It is of biological interest because several key biological reactions would occur spontaneously in this solvent, whereas they are vulnerable to reaction with water (hydrolysis).133 It does, however, share HCN's vulnerability to reacting with water and so would require a very dry environment.
- Hydrogen Sulfide (H2S) Hydrogen sulfide is a structural analog of water (since sulfur is located just below oxygen on the periodic table). Despite this similarity, H2S as a solvent has little in common with water. It is only weakly polar and so is a poor solvent. It doesn't support notable levels of hydrogen bonding, so its boiling point, range of liquidity, and thermal properties are all rather low.134 So while H2S is relatively common, it would make a poor choice as a life solvent.
- Hydrogen peroxide (H2O2) This compound has many properties that are similar to water, but with a larger range of liquidity. Its biggest problem is that it is a strong oxidizer, and so would rapidly convert most carbon compounds into carbon dioxide. Because of this, pure hydrogen peroxide would be a poor choice for a life solvent. Instead, astrobiologists have suggested that a mixture of water and hydrogen peroxide might be an effective combination. In particular, this mixture might be beneficial on a planet similar to Mars where its hygroscopic (water-absorbing) nature would be helpful in such a dry environment.135
- Hydrazine (N2H4) Hydrazine has many properties that make it an interesting competitor with water in many categories. One major problem, however, is that it reacts rapidly with oxygen—which is why it is used as rocket fuel. A second problem is that it is unlikely to exist in significant quantities due to its high reactivity.
- Methanol (CH3OH) Methanol is an organic solvent but the presence of the alcohol group (-OH) means that it is chemically similar to water. Because of this, it is an excellent polar solvent and supports hydrogen bonding. As a result, many of its properties are competitive with water or in a few cases greater (dielectric constant and range of liquidity). Its most promising feature, however, is that it may be relatively common having been detected in outer space and in comets. Some have even suggested that there may have been pools or even oceans of methanol on early Mars.136 However, there does not appear to be any on Titan, despite the widespread presence of methane.
Case 2: Hydrocarbon-like (non-polar) solvents
In laboratories, non-polar hydrocarbons are frequently employed because of their ability to dissolve most non-polar organic molecules (whereas water typically excludes them). In addition, their low reactivities make them less likely than water to interfere with sensitive chemical reactions. However, these solvents lack hydrogen bonding, which means that small hydrocarbons tend to have very low boiling points and so would require low-temperature environments. Larger hydrocarbons would have progressively higher boiling points, but are correspondingly less likely to occur in nature than smaller ones.
- Methane (CH4), Ethane (C2H6) Methane and its larger cousin ethane are of great interest to astrobiologists because of their widespread presence on the surface of Saturn's moon, Titan.137 High-energy radiation would cause methane and ethane to react with nitrogen in the atmosphere to produce a complex array of more complicated molecules.138 This leads to interesting speculation about possible life on Titan, but how do methane and ethane stack up as life solvents? One obvious difficulty is that they both have very low melting and boiling points as well as narrow ranges of liquidity. Methane's range of liquidity is one-fifth and ethane's is two-thirds of water's. A second major challenge is to achieve compartmentalization—to form something akin to cell membranes. This may be possible because certain polar organic molecules would be excluded from the non-polar methane/ethane (via the hydrophobic effect).139 Despite these challenges, life in liquid hydrocarbons has been seriously proposed.140
- Silane (SiH4), Silicon Tetracholoride (SiCl4), Silicon Tetraflouride (SiF4) These are primarily of interest with regard to supporting possible silicon-based life. One major problem is that these compounds are unlikely to form in significant quantities on planets. A further complication is that these compounds react with water and ammonia and so would require an environment devoid of both liquids.141 Consequently, it is difficult to imagine life utilizing any of these due to lack of availability.
Case 3: Non-traditional solvents
This third category is made up of everything that doesn't fit nicely into the previous groups. While they are rarely used as solvents by chemists, they are of astrobiological interest because they might be able to support life under specific planetary conditions that are too extreme for other solvents.
- Nitrogen (N2) Molecular nitrogen is abundant in the universe, but it has such a low boiling point that it is usually found in a gaseous state. However, it would be a liquid on very cold planets or moons located very far from their parent star (such as Uranus' moon, Triton). Such low temperatures would be too cold for traditional carbon chemistry, as well as limiting how much of a given compound could be dissolved. The only semi-serious possibility for life in these ultra-cold conditions is silicon biochemistry, which we already discussed.
- Hydrogen (H2) Hydrogen is the most abundant molecule in the universe, but has such an extremely low boiling point (253°C/423°F) that it would only be found in the liquid state on planets or moons extremely distant from their stars (around 20 times farther out than Neptune is in our solar system). At these extremely low temperatures, traditional chemistry would simply shut down, thus preventing almost any imaginable lifeforms from ever starting. Nevertheless, two authors have seriously proposed a strange form of life that they think might be able to live in liquid hydrogen.142
- Supercritical Hydrogen (H2) Gas giant planets, like Jupiter and Saturn, are primarily comprised of hydrogen. Under very high pressures (which for Jupiter is located in the atmosphere about 80 percent distant from the center of the planet), hydrogen is a supercritical fluid (meaning it behaves partially like a liquid and partially like a gas).143 Some have proposed that this fluid might serve as a host for life chemistry. One problem with this scenario is that the increased pressures are accompanied by higher temperatures. For some gas giant planets, there may be a "habitable zone" in which the hydrogen is a supercritical fluid—yet still cool enough (less than 227°C/441°F) that the carbon molecules needed for life would still be stable.144
Could Any of These Solvents Support Life?
Other planets offer a wide diversity of possible environments and many of these may support some type of liquid on or below their surfaces or in their atmospheres. Scientists have been investigating whether some of these liquids might be able to serve as an effective life solvent. Many of these can be ruled out on astronomical grounds—they simply would not be available in sufficient quantities on planets/moons. Others are ruled out on chemical grounds—they are just too limited as solvents. So although the number of possible life solvents is very large, only ammonia is able to avoid immediate disqualification based on these two grounds. We will therefore turn our full attention to studying ammonia in more detail.
Is Ammonia-based Life Possible?
Ammonia (NH3) was first proposed as a possible life solvent by J. B. S. Haldane in the Symposium on the Origin of Life in 1954.145 This idea was then developed and extended by V. Axel Firsoff.146 It remains the most popular candidate today and its proponents include Gerald Feinberg and Robert Shapiro,147 Peter Molton,148 Steven Benner,149 and William Bains.150 Even popular writes like Carl Sagan151 and Isaac Asimov152 have argued for the possibility of ammonia-based life.
Ammonia is an obvious choice for a possible alternative to liquid water. First, it is the most well-known and thoroughly studied polar solvent (after water). Second, ammonia is the fourth most abundant molecule in the universe,153 which means that it will be present in significant quantities on many planets and moons. We know that it was present on the early Earth and can be found on many planets in our solar system, such as on Jupiter, Saturn, Uranus, Neptune, and some of their moons. Third, ammonia is chemically and physically similar to water (see figure 9). This means that ammonia shares many of water's useful qualities—particularly its ability to form strong hydrogen bonds.
|Figure 9: Molecular structures of (a) ammonia and (b) water. Image credit: John Millam.|
Figure 9: Molecular structures of (a) ammonia and (b) water. Image Credit: John Millam
Pure versus Aqueous Ammonia
In discussing ammonia, we first need to carefully distinguish between its two main forms. What most people think of as ammonia is actually household ammonia (a common cleaning agent). This is not pure ammonia, but is a mixture of water and ammonia and so is more correctly referred to as aqueous ammonia or ammonium hydroxide (NH4OH). Household ammonia is convenient to handle because it is a liquid at room temperature, whereas, pure ammonia or anhydrous ammonia (NH3) is a gas. (Pure ammonia at atmospheric pressure is a liquid between 77.7°C and 33.4°C or between 108°F and 28°F, see table 2.)
Current research on ammonia-based life almost exclusively assumes that it starts and operates in an anhydrous (water-free) ammonia environment. Excluding water from such scenarios is motivated by the fact that water would introduce several complicating factors. First, aqueous ammonia is highly basic.154 Such conditions would be deleterious to many types of biomolecules.155 Second, the direct utilization of both water and ammonia would greatly complicate life chemistry, because biomolecules would need to be stable with respect to both water and ammonia (combining the disadvantages of both). One possible way around this challenge would be for the organism to excrete water, leaving only pure ammonia inside the organism. Given the inherent difficulty of differentiating between water and ammonia, it is difficult to conceive of this naturally occurring under plausible prebiotic conditions.156 For simplicity, however, we will restrict all our remaining discussion to considering life in pure ammonia.
Ammonia on Other Planets
While astrobiologists favor a nearly water-free ammonia environment for their alternative life proposals, ammonia in real planetary environments would generally be of the aqueous variety due to the ubiquitous presence of water in the universe. This is likely the case for the subsurface ocean on Saturn's moon, Titan.157 One proposed scenario for nearly pure ammonia is a planet or moon that is cold enough to freeze out any water as ice, yet not so cold that ammonia also freezes.158 The key problem with this idea is that mixing water and ammonia lowers their combined melting point—in other words ammonia in water acts as an antifreeze. Depending on the ratio of ammonia to water, the temperature needed to freeze out all of the water would need to be as low as 96°C/141°F at standard pressure.159 Therefore under most circumstances, ammonia on other planets would be contaminated with notable amounts of liquid water, which would pose problems for some of the biochemically essential reactions.
Currently, the best candidates for harboring mostly pure ammonia are the cloud layers of gas giant planets like Jupiter and Saturn.160 The outermost reaches of these planet's atmospheres are very cold, but as one moves farther in, they get progressively warmer, so there will be a region with the right temperature for small droplets of liquid ammonia. The discovery of ammonia in the clouds of Jupiter led Sagan and Salpeter to speculate about possible life there,161 but this scenario faces the immense problem of not having a solid surface or liquid oceans for life to develop on or in.162
Possible Biochemistry in Ammonia
Given that ammonia is likely present on many worlds, what kind of biochemistry might it support? Silane biochemistry probably wouldn't work because the ammonia would break the weak silicon-silicon bonds. Silicate and silicone biochemistries wouldn't have that problem, but these scenarios would require much higher temperatures—well beyond the range at which ammonia is a liquid. Boron-nitrogen biochemistry is well suited for an ammonia environment, because ammonia is good at dissolving boron and is a liquid at the lower temperatures needed for this type of biochemistry. Yet, this scenario remains unlikely due to the inherent difficulties of boron-nitrogen chemistry.
A carbon-based biochemistry currently seems to offer the best chance for ammonia-based life. Clearly carbon is capable of supporting complex biomolecules (unlike its competitors, silicon and boron). But we know that our terrestrial biochemistry wouldn't work in ammonia. For example, the lipid bilayers (that make up cell membranes) dissolve in ammonia and the phosphate bearing molecules needed for energy gathering and metabolism (such as ADP and ATP) would be destroyed.163 Therefore, ammonia-based life would require biomolecules that would be stable in ammonia yet could fulfill the same roles observed in terrestrial life.
Firsoff and later Molton proposed an alternative carbon-based biochemistry utilizing analogs of terrestrial biomolecules suited for an ammonia environment.164 Mainly this involves replacing oxygen atoms with nitrogen (plus a hydrogen atom to balance the charge). A few specific examples are shown in figure 10. Of course, actual ammonia-based life might develop biomolecules that are very different from the ones proposed here. Nevertheless, this proposal helps demonstrate that carbon-based biochemistry in ammonia is at least chemically conceivable.
|Biochemistry Type||Terrestrial Form||Ammonia-based Form|
Figure 10: Three classes of terrestrial biomolecules and their proposed counterparts for ammonia-based biochemistry.165 Structural differences are highlighted in red. R represents any organic functional group and n stands for the number of repeating units in the protein chain and is an integer. Image Credit: John Millam
Evaluating Ammonia as a Life Solvent
Now we come to the crux of the matter: how well does ammonia perform as a life solvent? The eight properties of a good life solvent will serve as a starting point for our evaluation. To help in understanding the strengths and weaknesses of ammonia as a solvent, we will compare it to water—currently the only known life solvent. For a side-by-side comparison of properties of water and ammonia, see table 2.
- Ammonia is common in the universe. One of ammonia's most important qualifications as a life solvent is that it is naturally abundant in the universe (as previously noted), so it should be present on many planets and moons. The difficulty is that in most cases ammonia would be found in the form of aqueous ammonia rather than anhydrous ammonia.
- Ammonia is a good solvent. Ammonia is a well-studied and frequently-used polar solvent; however compared to water, it is less polar (with a smaller dipole moment).166 This means that ammonia is not as effective at dissolving polar molecules and salts. On the other hand, this means that ammonia does a slightly better job than water at dissolving non-polar organic molecules—a potentially beneficial property since these types of molecules can play an important role within the cell.167 Beyond this, there are at least two areas where ammonia's behavior as a solvent differs notably from that of water. First, alkali metals can dissolve directly in ammonia without reaction to form so-called "blue solutions."168 In contrast, alkali metals react violently with water. Second, ammonia only dissolves sodium and potassium ions in small quantities. These ions are critical in terrestrial life for regulating cell membrane potential and for nerve transmission. An ammonia system might instead fulfill this role using cesium and/or rubidium ions, which ammonia readily dissolves.169 A major problem with this scenario is that cesium and rubidium are relatively rare, so they probably wouldn't be available in sufficient quantities. (For example in the Earth's crust, those two elements are at least a thousand times less abundant than sodium and potassium.)
- Ammonia has a relatively low range of liquidity. The melting and boiling points of ammonia are considerably lower than those of water, so it is generally assumed that it is only suitable for very cold temperature environments (where it would be a liquid). An even bigger problem is that ammonia (at standard pressure) has a low range of liquidity—less than half of water's range. This means that ammonia-based life would be more vulnerable to environmental temperature changes. One possible remedy for both of these issues is increasing the environmental pressure, which would raise ammonia's boiling point and correspondingly increase its range of liquidity. At even 60 atm, ammonia's boiling point is raised to 98°C/208°F, which would make it a liquid even at Earth's temperatures.170 Under those conditions, ammonia's range of liquidity would be an ample 175 °C/315°F. Such pressures can be found on planets with very thick atmospheres, such as that of Venus.
- Ammonia's hydrophobic effect is weaker. The hydrophobic effect in ammonia is considerably weaker than in water, which results in two significant problems. First, ammonia solutions would be less able to form cell membrane-like structures and those that do form would be weaker. Second, it would be harder for protein-like structures to adopt and maintain the precise three-dimensional structure required for proper functioning.
- Ammonia's dielectric constant. Ammonia's dielectric constant is around one-third of water's. This reduces ammonia's ability to dissolve salts and support ionic species in solution. This would also make keeping many large and complex molecules in solution more difficult. (On Earth, proteins, DNA, and RNA have electrically charged sites, which allow water to keep them in solution. Ammonia would be less able to keep these types of molecules in solution.) This also means that ammonia is not as good as water at supporting acid-based chemistry.171
- Ammonia's excellent thermal properties. Ammonia's thermal properties compare favorably with those of water (largely because both exhibit strong hydrogen bonding). For example, ammonia's heat capacity is slightly higher and its heat of fusion is slightly lower, although its heat of vaporization is only about half of water's. Taken together, ammonia is nearly as good as water at regulating heat and temperature.
- Ammonia has a low viscosity. Both ammonia and water have very low viscosities, which mean that they both flow freely and they both allow dissolved compounds to move about easily. Ammonia's viscosity is actually lower (about one-fourth of water's), but this difference is not considered significant.
- Ammonia has a weaker surface tension. Ammonia's surface tension is approximately one-fourth of water's value. This would mean that adsorption and capillary action (both discussed earlier) would be less effective in ammonia.
In addition to the issues already discussed, ammonia has at least one other major weaknesses as a life solvent.
- Flammable in oxygen. Because ammonia reacts strongly with oxygen, hypothetical ammonia-based life would almost certainly require an oxygen-free environment.
This survey shows that ammonia clearly does have a lot going for it. Ammonia is generally inferior to water, but only moderately so. Therefore, there is nothing that can categorically rule it out. Still the case for ammonia as a life solvent is far from a slam-dunk. First, large bodies of nearly-water-free liquid ammonia (comparable to terrestrial lakes and oceans) are highly improbable despite ammonia's large-scale abundance due to the wide-spread presence of water in the universe. Second, ammonia-based life would likely require much lower temperatures (or much higher pressures). Lower temperatures would limit rates of biochemical reactions. Third, ammonia's weaker hydrophobic effect would make protein folding and the development of cell-membrane-like structures much more problematic. Finally, ammonia's vulnerability to oxygen further limits where ammonia-based life might live. In conclusion, the weight of evidence argues against ammonia-based life.
Environmental Issues: Nitrogen versus oxygen gas
So far, we have considered ammonia purely in its role as a solvent, but there are some additional chemical and planetary considerations that are equally relevant. To begin with, the fate of ammonia is closely coupled to that of nitrogen gas (N2) in the environment whereas water is tied to oxygen gas (O2). Note that nitrogen and oxygen gasses refer to molecular nitrogen (N2) and oxygen (O2) as distinguished from the elements nitrogen (N) and oxygen (O). Even though ammonia and water are chemically close cousins, O2 and N2 are radically different in their chemical behavior and this has the following important consequences for ammonia-based life:
- Ammonia does not self-shield against ultraviolet light. When water is hit by high energy ultraviolet (UV) light coming from the sun, it dissociates into oxygen (O2) and hydrogen (H2) gasses. Some of this oxygen gas will make its way into the upper atmosphere and get transformed into ozone (O3). This results in the formation of an ozone layer that absorbs UV from the sun which keeps it from reaching the surface.172 This shields surface water from further dissociation, which is important for retaining water over the planet's multi-billion year history. (This dissociation mechanism was critical for the early Earth, although today oxygen for the ozone layer is mainly supplied by photosynthesis in plants. The ozone is very important for protecting people and other animals from the damage caused by UV radiation.) In contrast, ammonia dissociates into N2 and H2, neither of which block UV light.173
- Nitrogen gas is not useful for metabolism. In his ground-breaking speculation about ammonia-based life, V. Axel Firsoff wrote:
"The fact that we breathe oxygen is the consequence of the fact that we drink water. Jovian [ammonia-based] animals could breathe nitrogen and drink liquid ammonia. Whether they do remains to be seen."174 (our italics)
This is totally implausible, given what we know about nitrogen gas. Oxygen is well suited for terrestrial metabolism because it is very reactive and therefore readily participates in vital reactions.175 Nitrogen gas is the opposite, being extremely unreactive. Breaking nitrogen gas down into useful nitrogen compounds (nitrogen fixation) is such a difficult task that only a few terrestrial organisms are capable of doing it and those that do only produce limited quantities. It is therefore hard to imagine ammonia-based life processing nitrogen gas quickly and efficiently enough that it could support metabolism. This eliminates an otherwise ideal and readily available resource for such life, adding yet another complication for ammonia-based life.
- Loss of ammonia via decomposition to nitrogen gas. A number of processes convert water into oxygen gas (such as photosynthesis by plants and decomposition by UV light). Once produced, O2 is very reactive and most of it will quickly be converted back into water. (Today, Earth is able to maintain high levels of oxygen in the atmosphere because the oxygen gas consumed by living organisms and other chemical processes is constantly replaced through photosynthesis in plants.) Ammonia, in contrast, faces a real conundrum. While ammonia (and other nitrogen-bearing molecules) can be readily decomposed into N2 by UV light,176 the reverse process is very difficult. Only a few non-biological processes (mainly lightning) are powerful enough to break the very strong triple bond of N2. This means that if the planet's primordial supply of ammonia is decomposed to nitrogen gas over time, there would be few non-biological means to regenerate it. So even if a planet starts with significant quantities of ammonia, it may not be able to be keep it long enough to allow for even the possibility of ammonia-based life. In our solar system, ammonia is restricted to a few places where it is protected from decomposition due to UV radiation —the thick atmospheres of gas giant planets and the subsurface ocean of the moon Titan.
In the previous section, we established that ammonia seems inadequate to serve as a life solvent and the problems presented here only to serve to reinforce that assessment.
Much of the pioneering work on alternative biochemistry began in the 70s and 80s. At that time, too little was known to either support or rule out weird life possibilities. Even our knowledge of our own solar system was still very limited and extrasolar planets had not yet been discovered. Because of that, it was easy to assume that there was nothing particularly special about either carbon or water—they were simply what worked given the specific conditions on Earth. On that basis, one would expect that other possible life chemistries must exist. We simply hadn't discovered them yet since the majority of our knowledge was related to Earth conditions and terrestrial life.
That picture has changed considerably over the intervening decades. The emerging field of astrobiology is attempting to ground weird life claims on genuine scientific research and has filled in many missing gaps. However, all of this has done little to demonstrate that silicon or any other element has any real potential to support life. Similarly, astrobiologists have not found a solvent other than water that is likely to serve as a life solvent (although some still hold out hope for ammonia). Astrobiological research has, in fact, expanded (rather than diminished) our understanding of the uniqueness of carbon and water in supporting life chemistry.
This shift in perspective on weird life is reflected in statements made by Carl Sagan. Back in 1973, he coined the term "carbon chauvinism" to deride as anthropocentric the notion that life must necessarily be carbon-based.177 Yet in the same work, he admitted (albeit reluctantly) to being a carbon chauvinist, because none of the alternatives are as versatile as carbon. In an interview that same year, he proclaimed himself to be only partially a "water chauvinist," suggesting that ammonia or even simple hydrocarbons might serve as a life solvent.178 Nevertheless twenty years later and just two years before his death, Sagan conceded.
"Actually, focusing on organic matter and liquid water is not nearly so parochial and chauvinistic as it might seem. No other chemical element comes close to carbon in the variety and intricacy of the compounds it can form; liquid water provides a superb, stable medium in which organic molecules can dissolve and interact. What is more, organic molecules are surprisingly common in the universe. Astronomers find evidence for them everywhere, from interstellar gas and dust grains to meteorites to many worlds in the outer solar system. "179
He went on to discuss several reasons why silicon probably can't support life, ruled out hydrofluoric acid as a life solvent (because it is too rare), and only minimally considered the case for ammonia. He then concluded:
"For the moment, though, carbon- and water-based life-forms are the only kinds we know or can even imagine."180
Although Sagan never completely rejected the possibility of weird life, his conclusion is a profound one. Essentially, he acknowledged that the conventional view that carbon and water are necessary for life was grounded in scientific evidence, rather than resulting from a failure to adequately consider alternative chemistries. And if Sagan concluded that from just a few evidences, one can only wonder how much stronger his statement would have been if he had had all of the information available today.
So where does all this leave us? Based on the current state of research, there is strong (although not conclusive) scientific evidence for believing that life anywhere in the universe must be carbon- and water-based. Of course, one can never truly falsify weird life claims since there is still much that we do not know. Nevertheless, our current state of knowledge casts significant doubt over any such possibility.
- William Bains, "Many chemistries could be used to build living systems," Astrobiology 4:137-67 (2004).
- Philip Ball, Life's Matrix: A Biography of Water (Berkeley, CA: University of California Press, 2001).
- Steven A Benner, Alonso Ricardo, and Matthew A Cardigan, "Is there a common chemical model for life in the universe?" Current Opinions in Chemical Biology, 2004, 8:672-689.
- Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council, The Limits of Organic Life in Planetary Systems, The National Academies Press, 2007.
- Gerald Feinberg and Robert Shapiro, Life beyond Earth: the intelligent earthling's guide to life in the universe (William Morrow and Company, Inc, New York, 1980).
- V. Axel Firsoff, "An Ammonia-Based Life," Discovery 23:36-42 (January 1962).
- --, "Possible Alternative Chemistries of Life," Spaceflight 7 (July, 1965):132-136.
- --, Life Beyond the Earth;: A study in Exobiology (Hutchinson Scientific and Technical, London: 1963).
- Felix Franks, Water: A Matrix of Life, second edition (Cambridge, UK: Royal Society of Chemistry, 2000).
- Lawrence Joseph Henderson, The Fitness of the Environment (New York, NY: MacMillan Company, 1913). Reproduction by Nabu Press, 2010.
- R. A. Horne, "On the Unlikelihood of Non-Aqueous Biosystems," Space Life Sciences 3 (August 1971):34-41.
- Louis Neal Irwin and Dirk Schulze-Makuch, Cosmic Biology: How Life Could Evolve on Other Worlds (New York, NY: Springer, 2011).
- P. M. Molton, "Terrestrial Biochemistry in Perspective: Some Other Possibilities," Spaceflight 15 (April 1973):139-144.
- --, "Non-aqueous Biosystems: The Case for Liquid Ammonia as a Solvent," Journal of the British Interplanetary Society 27, 243-262 (1974).
- --, "The Multiplicity of Potential Living Systems Based on C, H, O, N," Journal of the British Interplanetary Society 28, 392-8, 1975.
- A. E. Needham, The Uniqueness of Biological Materials. (Oxford, London: Pergamon Press, 1965), (March, 1968):9-28.
- Norman R. Pace, "The Universal Nature of Biochemistry," PNAS, vol. 98, No. 3 (January 30, 2001), 805-8.
- Clifford Pickover, The Science Of Aliens (New York, NY: Basic Books, 1998).
- Kevin W. Plaxco and Michael Gross, Astrobiology: A Brief Introduction (Baltimore, MD: John Hopkins University Press, 2011).
- Fazale Rana, The Cell's Design: How Chemistry Reveals the Creator's Artistry (Grand Rapids, MI: Baker Books, 2008).
- Fazale Rana and Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off (Colorado Springs: Navpress, 2004).
- Hugh Ross, The Creator and the Cosmos: How the Latest Scientific Discoveries of the Century Reveal God, 3rd ed. (Colorado Springs: NavPress, 2001), 187.
- Dirk Schulze-Makuch and Louis Neal Irwin, Life in the Universe: Expectations and Constraints (Advances in Astrobiology and Biogeophysics) (New York, NY: Springer, 2008).
- Peter Ward, Life as We Do Not Know It (London: Penguin Books, 2005).
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- Origin of life: latest theories/problems
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- Book Review: Origins of Life: Biblical and Evolutionary Models Face Off
Origins of Life: Biblical and Evolutionary Models Face Off by Fazale Rana and Hugh Ross. Probably the single most potent scientific argument against atheism is the problem with a naturalistic origin of life. This very problem led me to become a deist as a biology major at USC in the early 1970's. The problems for atheists have gotten no better since that time. In fact, the last 30+ years of research have turned up even more problems than those that existed when I first studied the theories. Fuz Rana (a biochemist) and Hugh Ross (an astrophysicist) have teamed up to write the definitive up-to-date analysis of the origin of life. The book examines the origins of life from the perspectives of chemistry, biochemistry, astronomy, and the Bible. A biblical creation model is presented along side the naturalistic models to help the reader decide which one fits the data better. This is an excellent book to give to your unbelieving friends, since it presents a testable creation model that is clearly superior to any naturalistic model.
- Full permission is given to reproduce or distribute this document, or to rearrange/reformat it for other media, as long as credit is given and no words are added or deleted from the text.
- Dr. John Millam received his PhD in theoretical chemistry from Rice University in 1997, and currently serves as a programmer for Semichem in Kansas City.
- Mr. Ken Klos received his MS in environmental studies from the University of Florida in 1971, and worked as an environmental/civil engineer for the state of Florida.
- Timothy Ferris, "Seeking an End to Cosmic Loneliness," The New York Times Magazine, October 23, 1977, 97.
- I. S. Shklovskii and C. Sagan, Intelligent Life in the Universe (San Francisco, CA: Holden-Day, 1966), 411.
- Carl Sagan and Frank Drake, "The Search for Extraterrestrial Intelligence," Scientific American, 232, May 1975, 80.
- Andrew J. Lepage, "Where They Could Hide: The Galaxy Appears Devoid of Supercivilizations, but Lesser Cultures Could Have Eluded the Ongoing Searches," Scientific American, July 2000, 40-41.
- Ian Crawford, "Where Are They? Maybe we are alone in the galaxy after all," Scientific American, July 2000, 38-43.
- Hugh Ross, Ken Samples, and Mark Clark, Lights in the Sky & Little Green Men (Colorado Springs, CO: NavPress, 2002).
- Kevin W. Plaxco and Michael Gross, Astrobiology: A Brief Introduction (Baltimore, MD: John Hopkins University Press, 2011), 266-271.
- Jeff Zweerink, "A Recap of Unexpected Exoplanet Finds?" Today's New Reason To Believe on May 22, 2014.
- Sam Flamsteed, "Impossible Planets," Discover, September 1997, 7883. Hugh Ross, The Creator and the Cosmos, 3rd ed. (Colorado Springs: NavPress, 2001), 187.
- "What were 2 parameters in 1966 grew to 8 by the end of the 1960s, to 23 by the end of the 1970s, to 30 by the end of the 1980s, to the current list of 123 [as of 2001]," Hugh Ross, 187 with a partial listing on p. 188-193. This subsequently jumped to 153 as of 2002, Hugh Ross, Ken Samples, and Mark Clark, p. 185-189. As of 2008, the list stands at 402 fine-tuned parameters (Hugh Ross, "Part 2. Fine-Tuning for Intelligent Physical Life," accessed September 18, 2014).
- Peter D. Ward and Donald Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe (New York: Copernicus Books, 2000).
- Stephen Webb, If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life (New York, New York: Copernicus, 2002). See also Robert Naeye, "OK, Where Are They?" Astronomy, July 1996, 38-43.
- Natalie Wolchover, "Will We Really Find Alien Life Within 20 Years?" (accessed September 18, 2014).
- Plaxco and Gross, Astrobiology: A Brief Introduction, 202-225. See also Clifford Pickover, The Science Of Aliens (New York, NY: Basic Books, 1998), 61-78 and Fazale Rana and Hugh Ross, Origins of Life (Colorado Springs: Navpress, 2004), 171-181.
- Peter Ward, Life as We Do Not Know It: The NASA Search for (and Synthesis of) Alien Life (London: Penguin Books, 2005), 1-8, 136-7.
- Ward, Life as We Do Not Know It, pp. 66-69.
- Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council, The Limits of Organic Life in Planetary Systems, The National Academies Press, 2007, 44-46.
- Gerald Feinberg and Robert Shapiro (Life beyond Earth: the intelligent earthling's guide to life in the universe, William Morrow and Company, Inc, New York, 1980) provide one of the most extensive and imaginative discussions of possible alien life, including "weirder life." They propose (1) life among the clouds of gas-giant planets; (2) plasma-based life inside stars; (3) life on the surface of neutron stars; (4) radiant life (electromagnetic radiation ordered by interacting with matter in dense interstellar gas clouds); and (5) life in liquid hydrogen.
- Ward, Life as We Do Not Know It, p. 63.
- Guillermo Gonzalez and Jay W. Richards, The Privileged Planet: How Our Place in the Cosmos Is Designed for Discovery (Washington, DC: Regnery Publishing, Inc., 2004), 127-136.
- William Bains, "Many chemistries could be used to build living systems," Astrobiology 4:137-67 (2004).
- Ibid, 140.
- Fazale Rana, The Cell's Design: How Chemistry Reveals the Creator's Artistry (Grand Rapids, MI: Baker Books, 2008).
- Louis Neal Irwin and Dirk Schulze-Makuch, Cosmic Biology: How Life Could Evolve on Other Worlds (New York, NY: Springer, 2011), 16.
- Dirk Schulze-Makuch and Louis Neal Irwin, Life in the Universe: Expectations and Constraints (Advances in Astrobiology and Biogeophysics) (New York, NY: Springer, 2008), p 91.
- Louis Neal Irwin and Dirk Schulze-Makuch, "Assessing the Plausibility of Life on Other Worlds," Astrobiology, 1:144 (2001).
- For a pictorial representation of the relative size of various biomolecules, see poster from the RCSB Protein Data Bank.
- Rana, The Cell's Design, p. 53-67.
- Ibid, 56-57.
- Some transition metals are classified as micronutrients because they are required in minute quantities in many living organisms. For example, iron is used in hemoglobin to bind oxygen so that it can be transported throughout the body. Another example is molybdenum which is used in certain bacterial enzymes for nitrogen fixation (where nitrogen gas in the atmosphere is converted into biologically useful ammonia).
- Silicon, in particular, is needed in many organisms albeit in trace amounts. For example, plants (particularly grasses) utilize it in their metabolism. A few exceptional creatures, such as diatoms, directly incorporate it as structural material for their skeletons. Schulze-Makuch and Irwin, Life in the Universe, 97-100.
- The technical meaning of an element's period is that it equals the number of electron shells occupied by its electrons. These shells are like the layers of an onion with each subsequent shell being located farther out from their nuclei. So carbon in the second row/period has two electron shells, while silicon in the third row has three shells and germanium below that has four electron shells.
- Elements in the second row of the periodic table (e.g., carbon, nitrogen, and oxygen) are strictly limited to forming no more than four bonds. Elements in rows three and below (e.g., silicon, phosphorus, and sulfur) can sometimes exceed this limit and form structures with five or even six bonds. These cases are referred to as being "hypervalent" because they go beyond the normal four-bond (valence) limit.
- J. C. Bailar Jr, et al., Chemistry, 2nd ed., (Orlando, FL: Academic Press, 1984), 886.
- Multiple bonds among main group elements are primarily restricted to carbon, nitrogen, and oxygen located in period 2. Prior to the 1980s, it was widely believed that only elements in period 2 could form multiple bonds. Chemists now recognize that phosphorus, sulfur, and selenium can occasionally form double bonds (Ibid, 252). Silicon and boron are also known to occasionally form multiple bonds, but these are generally very weak and highly reactive.
- Ibid, 886.
- Ward, Life as We Do Not Know It, 63.
- Norman R. Pace, "The Universal Nature of Biochemistry," PNAS, vol. 98, No. 3 (January 30, 2001), 805.
- Michael J. S. Dewar, "Why Life Exists," Organometallics, Vol. 1, No. 12 (1982), 1705-8.
- Plaxco and Gross, Astrobiology, 11-12.
- Michael J. S. Dewar and Eamonn Healy, "Why Life Exists," 1705.
- R. H. Dicke, "Dirac's Cosmology and Mach's Principle," Nature 192, 1961, 440
- Pace, "The Universal Nature of Biochemistry," 805-8.
- Carl Sagan, The Cosmic Connection: An Extraterrestrial Perspective (Cambridge: Cambridge University Press, 2000), 41-49. Originally published by Doubleday in 1973.
- Sagan wryly noted that "much more attention has been paid to carbon organic chemistry than to silicon or germanium organic chemistry, largely because most biochemists we know are of the carbon, rather than the silicon, variety" (Ibid, 47).
- Gerald Feinberg and Robert Shapiro use the term "carbaquists" (i.e., water and carbon chauvanists) to deride those who believe that life fundamentally requires both carbon and water. Feinberg and Shapiro, Life beyond Earth, 25, 225-237.
- Sagan, The Cosmic Connection, 47
- From an address by J. E. Reynolds quoted in Howard W. Post, Silicones and Other Organic Silicon Compounds (Reinhold Publishing Corp., NY: 1949), 1.
- Sir Harold Spencer Jones, Life on Other Worlds (New American Library, NY: 1951), 27.
- Bains, "Many chemistries," 152-160.
- P. M. Molton, "Terrestrial Biochemistry in Perspective: Some Other Possibilities," Spaceflight 15 (April 1973):140.
- Steven A Benner, Alonso Ricardo, and Matthew A Cardigan, "Is there a common chemical model for life in the universe?" Current Opinions in Chemical Biology, 2004, 8:675-6.
- V. A. Firsoff, "Possible Alternative Chemistries of Life," Spaceflight 7 (July, 1965):133. See also V. A. Firsoff, Life Beyond the Earth;: A study in Exobiology(Hutchinson Scientific and Technical, London: 1963), 133-139.
- For a technical discussion of the differences between carbon and silicon, see Andrew Barron, "Comparison Between Silicon and Carbon," Connexions, November 23, 2009.
- Bains, "Many chemistries," 154-5.
- Schulze-Makuch and Irwin, Life in the Universe, 94-106.
- Silicon is located in the row below carbon on the periodic table and consequently has a larger atomic radius (since radius increases as you go down the table). This larger radius means that atoms bonding with silicon atoms cannot get close enough to each other to generate strong multi-bond interactions. As a general rule, only elements in the second row of the periodic table (mainly carbon, nitrogen, and oxygen) form stable multiple bonds. While silicon can actually form multiple bonds—these bonds are not very stable. For perspective, it took chemists until 1981 to synthesize a molecule containing a silicon-silicon double bond and the triple bond was not created until 2004. So while silicon technically can form multiple bonds, its ability is too limited to be relevant to this discussion.
- Bains, "Many chemistries," 154. See also Plaxco and Gross, Astrobiology, 11-12.
- Low temperatures mean less energy is available for reactions which would slow reaction rates and therefore help stabilize these structures.
- Molton, "Terrestrial Biochemistry in Perspective," 140.
- Some suggested solvents are methane (CH4) and methyl alcohol (CH3OH) (Schulze-Makuch and Irwin, Life in the Universe, 104). Other possibilities include silane (SiH4), silicon tetrachloride (SiCl4), and even silicon tetrafluoride (SiF4) (Molton, "Terrestrial Biochemistry in Perspective," 141).
- Molton, "Terrestrial Biochemistry in Perspective," 140.
- The electronegativities are 1.8 (silicon) < 2.1 (hydrogen) < 2.5 (carbon). Bailar et al., Chemistry, 290. For a discussion of the chemical significance of this difference, see Andrew Barron, "Comparison Between Silicon and Carbon," Connexions, November 23, 2009.
- Yingmei Qi and Scott M. Sieburth, Amino Acids, Peptides and Proteins in Organic Chemistry, Modified Amino Acids, Organocatalysis and Enzymes (Amino Acids, Peptides and Proteins in Organic Chemistry (VCH)) (Volume 2), Andrew B. Hughes (ed.), Wiley-VCH, 2009, 261-2.
- Benner et al., "Is there a common chemical model for life in the universe?," 675.
- These polysilanes are reported to have molecular weights up to one million daltons (where one dalton is the mass of a hydrogen atom). That translates to length of roughly 40,000 polymeric units. Robert D. Miller and Josef Michl, "Polysilane high polymers," Chemical Reviews 89(6), 1359-1410 (1989).
- Some may object that silicon can almost certainly form more complex polymeric structures than are currently known. While true, that doesn't really help support silane-based life. Given the absence of large complex silane structures in nature and the extreme difficulty in forming them under even ideal laboratory conditions, it is extremely difficult to imagine even the precursors of silicon-based life forming under plausible conditions.
- Pace, "The Universal Nature of Biochemistry," 805.
- Schulze-Makuch and Irwin, Life in the Universe, 102, 105-6.
- Feinberg and Shapiro, Life beyond Earth, 252-6.
- Bains notes that silicon being locked up as silicate rocks is at least partially a result of terrestrial conditions and so this would not be equally true on all planets (Bains, "Many chemistries," 157-159). Moreover even on Earth, not all silicon is tied up as silicates. Nevertheless, very little silicon can be expected to be chemically available in a planetary environment despite silicon's great abundance in the crust.
- Carbon dioxide (CO2) consists of a carbon atom double bonded to two oxygen atoms. Since each of the three atoms has its optimal number of bonds, it is a discrete molecule allowing it to be a gas. Silicon does not support strong multiple bonds and therefore silicon dioxide cannot support the same configuration as carbon dioxide. Instead, each silicon atom bonds to four oxygen atoms, which then bond to other silicon atoms, which in turn bond to still other oxygen atoms, and so on. In this way, silicon dioxide is a crystalline solid (i.e. rock) rather than a free molecule.
- Of course, silicate-based life need not function like Earth life. It might develop in a reducing environment and so might exhale silane (SiH4) instead, which is similar to methane and is a gas. While that is likely a possibility for hypothetical life based on silanes, this scenario seems unlikely for life based on silicates due to the necessary inclusion of oxygen.
- For perspective, an average man weighing 60-70 kg (130-150 lbs) exhales about 750-900 grams (1.7-2.0 lbs) of carbon dioxide a day. Lawrence Joseph Henderson, The Fitness of the Environment (New York, NY: MacMillan Company, 1913), 133. Reproduction by Nabu Press, 2010.
- Schulze-Makuch and Irwin, Life in the Universe, 106.
- Ibid, 106.
- Ibid, 105.
- Post, Silicones and Other Organic Silicon Compounds, 2.
- Schulze-Makuch and Irwin, Life in the Universe, 105.
- Ward, Life as We Do Not Know It, 74-75.
- Bains, "Many chemistries," 152-154.
- Carl Sagan, "The Search for Extraterrestrial Life," Scientific American (October 1994), 93.
- Bailar et al., Chemistry, 983-984, 993-995.
- Firsoff, "Possible Alternative Chemistries of Life," 135-6.
- Ibid, 134.
- Some have suggested ammonia as an alternative solvent, since it is good at dissolving boron and is generally found at the lower temperatures needed for boron-nitrogen chemistry (Firsoff, Life Beyond the Earth, 130). Ammonia's potential as a life solvent will be discussed later in this paper.
- Ward, Life as We Do Not Know It, 64.
- Gonzalez and Richards, The Privileged Planet, 127-136.
- For a more detailed discussion of why a liquid medium is necessary for intelligent life, see (1) Benner et al., "Is there a common chemical model for life in the universe?," 676-7; (2) Plaxco and Gross, Astrobiology, 14; (3) Bains, "Many chemistries," 139; (4) Dirk Schulze-Makuch and Louis Neal Irwin, Life in the Universe, 109 and (5) Ward, Life as We Do Not Know It, 71-72. Of these, only the first leaves a little room for non-liquid mediums.
- Benner et al., "Is there a common chemical model for life in the universe?," 676.
- Ibid, 676.
- Ward, Life as We Do Not Know It, 70.
- Plasma and dense gases have been suggested as an alternative to traditional liquid solvents (Feinberg and Shapiro, Life beyond Earth, 212, 380-386). Supercritical fluids (SCF) represent another particularly interesting possibility, because they tend to be good solvents and are increasingly being used in industrial applications. Their main drawback as a possible life solvent is that they rarely occur naturally on small rocky planets. Moreover, the properties of SCFs are very sensitive to the environmental temperature and pressure, which would pose serious problems for any living things dependent upon them (Bains, "Many chemistries," 160-2). A third possibility is life starting on a solid mineral surface that helps catalyze reactions (Ward, Life as We Do Not Know It, 98-100). This idea has been postulated in some origins of life scenarios. The solid surface, however, only serves as a temporary intermediate and the life form would still be liquid based, so will not be discussed here.
- V. Axel Firsoff, "An Ammonia-Based Life," Discovery 23: 39-40 (January 1962). See also Plaxco and Gross, Astrobiology, 14-18.
- Both DNA and RNA have a phosphate backbone with repeating negative charges along their length. Likewise, amino acids in solution can spontaneously change to their zwitterion form where one of their hydrogen atoms shifts position thus creating both a positive and a negative charge center. In both cases, water's very large dielectric constant is critical for stabilizing these charge centers, which help keep the large molecules in solution.
- Benner et al., "Is there a common chemical model for life in the universe?," 681.
- Dr. John Millam and Ken Klos, "Water: Designed for Life, Part 1 (of 7)" Today's New Reason To Believe on May 20, 2013. Part 1 contains links to the remaining parts.
- The reason water is so abundant is that it is composed of the first and third most abundant elements in the universe. (The second most abundant element is helium, but it does not form any compounds and so can be safely ignored.)
- Bailar et al., Chemistry, 425.
- We can find pressures like that at ocean depths around 2 km or 1.2 miles. At even greater depths and correspondingly higher pressures, water can become a supercritical fluid (where it behaves partially as a liquid and partially as a gas). For example, we see this at hydrothermal vents at depths of 3 km or 1.8 miles with temperatures as high as 464°C/867°F.
- Schulze-Makuch and Irwin, Life in the Universe, 92.
- For a quick three-page summary of how water's eccentric nature undergirds almost every area of life, see Felix Franks, "Water: The Unique Chemical," Chemistry in Britain 12, 278 (1976). See also, R. A. Horne; "On the Unlikelihood of Non-Aqueous Biosystems," Space Life Sciences 3 (August 1971):34-41. See also, A. E. Needham, The Uniqueness of Biological Materials (Oxford, London: Pergamon Press, 1965), 9-28.
- Henderson, The Fitness of the Environment, 72-132.
- Feinberg and Shapiro, Life beyond Earth, 230-1. See also Benner et al., "Is there a common chemical model for life in the universe?," 681-4.
- Philip Ball, Life's Matrix: A Biography of Water (Berkeley, CA: University of California Press, 2001), 210-215. See also Plaxco and Gross, Astrobiology, 211-215.
- Water's chemical reactivity results from its oxygen atom acting as a powerful nucleophile in reactions. Oxygen is such an effective nucleophile because it has a large electronegativity (second only to fluorine). The hydrogen atoms can also act as acidic protons. Benner et al., "Is there a common chemical model for life in the universe?," 679.
- Ibid, 679.
- Three of the bases of DNA (cytosine, guanine, and adenine) contain an amine group that is vulnerable to reacting with water (hydrolytic deammonization). In their nucleoside form (a base combined with a sugar), each has a half-life of around 70 years in water at 25°C or 77°F.
- Felix Franks, Water: A Matrix of Life, second edition (Cambridge, UK: Royal Society of Chemistry, 2000), 144.
- Carbon dioxide is reactive because the carbon is an effective electrophilic center. Bicarbonate, however, is much less reactive, because access to the electrophilic carbon is blocked by the anionic carboxylate group. Benner et al., "Is there a common chemical model for life in the universe?," 683.
- Fazale Rana, The Cell's Design: How Chemistry Reveals the Creator's Artistry, 262-5.
- Bains, "Many chemistries," 137-167.
- Simplified version of Figure 5 from Bains, "Many chemistries," 145
- The sulfuric acid haze is located between 38 and 72 km above the surface of Venus with temperature ranging from 150°C/302°F down to 45°C/49°F. Irwin and Schulze-Makuch, Cosmic Biology, 153-7.
- Bains, "Many chemistries," 140.
- Ibid, 140.
- Ibid, 146.
- S. R. Taylor, Destiny or Chance: Our Solar System and its Place in the Cosmos (Cambridge, Great Britain: Cambridge University Press, 2000), 123-6.
- Schulze-Makuch and Irwin, Life in the Universe, 109-132.
- Benner et al., "Is there a common chemical model for life in the universe?," 676-680.
- One exception is molecules with polar sections that are highly symmetrical (like carbon dioxide, CO2), such that the partial charge in one part is exactly balanced out by the charge in another part leaving it non-polar. Also, some molecules (e.g., soaps) contain both polar sections and non-polar sections (amphiphilic).
- Plaxco and Gross, Astrobiology, 14.
- The main alteration would be to replace oxygen atoms in proteins with nitrogen (plus a hydrogen to balance the charge). Firsoff, "An Ammonia-Based Life," 36-42. See also Peter Molton, "Non-aqueous Biosystems: The Case for Liquid Ammonia as a Solvent," Journal of the British Interplanetary Society 27, 243-262 (1974).
- Plaxco and Gross, Astrobiology, 17.
- Irwin and Schulze-Makuch, Cosmic Biology, 155
- Benner et al., "Is there a common chemical model for life in the universe?," 678.
- Schulze-Makuch and Irwin, Life in the Universe, 120.
- Ibid, 120-121. Pyrimidines and purines are of biological significance because they are the components of the nucleobases of DNA and RNA.
- Ibid, 121.
- Formamide facilitates the formation of ATP (from ADP and phosphate), nucleosides (from ribose borates and nuclear bases), and peptides (from amino acids). Benner et al., "Is there a common chemical model for life in the universe?," 679.
- Firsoff, "Possible Alternative Chemistries of Life," 135-136.
- When the Viking landers sampled Martian soil in 1976, the life detection experiments gave inconclusive results. Some of the indicators suggested the presence of life, while other details argued against that conclusion (Schulze-Makuch and Irwin, Life in the Universe, 183-191). Most scientists today hold that the anomalous findings were the result of unusual soil chemistry, not life. However, one recent hypothesis is that Martian organisms based on water and hydrogen peroxide might be able to explain all of the Viking results (Ibid, 190). This claim, while interesting is very speculative.
- Methanol could have formed from the oxidation of methane in the atmosphere of early Mars. One possible evidence for this is the presence of hematite deposits on Mars. The most likely explanation for the presence of this mineral is iron precipitating out of a liquid environment. Most scientists see this as evidence for liquid water on the early Martian surface; however, one group has suggested a methanol origin for the hematite. Yan Tang, Quinwang Chen, and Yujie Huang, "Early Mars may have had a methanol ocean," Icarus 180:88-92 (2006).
- Ward, Life as We Do Not Know It, 223-227.
- Ibid, 229-231.
- This has been demonstrated, for example, with (polar) acetonitrile (C2H3N) in (non-polar) hexane. Benner et al., "Is there a common chemical model for life in the universe?," 679.
- Feinberg and Shapiro, Life beyond Earth, 250-252.
- Molton, "Terrestrial Biochemistry in Perspective," 140.
- The proposed creatures (known as H-bits) would not use regular chemical reactions for energy, but would exploit the subtle difference in energy between the two spin states of hydrogen. Feinberg and Shapiro, Life beyond Earth, 386-393.
- Feinberg and Shapiro, Life beyond Earth, 318-328.
- Benner et al., "Is there a common chemical model for life in the universe?," 679-680.
- J. B.S Haldane, "The Origins of Life," New Biology 16:12-27 (1954).
- Firsoff, "An Ammonia-Based Life," 36-42. See also Firsoff, Life Beyond the Earth, 122-128. Firsoff, "Possible Alternative Chemistries of Life," 132-136.
- Feinberg and Shapiro, Life beyond Earth, 246-250.
- Molton, "Terrestrial Biochemistry in Perspective," 140. Molton, "Non-aqueous Biosystems," 243-262. Molton, "The Multiplicity of Potential Living Systems Based on C, H, O, N," Journal of the British Interplanetary Society 28, 392-8, 1975.
- Benner et al., "Is there a common chemical model for life in the universe?," 672-689.
- Bains, "Many chemistries," 137-167.
- Tom Head (ed.), Conversations with Carl Sagan (Literary Conversations), (Jackson, MS: University Press of Mississippi, 2006), 10-11.
- Isaac Asimov, " Not as We Know it the Chemistry of Life," Cosmic Search, Issue 9, Volume 3, Number 1 (Winter 1981), 5.
- Plaxco and Gross, Astrobiology, 14. The widespread presence of ammonia is the result of the high cosmic abundance of the elements nitrogen and hydrogen. It is, however, less common than water because nitrogen is less common than oxygen in the universe.
- For perspective, a 1.0 M aqueous solution of ammonia has a pH of 11.6. Saturn's moon, Titan, is believed to have large subsurface ocean of aqueous ammonia with a pH > 10.5 (Ward, Life as We Do Not Know It, 228). This high level of basicity would cause extreme difficulty for life originating there. However, we do know that at least one extremophilic bacteria (an alkaliphile) can survive in water with a pH as high as 12 (by maintaining an internal pH of 8.6). So, it is at least possible for an organism to live under such basic conditions.
- The hydroxide anion (OH-), arising from a high pH, is a powerful nucleophile and destroys many molecules which are essential to metabolic pathways. Some organisms are able to survive in high pH environments by pumping out hydroxide ions or pumping in hydrogen ions in order to maintain a neutral pH inside their cells. Schulze-Makuch and Irwin, Life in the Universe, 56-7.
- Terrestrial (water-based) life faces the analogous challenge of eliminating ammonia produced during metabolism. In humans, this is handled by the kidneys. This demonstrates that it should be possible for a hypothetical organism to accomplish the opposite process—excreting water while retaining ammonia (Molton, "Non-Aqueous Biosystems," 245-6). However, such mechanisms are extremely complex and therefore would be exceedingly unlikely to occur naturally under prebiotic conditions.
- Ward, Life as We Do Not Know It, 220-227.
- Firsoff, Life Beyond the Earth, 122.
- Molton, "Non-Aqueous Biosystems," 245-246.
- Molton, "The Multiplicity of Potential Living Systems," 394-5.
- Carl Sagan and E. E. Salpeter, "Particles, Environments, and Possible Ecologies in the Jovian Atmosphere," Astrophysical Journal Supplement Series 32:737-755 (December 1976).
- One major challenge for life in the clouds of Jupiter-like gas giant planets is that gravity and strong air currents would naturally pull any organism toward the planet's center. This would result in much higher temperatures and pressures that would destroy such life. Sagan and Salpeter suggested "bubble life" capable of maintaining a stable altitude like a miniature air balloon. This is an advanced feature, so the precursors to such life would not have such a buoyancy mechanism and so would likely be quickly destroyed, before they could ever take hold. Schulze-Makuch and Irwin, Life in the Universe, 137-140.
- Ward, Life as We Do Not Know It, 72-73, 212-3.
- Firsoff, "An Ammonia-Based Life," 41-42. Molton, "Non-Aqueous Biosystems," 250-261. Molton, "Terrestrial Biochemistry in Perspective," 139. See also Benner et al., "Is there a common chemical model for life in the universe?," 678.
- Examples taken from Molton, "Terrestrial Biochemistry in Perspective," Table 1, 139.
- The main reason for these differences is that the nitrogen in ammonia is less electronegative than the oxygen in water. That means that the bonds in ammonia are less polar than those in water.
- Benner et al., "Is there a common chemical model for life in the universe?," 677-8.
- Firsoff, "An Ammonia-Based Life," 41; Firsoff, Life Beyond the Earth, 125-126. Firsoff, "Possible Alternative Chemistries of Life," 135.
- Molton, "Non-Aqueous Biosystems," 249-250.
- Molton, "Terrestrial Biochemistry in Perspective," 140.
- Schulze-Makuch and Irwin, Life in the Universe, 118.
- UV light is divided into four energy regions: UVa (low energy), UVb (moderate energy), UVc (high energy), and vacuum UV (very high energy). Oxygen and other molecules can block most of the damaging UVc and all of the vacuum UV coming from the sun. Ozone is required to eliminate harmful UVb and any remaining UVc. Healthy UVa light is allowed through.
- Schulze-Makuch and Irwin, Life in the Universe, 118.
- Firsoff, "An Ammonia-Based Life," p. 42.
- Oxygen's reactivity, however, does come with a price. A side product of using oxygen in metabolism is free radicals which can cause damage inside cells. Our bodies protect against this mainly through the production of antioxidants.
- W. H. Kuhn and S.K. Atreya, Ammonia Photolysis and the Greenhouse Effect in the Primordial Atmosphere of the Earth. Icharus 37 (1979), 207. J.S. Levine, The photochemistry of the paleoatmosphere. J. Mol. Evol. 18 (1982), 161.
- Sagan, The Cosmic Connection, 46-47.
- Interview of Carl Sagan by Timothy Ferris in 1973. Tom Head (ed.), Conversations with Carl Sagan, 10-11.
- Sagan, "The Search for Extraterrestrial Life," 93.
- Ibid, 93.
Last Modified September 23, 2014