In classic science fiction serials, such as Star Trek, Doctor Who, and Stargate SG-1, intrepid heroes regularly travel to other worlds. These planets are almost always depicted as being fundamentally similar to our own with the same gravity, the same breathable atmosphere, and frequently even the same plants and trees. Of course, that is fiction. We now know that the great majority of other known planetary systems are distinctly different from our own. What consequences might these planetary differences have for life that hypothetically could arise on them?
Consider for a moment the Andorians of Star Trek. Their home world, Andoria, is said to be an icy moon orbiting a Jupiter-like gas giant planet. Such cold conditions would likely keep water in a frozen state. Ammonia, however, has a lower freezing point than water and so might be able to remain a liquid under those conditions. Therefore, if Andorian-like beings actually existed, ammonia inside their cells might serve the same purposes as water does for humans on Earth.
If our fictional Andorians actually existed, they would be classified as “weird life”—forms of life that are fundamentally different from known life. All known life utilizes water as a life solvent (the liquid medium inside of cells that hosts and supports biochemistry). Previously, we considered what was necessary for any substance to serve as a life solvent. Based on these considerations, we concluded that ammonia is the most promising alternative life solvent. Consequently, we will provide a detailed analysis of ammonia to see if it has what it takes to be a life solvent.
Ammonia as a Life Solvent
Ammonia (NH3) is an obvious choice for a possible alternative to liquid water. First, it is the most well-known and thoroughly studied solvent (after water). Second, ammonia is the fourth most abundant molecule in the universe, which means that it will be present in significant quantities on many planets and moons. Third, ammonia is chemically and physically similar to water (see figure 1). This means that ammonia shares many of water’s useful qualities.
Figure 1: Molecular structures of (a) ammonia and (b) water. Image credit: John Millam
Pure vs. Aqueous Ammonia
In discussing ammonia, we first need to 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 is more correctly referred to as aqueous ammonia. 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 (-108°F) and -33.4°C (-28°F).
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. However, the problem is that water is so universally abundant that under most planetary conditions we would find aqueous ammonia instead. For simplicity, however, we will restrict all our remaining discussion to considering life originating in and supported by pure ammonia.
Evaluating Ammonia as a Life Solvent
To evaluate ammonia’s potential to serve as a possible life solvent we will consider the eight properties of a good life solvent. In addition, we will compare it to water, which is currently the only known life solvent. (For complete details, see our full paper.)
- Ammonia is common in the universe. One of ammonia’s most important qualifications is that it is naturally abundant in the universe, 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 polar solvent; however, compared to water, it is less polar. This results in ammonia not being as effective as water at dissolving polar molecules and salts. On the other hand, it results in ammonia doing a slightly better job than water at dissolving nonpolar organic molecules—a potentially beneficial property because these types of molecules can play an important role within the cell.
- Ammonia has a relatively low range of liquidity. Consequently, ammonia-based life would be more vulnerable to environmental temperature changes.
- Ammonia has a weaker hydrophobic effect. The hydrophobic effect in ammonia is considerably weaker than in water, resulting in two significant problems. First, ammonia solutions would be less able to form cell membrane–like structures, and those that did 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 has a weaker dielectric constant. Ammonia’s dielectric constant is around one-third of water’s. This feature reduces ammonia’s ability to dissolve salts and support ionic species in solution. It also makes keeping many large and complex molecules (those equivalent to proteins, DNA, and RNA in terrestrial life) in solution more difficult.
- Ammonia has excellent thermal properties. Ammonia’s thermal properties compare favorably with those of water, although its heat of vaporization is approximately half of water’s.
- Ammonia has a low viscosity. Both ammonia and water have very low viscosities, which means that they both flow freely and allow dissolved compounds to move about easily.
- Ammonia has a weaker surface tension. Ammonia’s surface tension is approximately one-fourth of water’s value. As a result, adsorption (which helps concentrate biomolecules on a cell’s surface) and capillary action would be less effective in ammonia than in water.
In addition to its weaknesses as a life solvent, ammonia has at least two other problems that we need to consider.
- Ammonia is flammable in oxygen. Because ammonia reacts strongly with oxygen, hypothetical ammonia-based life would almost certainly require an oxygen-free environment.
- Ammonia does not self-shield against ultraviolet (UV) light. When water is hit by high-energy UV light, it dissociates into O2, which can be further transformed into ozone (O3). Ozone absorbs UV light, thus shielding water from further dissociation. In contrast, ammonia is dissociated into nitrogen (N2), which offers no such protection.
Life based on liquid ammonia is an intriguing possibility. Indeed, ammonia compares well to water in many ways. While ammonia as a life solvent cannot be categorically ruled out, it does have a number of significant problems that cannot be overlooked. We are sure that Star Trek fans will be disappointed to learn that Andorians (or other ammonia-based life) are unlikely to exist anywhere in the universe.
Cosmologist Carl Sagan was an early proponent of weird life. He dismissed the notion that life inherently requires both carbon and water as a form of “chauvinism.”1 However, just two years before his death in 1996, he conceded,