In Star Trek: The Next Generation, the crew of the Enterprise regularly encounters exotic alien life-forms. Many of these extraterrestrials are truly “weird”—distinctly different from life as we know it. In one particular episode, a crystalline life-form refers to humans as “ugly bags of mostly water.”1 “Ugly” is a matter of perspective, but describing humans as “bags of mostly water” is reasonably accurate. We are composed of 60 percent water approximately. All Earth-life utilizes water as an essential component, but some people speculate that perhaps weird life might rely on ammonia or some other liquid. Could such an idea cross from science fiction to science fact?
Water inside our cells serves to dissolve the biomolecules (e.g., proteins, DNA, and lipids), nutrients, and salts needed for cells to function and serves as a medium to bring these chemicals together into a common environment in which all these molecules can interact efficiently. This facilitates the biochemical reactions needed for life. Water also excludes certain fat molecules (lipids), which drives the formation of cell membranes. Accordingly, we refer to water as being a life solvent; many critical life reactions would be impossible without it.
Some scientists have proposed that life could arise on other planets in the universe utilizing a liquid other than water as a life solvent. If these hypothetical creatures existed, we would classify them as weird life because they would have to be fundamentally different from life as we know it. (For more details, see our full paper on this subject.)
Water as the Solvent of Life
Given that water is the only known life solvent, it is useful to consider how it optimally fulfills this role. Against this benchmark, we can compare other possible solvents. A full review of water’s exceptional properties is beyond the scope of this paper. Instead, we refer readers to our seven-part series titled “Water: Designed for Life.” Those articles define important terms, provide helpful background information, and explain the roles of water’s key properties. Here we will summarize only the most relevant details:
- Water is ubiquitous. Water is one of the most abundant molecules in the entire universe. As such, we can expect to find it 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.
- Water is the “universal solvent.” Water is considered the most effective known solvent, which has earned it the title “universal solvent.” It is 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 molecule classes that generally do not dissolve in water. But even this limitation has a positive benefit: the hydrophobic effect (which we will discuss below).
- Water has a large range of liquidity. Under terrestrial conditions, pure water is a liquid over a rather impressive temperature range: 0–100°C (32–212°F). Adding common salts can reduce water’s melting point to as low as -23°C (-10°F). Increasing the external pressure to 215 times atmospheric pressure increases the boiling point to 374°C (706°F). This means that water has a potential range of liquidity of 397°C (716°F).
- Water has a strong hydrophobic effect. Water has the strongest hydrophobic effect of all naturally occurring molecules, 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 of the protein 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 is important for life solvents; and few liquids can even come close to rivaling water’s thermal properties (i.e., properties dealing with heat and temperature). Specifically, water has exceptionally high values for: 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 known to be capable of supporting life chemistry, it is noteworthy that water is exceptionally well suited to facilitate carbon chemistry for at least two reasons. First, carbon forms very strong bonds with both hydrogen and oxygen, which compose water. 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, lower temperatures would result in slower chemical reactions, which would hinder the development of life. For water to have a much higher boiling point would not be helpful because most carbon compounds degrade at temperatures above 200°C (392°F). Therefore, water is a liquid at the optimal temperature range for carbon chemistry.
Water is an amazing liquid. As a life solvent, it is without equal. Water’s additional extraordinary properties are too numerous to include in this article. In our opinion, they are a clear indication that water is designed for life.
In scientific literature, water has been referred to as the “vital fluid” and the “elixir of life” for good reasons.2 Nothing else even comes close to matching its ability to support life. For this reason, in its search for extraterrestrial life, NASA has largely embraced a “follow the water” strategy of looking for liquid water on other planets. However, not everyone agrees that water is the only possible life solvent. We will explore this possibility in a future article.
Dr. John Millam
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.