A new study published in the online journal Life says a hotter, pressurized form of carbon dioxide could harbor life in a similar way water does on Earth. This is an interesting find, theoretical though it is, because it might obviate the need for water to be present for life to exist on other planets. In fact, of the more than 2,700 exoplanet candidates, more than 2,000 are massive enough to have such carbon dioxide present on their surface.
At about 305 kelvin (90 degrees fahrenheit/32 degrees celsius) and 73-times Earth’s atmospheric pressure, carbon dioxide becomes supercritical, a form of matter that exhibits the physical properties of both liquids and gases. Its properties are very different from what they usually are in its common state—in the same way highly pressurized water is acidic but normal water isn’t. Supercritical carbon dioxide is often used as a sterilization agent because it can deactivate microorganisms quickly at low temperatures.
As the study’s authors found, some enzymes were more stable in supercritical carbon dioxide because it contains no water. The anhydrous property also enables a “molecular memory” in the enzymes, when they ‘remember’ their acidity from previous reactions to guide the future construction of organic molecules more easily. Moreover, as stated in the paper:
… the surface tension in carbon dioxide is much lower than that of water, whereas the diffusivity of solutes in scCO2 is markedly higher [because of lower viscosity]. Thus, scCO2 can much easier penetrate [cell membranes] than subcritical fluids can.
The easiest way—no matter that it’s still difficult—to check if life could exist in supercritical carbon dioxide naturally is to check the oceans at about a kilometer’s depth, where pressures are sufficient to entertain pockets of supercritical fluids. As the authors write in their paper, supercritical carbon dioxide is less dense than water, so they could be trapped under rocky formations which in turn could be probed for signs of life.
A similarly accessible place to investigate would be at shallow depths below the surface of Venus. Carbon dioxide is abundant on Venus and the planet has the hottest surface in the Solar System. Its subsurface pressures could then harbor supercritical carbon dioxide. Dirk Schulze-Makuch, a coauthor of the paper and an astrobiologist at Washington State University, notes:
An interesting twist is that Venus was located in the habitable zone of our Solar System in its early history. [Him and his coworkers] suggested the presence of an early biosphere on the surface of this planet, before a run-away greenhouse effect made all life near the Venusian surface all but impossible.
The probability that Venus could once have harbored life is as strange as it is fascinating. In fact, if further studies indicate that supercritical carbon dioxide can play the role of a viable bio-organic solvent, the implications will stretch far out into anywhere that a super-Earth or gas-giant is found. Because its reactions with complex organic molecules such as amines will not be the same as water’s, the life-forms supercritical carbon dioxide could harbor will be different—perhaps more primitive and/or short-lived. We don’t know yet.
This study continues a persistent trend among astrobiologists since the 1980s to imagine, and then rationalize, if and how life could take root in environments considered extreme on Earth. After the NASA Kepler space telescope launched in 2009 and, in only four years of observation, yielded almost 4,100 exoplanet candidates (more than a thousand confirmed as of now), astrobiologists began to acquire a better picture of the natural laboratories their hypotheses had at their disposal, as well as which hypotheses seemed more viable.
In August this year, Schulze-Makuch himself had another paper, in Science, that discussed how a lake of asphalt in Trinidad harbored life despite a very low water content (13.5%), and what this said about the possibilities of life on Saturn’s moon Titan, which exhibits a similar chemistry on its surface. The Science paper had cited another study from 2004. Titled “Is there a common chemical model for life in the universe?” it contained a pertinent paragraph about why the search for alien life is important as well as likely endless:
The universe of chemical possibilities is huge. For example, the number of different proteins 100 amino acids long, built from combinations of the natural 20 amino acids, is larger than the number of atoms in the cosmos. Life on Earth certainly did not have time to sample all possible sequences to find the best. What exists in modern Terran [i.e. Earth-bound] life must therefore reflect some contingencies, chance events in history that led to one choice over another, whether or not the choice was optimal.