We’re all familiar with ordinary carbon dioxide—the greenhouse gas that we exhale, and that plants use for photosynthesis. However, there’s another kind of carbon dioxide, called “supercritical,” that’s quite different from the gas we see under normal conditions on Earth’s surface. Any fluid has a “critical point” of combined temperature and pressure, above which there’s no longer a clear boundary between the liquid and gas phase. For carbon dioxide, the critical point is 31.1oC (88oF) and 73.8 bar pressure (one bar corresponds to Earth’s atmospheric pressure at sea level). Not terribly hot, but equivalent in pressure to what you’d experience nearly half a mile under the ocean’s surface.
Could life exist in an environment where CO2 has gone supercritical? That’s a question Ned Budisa and I address in a paper just published in the journal Life as part of a special edition on Planetary Exploration: Habitats and Terrestrial Analogs.
Carbon dioxide is usually not considered a suitable solvent for life, at least not on Earth. But its chemical and physical properties change quite dramatically when it becomes supercritical. Above its critical point, CO2 mixes well with a variety of organic compounds. It participates in a number of organic synthesis reactions, and some bacteria and their enzymes have been shown to be active with supercritical CO2 as a solvent.
These bio-friendly properties are relevant not just for alien worlds, but also in certain exotic environments on Earth. For example, microbial life has been found near subsurface accumulations of liquid carbon dioxide under Earth’s oceans. The microbes may be using at least some of the biologically advantageous properties of supercritical carbon dioxide.
Since carbon dioxide is a common compound in planetary atmospheres—including those of neighboring worlds like Venus and Mars—supercritical carbon dioxide may be biologically significant on many planets.
This is especially true for Venus. Venus is nearly as massive as Earth and has an atmosphere of 92 bar, mostly consisting of carbon dioxide. At one point in its early history, Venus was located in the habitable zone of our solar system, meaning that liquid water was stable on its surface. That raises the possibility that it had an early biosphere, before a runaway greenhouse effect heated the planet and pushed surface temperatures up to the level of an oven on broil. It is unlikely that life could have survived on early Venus by using supercritical carbon dioxide rather than water as a solvent, but perhaps some organic remnants from an earlier biosphere, if it existed, could have been preserved in that medium.
The importance of supercritical carbon dioxide extends far beyond Venus, though, since many newly discovered exoplanets are “Super Earths” with several times the mass of Earth. In these high-pressure environments, supercritical carbon dioxide might be common. So, while supercritical fluids have been studied for their biochemical and biotechnological applications, astrobiologists may want to start paying closer attention, too.