Astrobiologists like to argue about the various parameters required for planetary habitability, but one thing they tend to agree on is that water must be present. A new theory upends this assumption by suggesting that alien life could thrive on "supercritical carbon dioxide" instead.

When chemists talk about supercritical fluids (SCFs), they're describing fluids that have exceeded their temperature and pressure thresholds. Once you get past this "critical point," liquids and gases cannot co-exist. Indeed, matter can be pushed to temperatures and pressures to the point where it's impossible to distinguish whether it's a liquid or a gas. Thus, SCFs don't have a definite phase. Owing to its unique properties, they can dissolve materials like a liquid, but flow like a gas.

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Carbon dioxide becomes supercritical when its temperature exceeds 305 degrees Kelvin (about 88 F) and its pressure goes beyond 72.9 the standard atmosphere (atm) at sea level (the kind of pressure you'd find a half-mile beneath the ocean surface). Supercritical CO2 is used for a number of things, including decaffeinating coffee beans and even dry cleaning.

But if astrobiologists Nediljko Budisa and Dirk Schulze-Makuch are correct, supercritical CO2 might be capable of acting as a life-sustaining solvent in a planetary environment. Writing in Space, science writer Charles Q. Choi explains:

Ordinarily, carbon dioxide is not considered a viable solvent to host the chemical reactions for life, but the properties of supercritical fluids can differ quite significantly from the regular versions of those fluids — for instance, while regular water is not acid, supercritical water is acidic. Given how substantially different supercritical carbon dioxide is from regular carbon dioxide in terms of physical and chemical properties, scientists explored whether it could be suitable for life.

"I always have been interested in possibly exotic life and creative adaptations of organisms to extreme environments," said study co-author Dirk Schulze-Makuch, an astrobiologist at Washington State University in Pullman. "Supercritical CO2 is often overlooked, so I felt that someone had to put together something on its biological potential."

The researchers noted that enzymes can be more stable in supercritical carbon dioxide than in water. In addition, supercritical carbon dioxide makes enzymes more specific about the molecules they bind to, leading to fewer unnecessary side reactions.

Surprisingly, a number of species of bacteria are tolerant of supercritical carbon dioxide. Prior research found that several different microbial species and their enzymes are active in the fluid.

Fascinatingly, the atmospheric pressure of Venus is about 90 times greater than that of the Earth, with an average temperature of 467 degrees C. About 97% of its atmosphere is carbon dioxide. It's possible, therefore, that the atmosphere of Venus is a SCF. And indeed, the researchers speculate that organic remnants of life could still be preserved in such a fluid.

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Other possibilities include super-Earths — planets with 10 or more time the mass of our planet. These exoplanets would have stronger gravitational pulls, resulting in higher atmospheric pressures, and by consequence, SCFs.

Read Choi's entire article at Space. And check out the scientific study at the journal Life.

Image: NASA