Supercritical fluids (SCFs) are strange; mashed by high temperature and pressure into a weird transitional not-quite-liquid, not-quite-gas state, they can possess elements of both liquids and gases. They are like gases that dissolve, liquids with no surface tension, and yet in a way they’re like neither. This gives them properties that they otherwise wouldn’t have, and makes them incredibly useful for a wide range of practical applications.
If you view a large body of supercritical fluid from space, it’s going to look like the atmosphere of a hot, high-temperature planet. io9’s George Dvorsky uses Venus as an example:
[T]he 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.
Space.com’s Charles Q. Choi has recently highlighted the work of the pioneering American astrobiologist Dirk Schulze-Makuch, who has made a compelling argument that dry planets can sustain life in supercritical fluid atmospheres—specifically, supercritical fluid atmospheres that contain large amounts of carbon dioxide which, in its supercritical state, appears to provide a habitable medium for life in much the same way that water does.
This possibility would dramatically increase the odds that there could be life on other planets (we are once more unable to completely rule out the possibility of life on Venus, for example), but it also complicates our efforts to evaluate potential habitats by looking at their chemical signatures. If the Schulze-Makuch theory holds up, there could be a wide range of “uninhabitable” atmospheric chemicals that are actually quite habitable in a supercritical state—and life on planets containing these sorts of atmospheres may turn out to be much more common, on a cosmic level, than life on planets containing water. We just don’t know yet. Extraterrestrial life, it seems, can emerge in any number of unpredictable ways; the only thing we can confidently say about it is that we haven’t (officially) found it yet.