Appropriate liquid/solvent for life in my underground environment on Venus
Partly inspired by this question
My Venusians are happy in their cloud top city when a small group somehow (with lots of handwaving) crashes to the surface, falls underground, and ends up in a hidden cavern. Our Venusians are in some sort of vehicle that can survive the local conditions, and like caves on earth the conditions in this underground cavern are similar to those above ground. To everyone's surprise they find life living in "pools" in the cavern.
I'm going with pools because (duh!) everyone likes/expects there to be underground lakes/rivers. Also I generally expect liquids to be a requirement for life anyway, as their ability to dissolve other chemicals and act as a medium to speed up chemical reactions is very important for all earth life.
Obviously though these "pools" are not composed of liquid water. Earth gets water lakes, Titan gets methane lakes, but what does Venus get? Given what we know of Venus, are there any plausible candidates for chemicals that would be liquid at VSTP (Venus Standard Temperature and Pressure) and might actually be around in enough quantities to form pools?
There is already a lot of handwaving going on, so if need be I'm happy to loosen the "present in sufficient quantities" requirement.
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1 answer
Supercritical carbon dioxide
Once upon a time, Venus may have had seas of supercritical $\text{CO}_2$ ($\text{scCO}_2$) thanks to a higher surface temperature (by a few hundred Kelvin) and surface pressures (by a factor of 3 or so). However, now that the atmospheric pressure has dropped to about 9.3 MPa, this is no longer feasible aboveground; while there is plenty of $\text{scCO}_2$ to go around, you're unlikely to find pools of it anymore on the surface.
In subsurface oceans, however, supercritical $\text{CO}_2$ could still exist, and it would be a decent solvent for some enzymes. Trace amounts of water would be required, but Venus does indeed have such trace amounts in its atmosphere. Under the right conditions, $\text{scCO}_2$ may fit your requirements.
The enzymes
A number of enzymes react well with $\text{scCO}_2$, including
- Lipases, which are involved in the hydrolysis of fats
- Phosphatases, although these typically function optimally with water as a solvent
- Dehydrogenases, used in certain oxidation reactions; these may involve NAD$^+$ (used in glycolysis) and NADP$^+$
- Oxidases, which are used in oxidation-reduction reactions, such as part of the electron transport chain
- Amylases, used to form sugars from starch
We need to be careful, though, as these enzymes can denature and lose their structure at many of the temperatures at which $\text{CO}_2$ is supercritical. Furthermore, under some conditions, $\text{scCO}_2$ can inhibit enzyme function, which is why it can be used for sterilization.
Experimental cases
Apparently (see the previous paper), $\text{scCO}_2$ has been shown to increase reaction rates in several types of bacteria; for example, it helped E. coli and Saccharomyces cerevisiae (a yeast) using $\alpha$-amylase, the most important amylase in most animals. This occurred at 20 MPa and 308 K.
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