Venus is, in my humble opinion, one of the most fascinating planets in our entire solar system.

Granted, it’s not exactly the kind of place you might go for a picnic. In fact, if you went there without any sort of protection, it’s hard to say what would kill you first; you’d be simultaneously roasted, crushed and dissolved. Nonetheless, our friendly neighbourhood acid ball is a curious place…

One of the most interesting things about Venus is it’s atmosphere. The bizarre sulfur chemistry going on in the clouds is a mystery. Sulfrous chemicals that shouldn’t be found together are sprinkled liberally throughout the venusian cloud decks. Sulfuric acid droplets fall as rain, evaporating miles above the planet’s surface in a corrosive parody of the virga that falls over Earth’s tropical rainforests. The sulfuric acid too, must be replenished somehow. If the planet was completely inactive, those beautifully deadly clouds simply wouldn’t exist. Even more bizarre is the heavy metal snow that some believe falls on the planets mountaintops (one explanation for why they’re abnormally reflective). But by far, the most interesting thing about Venus’s atmosphere comes down to what it contains the most of.

Nearly 97% of the venusian atmosphere is carbon dioxide. And Venus has a lot of atmosphere — it’s well over 90 times as massive as Earth’s. With all of that mass being compressed downwards by gravity, its surface pressure reaches around 93 bar. That pressure combined with the fact that CO 2 is an effective thermal insulator makes the surface temperature 740 Kelvins. Just to compare, lead melts at around 600K. That temperature and pressure pushes CO 2 past what chemists call it’s “critical point”, causing it to behave rather strangely. Too hot to act like a liquid, but under too much pressure to act like a gas, this CO 2 is actually neither! Halfway between the two, it’s a supercritical fluid. Here on Earth, supercritical CO 2 (sometimes called sc-CO 2 for short) has a lot of uses. Among other things, it can be used in dry cleaning and to decaffeinate coffee (it’s a very good solvent).

Could the Venusian highlands stand out from a kind of “supercritical ocean” into the more normal gassy CO 2 , the way Earth’s continents stand above the surface of the water? The highest peak on Venus, Maxwell Montes is roughly 11km tall. That’s higher than Everest! Could that be high enough?

This image shows the surface of Venus, coloured according to elevation. Blue are lowlands, while red are highlands. The large land mass in the image is the Aphrodite Terra. Roughly the size of Africa, it’s the lower of the two great venusian “continents” — with the other being the more mountainous Ishtar Terra. Of course, I use the term “continent” loosely, as Venus has no plate tectonics. It works as an analogy though…

What we don’t know is exactly how much of the atmosphere is sc-CO 2 . The transition from gas into supercritical fluid is most likely a smooth one, and not a clear boundary like Earth’s watery oceans, but beyond that we just don’t know. Not much has been written on the subject.

What implications might all of this have for the planet’s geology and atmospheric chemistry? And what about that snow — could it explain why only the mountaintops are so reflective? Could it help explain why Venus’s surface appears to be so young? Perhaps we need to take a closer look at the Venusian highlands. The two continents weren’t really looked at by the russian Venera probes, which landed on lower ground. Magellan mapped the surface of Venus with radar, but that wouldn’t show interesting rock formations or the actual snow made from lead sulfide (though it is, I believe, how we know those mountaintops are so shiny). Lead sulfide, incidentally is more commonly known as the mineral, galena. If this is indeed what the snow on the venusian mountaintops is made from, they must look beautiful. Who knows how different the planet might be above its supercritical ocean.

Incidentally, if anyone reading knows more about this, if you can point me towards any papers or articles about it I’d love to read them. In the meantime, while most of humanity are looking outwards towards Mars, I hope I’m not the only one who’d still like to look back towards Venus…