A Supercritical Ocean?

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 CO2 is an effective thermal insulator makes the surface temperature 740 Kelvins. Just to compare, lead melts at around 600K. That temperature and pressure pushes CO2 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 CO2 is actually neither! Halfway between the two, it’s a supercritical fluid. Here on Earth, supercritical CO2 (sometimes called sc-CO2 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 CO2, 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-CO2. 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…

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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11 Responses to A Supercritical Ocean?

  1. Pingback: Acid Lover’s Paradise | Supernova Condensate

  2. jukovz says:

    Lowland on Venus seems like an ocean. Venus planet Greenpeace planet if this image is not lies there should be a life.
    восстановление зрения

  3. invaderxan says:

    Re: Nicely put
    Wow… 2-3km? That’s really quite a good analog, isn’t it? I didn’t realise sc-CO2 had actually been considered as a medium for life — though I do know that aphiphilic molecules (lipid analogs) can self organise into micelles in sc-CO2 similarly to in water. In sc-CO2 though, the micelles are inside-out and may be filled with water or other polar molecules.
    As for speculative biochemistries or the potential for life in such an “ocean” I’m really not sure. It’s interesting to consider, but perhaps we need more information to draw any good conclusions just yet. Though it’s always fun to speculate…
    Geologically though, it’s certainly worth considering alternatives to the “catastrophic resurfacing” that many scientists consider to explain Venus’s surface. Frankly, that idea never quite sat comfortably with me…

  4. Anonymous says:

    Nicely put
    Hi InvaderXan
    Super-critical fluids have been posited as a possible medium for life, and super-c CO2 is a good organic solvent. It’s defined as super-critical when the temperature & pressure are past the critical point. Thus it isn’t super-critical anymore when the CO2 partial pressure drops below 75.8 bar (at the 78.55 bar level) somewhere between 2-3 km up. Since Earth’s oceans average ~2.67 km deep, if spread over the whole surface area, the super-critical “ocean” of Venus is comparable.
    The temperature is too high for stable biochemistry as we know it. I think it’s too hot for even the speculative biochemistries that have been proposed based on sulfuric acid and sulfur. If there was enough sulfuric acid present – Venus is as dry in sulfuric acid as it is in water – then a liquid ocean of the stuff could exist up to 654 C. Molten sulfur might work, but that seems in short supply too. The vapour pressure of any of those at ambient temperatures is too high for liquid to remain – it just evaporates rapidly. Not sure what could exist in that ‘ocean’ in sufficient concentrations to make for viable life.

  5. invaderxan says:

    Isn’t it just? It’s been mentioned in passing in literature, but seems to have largely escaped peoples attention. David Grinspoon mentions it in his book, Venus Revealed, where he also notes that little research has been done on the subject…

  6. Interesting! I hadn’t known about the supercritical fluid deal. Quite interesting.

  7. invaderxan says:

    Give it maybe a hundred years for us to both figure out how and be good enough to engineer one — then quite possibly. :)

  8. ryttu3k says:

    Yeah ^_^ What do you think about floating cities, doable?

  9. invaderxan says:

    I thought you might like it… ;)
    Oh yes — Earth air is a lifting gas on Venus. Plus, the atmosphere at a certain height on Venus is the most Earth-like in the whole solar system.
    Admittedly though, it’s true — human technology isn’t quite good enough to tame Venus just yet. Perhaps that just serves to make it even more interesting…

  10. ryttu3k says:

    That is so cool o.o
    I think people look towards Mars because it’s, you know, a bit less work to terraform XD (Just a bit ^_~) It wouldn’t take huge amounts of specialised equipment to walk on Mars, no more than it would to walk on the moon. Venus is an order of magnitude or six away in difficulty – the heat, the pressure, the sulphuric acid… yeah. Mars is a lot more likely to support a human population.
    (Although, have you ever heard of the Venus cloud city idea?)
    Still! Venus is extremely cool. …Well, no, it’s hot, but you get the idea XD
    ETA: Here’s some on the aerostat habitat. I actually used this as the home of one of my characters in NaNoWriMo 2006 XD

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