Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:
THALASSOGEN
I stumbled upon this term while researching my recent Molecular Monday post on ammonia. The word thalassogen comes from the Greek words for “sea” and “creation,” and it was coined by one of the great luminaries of both science and science fiction: Isaac Asimov.
Basically, a thalassogen is a chemical substance that could, under realistic circumstances, form an ocean on a planet or moon. Obviously water qualifies. Just look at Earth. But what other substances could we call thalassogenic?
First, we need something that can be liquid and is capable of remaining in a liquid state across a reasonable wide range of temperatures and pressures.
We also need a chemical that is reasonably plentiful in the universe. According to Asimov, that rules out something like mercury. Mercury does a great job being a liquid, but it’s so rare that we can’t realistically expect to find a world covered in mercury oceans.
Asimov also wrote that “ideally” a thalassogen should be able to transition from liquid to both solid and gaseous states without too much difficulty. That way, we could have something analogous to Earth’s hydrocycle, with clouds and rain and snow and glaciers. Please note: that’s ideal, but not necessarily a requirement.
In my opinion, the most sensible way to use this term is to say that a substance is (or could be) a thalassogen in a specific environment. So methane is a thalassogen on Titan, but not Earth. You might also say water is a thalassogen on Earth but not on Venus. Or water is a thalassogen beneath the surface of Europa, but not on Europa’s surface.
So as we venture out into space, what sorts of chemicals might we find acting as thalassogens on alien worlds? Asimov suggested water, ammonia, and methane as the most likely candidates. Other possibilities include carbon dioxide, sulfur dioxide, and sulfuric acid. We should also consider mixtures of these and other chemicals.
And who knows? Given some of the strange, improbable-seeming exoplanets we’ve discovered so far, maybe Asimov was a little too quick to rule out mercury.
Planet Pailly 
It’s interesting that for most of these alternatives, their liquid range is lower and much narrower than water’s. The narrower part seems like it would constrain the conditions and time periods (geologically speaking) where oceans of it might exist.
The exception is sulfuric acid, whose range is higher and wider than water’s. If I recall correctly, there is sulfuric acid rain on Venus, and sulfuric acid vapor in the atmosphere. Although according to its Wikipedia page, sulfuric acid has a tendency to react with and dilute itself in water, which may limit its prevalence past a certain distance from a star.
On mercury, given that they think there may be liquid iron rain in some environments, it does seem premature to rule it out.
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Yeah, I get the sense that oceans of liquid water, or possibly liquid water mixed with other things, would be the most common kind of oceans we’d find. Still, other possibilities exist. Especially if we increase atmospheric pressure, which can expand the liquid ranges of most of these chemicals.
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Good point on pressure, which I imagine will indeed be higher on super-Earths. That didn’t occur to me.
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I forget about air pressure a lot too. It’s tough getting out of the Earth-normal mindset when you’re trying to picture environments on other planets.
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I still think it’s fair to rule out Mercury. Since Iron is where stars stop producing elements outside of going supernova, heavy metals like Mercury are pretty rare. In the unlikely event that enough Mercury is gathered to potentially form an ocean, the ratio of heavy elements to light elements is still going to be preserved, and you’re going to have a lot more hydrogen, carbon, silicon, and the likes. And if a planet forms, the heavier elements are going to sink and form the core, while lighter elements will form the upper mantle and crust. That’s why our core is iron-nickle and our crust is silicon, and carbon planets, though vastly different from what we know, are also so common in the galaxy. Still, observations have continued to prove theory wrong. I am curious about this liquid iron rain. I know brown dwarfs have some pretty interesting iron-based “surfaces,” but I’m not too familiar with any details.
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Oh I totally get why Asimov wanted to rule that one out. It would take some pretty weird, wacky circumstances to create an ocean of liquid mercury. However, since Asimov’s time we’ve discovered at least a few weird, wacky exoplanets. So I think the possibility exists, even if it’s a very remote possibility.
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My tricorder readings appear to indicate M-class planets in this system. Short range analysis reveals a planet 18 minutes away with nitrogen oxygen atmosphere suitable for further investigation. I’m not sure about this mysterious sea of thalassogen but its molten lava substance is sustaining an extremely unearthly lifeform. I don’t think we should disturb it but it is possible?
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I think I have heard of exoplanets with molten rock on the surface. I hadn’t really thought of that, but maybe that counts as an ocean?
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