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:
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.