Hello, friends!
All month long, we’ve been talking about Europa, the sixth moon of Jupiter. Scientists are 99% sure that there’s an ocean of liquid water beneath Europa’s icy crust, and speculation runs rampant about possible alien life swimming around in that subsurface ocean.
I’m currently reading a book called The Zoologist’s Guide to the Galaxy, by Arik Kershenbaum. The book takes the fairly uncontroversial stance that the same evolutionary processes that shaped life on Earth would shape life on other worlds (uncontroversial among the scientific community, at least). Specific details about biochemistry or genetics might be wildly different, but general principles like natural selection are likely universal.
Other science writers follow the same premise when imagining what we might find beneath the surface of Europa. The environment is presumed to be very similar to the deepest, darkest reaches of Earth’s oceans. Therefore, the same evolutionary pressures should apply, and Europa-life should have much in common with the deep ocean creatures we find here on Earth.
For example, Europa-life would probably cluster around hydrothermal vents, or similar geological hot spots, at the bottom of the ocean. It’s nice and warm there, and there are plenty of tasty nutrients billowing up from the rocky mantle.
Another example: abyssal gigantism, which is the tendency for organisms in the deep ocean to grow to enormous sizes (compared to their shallow water cousins). Scientists aren’t 100% sure why abyssal gigantism happens, but it may have something to do with metabolic efficiency. If life in Earth’s deep oceans needs to be gigantic for the sake of metabolic efficiency, then Europa-life would probably be gigantic too.
A lot of science writers also predict that bioluminescence will be common on Europa. It’s fairly common here on Earth, especially in the deepest, darkest regions of Earth’s oceans. And as you can see in this totally legit photo from the Mariana Trench, bioluminescence is really pretty.
But while predictions about abyssal giants and hydrothermal vents make a certain logical sense to me, I’m not convinced bioluminescence makes sense on Europa. As I understand it, life on Earth developed eyes first, and bioluminescence came later.
Having some sort of light-detecting organ makes sense on a world where there’s plentiful sunlight. There’s an obvious evolutionary advantage to having eyes here on Earth. And then, if some Earth-creatures decided to swim down to the bottom of the ocean, it makes sense for them to develop bioluminescence in order to help them see each other and the environment around them (or to help them lure in food).
But the ocean on Europa lies beneath a thick shell of ice. There’s no sunlight there. There has never been sunlight there. So what is the evolutionary advantage of having eyes? And if there’s no evolutionary advantage to having eyes, what would be the evolutionary advantage of bioluminescence?
Whenever Europa-life is depicted in science fiction, it’s almost always lit up in bold, bioluminescent colors. A lot of science communicators seem to envision Europa-life that way too. And why wouldn’t they? To see all those strange alien creatures waving their glow-tentacles around—that would be an awe-inspiring sight! But as awesome as it would be to see Europa-life in all its bioluminescent glory, I cannot think of a good reason why Europa-life would evolve that ability. Can you?
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I haven’t finished reading The Zoologist’s Guide to the Galaxy yet, but what I’ve read so far is good, thought-provoking stuff. If you’re interested in what alien life might really be like, scientifically speaking, then I’d say this book is worth a look.
Planet Pailly 
you certainly made a fine argument against bioluminescence, although now I’m curious what sort of sonar-using critters there may be on Europa.
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And that, my friend, is the very thing I want to put into a story someday: the moment somebody first puts a microphone in Europa’s ocean and hears something like whale songs.
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I agree on bioluminescence being unlikely. I also wonder about the depth of the ocean on Europa (40-100 miles according to Google). The pressures in the lower portions must be pretty intense, even with the low gravity. How likely is life to develop in either the deep high pressure regions, or the upper regions without a nearby seafloor? (I wonder the same thing about water world exoplanets with oceans hundreds of miles deep.)
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I hadn’t thought about that. The pressure at the bottom of Earth’s oceans can be pretty intense too, though, so I doubt that would be a dealbreaker for life on Europa. And I seem to remember that pressure might be one of the factors behind abyssal gigantism. Something about water pressure helping support the extra weight of bigger, bulkier animals, or something like that.
But there’s surely a limit. If we’re talking about those water world exoplanets, a lot of those places do not sound habitable to me at all.
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Good point. There’s life even at the bottom of the Mariana Trench, which is seven miles down and has over 1000 atmospheres of pressure. With Europa’s lower gravity (13%), the pressure might be low enough at its seafloor for life, at least in the shallower regions or scenarios.
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So I found a paper that says that the pressure at the bottom of Europa’s ocean (100 to 200 km deep) would be equivalent to the pressure at 13 to 26 km deep on Earth. You’re right, Europa’s lower gravity makes a big difference.
Still, the Mariana Trench is about 10 km deep at its deepest point. So the pressure at the bottom of Europa’s ocean is greater than it would be anywhere on Earth.
https://www.jstage.jst.go.jp/article/bss/12/2/12_2_126/_article
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All life is bio-illuminescent – even us it’s just that it’s very feint and over time we’ve evolved away from it as we don’t need it anymore. Any life down there will be very likely to glow
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If you mean glowing in infrared, yeah I guess that’s true. That’s still not quite the bright and glorious bioluminescence portrayed in Sci-Fi.
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No it’s actually in the visible list spectrum it’s just really, really faint
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I think I will read this book!
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The Zoologist’s Guide to the Galaxy? It’s a worthy read. And if you’re looking for inspiration for Sci-Fi and/or fantasy creatures, it’ll really get the creative juices flowing.
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Oo, that’s amazing!! I’ll let you know what I thought once I’m done reading 🙂
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Does anyone wonder how humans claim to be the most evolved organisms when some ocean animals have bioluminesence and plants use the sun to make their food?
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Fair points! There seems to be this idea that human life (or human-like intelligent life) is evolution’s end goal. But evolution doesn’t really have an end goal besides survival and producing offspring.
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It seems like animals and plants evolved at higher rate too…even higher than us…many animal and plant species, only one species of human.
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True, although there were other species of human in the past, like the neanderthals. But they all died out. We’re the only ones left.
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