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Ahh, the seas of Europa. A major topic of debate in the astrobio community. As cool as it would be to imagine giant marine organisms drifting through the European depths, it's probably not the case. If life exists below the surface of Jupiter II, it will probably be fairly small -no more than a meter or two in size- and the reason for that is energy.
Europa, like all planets and moons, is an open energy system, receiving energy from outside the system in various ways. On Earth, the main form of energy input when it comes to the biosphere is sunlight, with some chemical processing taking over in places where the sunlight doesn't reach. The rest of Earth's energy input comes from tidal interaction with the other bodies of the solar system, especially the Moon, which manifests via friction as heat. That heat, as far as we know, is only exploited indirectly by organisms.
Europa, by contrast, gets MOST of its energy from tidal interaction with Jupiter and the other moons in the Jupiter sphere, transforming via friction into heat at its core and convecting up through the crust. Europa gets very little sunlight, and the light it does catch falls on an airless surface separated from the ocean by kilometers of hard ice. Europa actually gets more energy from particle radiation caught in Jupiter's magnetic field than it does from the sun. Overall, the Europan energy system likely has less energy than Earth's, and in different forms.
Europa's ocean is extremely deep, estimated at something like 100 kilometers from the underside of the ice to the rocky crust. While it appears to be rich in various complex life-friendly compounds like salts, tholins, and molecular oxygen, the fact of the matter is that almost all of the water column would be empty space. The likeliest forms of autotroph in Europa would be chemosynths (gaining energy from chemical reactions) and thermosynths (gaining energy from heat gradients). Both of these would probably have to stick to points on the ice subsurface and ocean floor where energy is abundant, like slush zones and geothermal vents, in order to survive. That all said, energy would be hard to distribute, and organisms in environments like that tend to be on the small side. Let's dive into speculation for a bit.
In terms of stratification, the Europan ocean has essentially 3 zones -4 if you count the ice itself.
Ice crust (~3 km) – The ice crust itself is sort of a gradient of solidity from soft almost slushy ice to harder than granite on the surface. If there are any organisms reliant on sunlight or radiation for energy, they’ll exploit fissures that lead to the surface to stay sheltered while utilizing the water and weak sunlight. However, being exposed to the vacuum of space is extremely bad for most lifeforms we know of, so this is very unlikely. It seems like a solid bet that the ice crust will be lifeless.
Ice shelf pseudo-benthic zone (~2 km) – The underside of the ice could have a sort of pseudo-benthic zone: given no interference, debris will fall toward the ocean floor (very slowly due to the light gravity); however there is surface substrate to anchor to and a temperature gradient to be exploited. I imagine thermosynths and chemosyths could form meadows and groves hanging from the underside of the ice, leading to the existence of grazers and therefore a modest food web. The return of nutrients into the system would be a challenge, but not an insurmountable one.
Open sea abyssal zone (~96 km) – As mentioned before, the vast open abyssal zone is likely to be akin to a desert, similar to the abyssal zone on Earth. I imagine a thin scattering of small roaming detritivores, and larger pelagic predators that hunt them, and if the nutrient density is high enough there could also be a few species of large filter-feeders.
Seafloor benthic zone (~2 km) – The benthic zone will be similar to the deep benthic zone of Earth: sparsely populated by seafloor detritivores with thriving biodiverse hotspots (hah) around geothermal venting sites. It is also likely there will be modest-sized predators, but anything exceptionally large seems unlikely.
Life within the subsurface ocean of Europa would be very alien, especially in deeper waters. Although the extremely low gravity produces a much gentler pressure gradient, the sheer depth of the oceans would produce a seabed pressure of 2500 atmospheres –28 times the surface pressure of Venus and well over twice the pressure in the Challenger Deep within the Mariana Trench. Anything living in this strange, sunless sea would rely exclusively on senses other than sight: sound, smell, electrical, or even infrared thermal sense. They will be bizarre, secretive lifeforms, unlike anything Earth has seen for hundreds of millions of years; sea monsters in their own right... but alas, probably no bigger than a seal.
It wrinkles my brain that Jupiter’s moon Europa has oceans that are sixty miles deep, while Earth’s oceans only reach seven miles deep at most. I’m willing to bet good money that there’s life in Europa’s oceans. Like five bucks. You hear me, NASA? I bet you five bucks that there’s life on Europa… Now that there’s money and reputation on the line, I bet they send a mission there real quick.