There has been a recent flurry of articles postulating the existence of life in warm oceans that exist under the the frozen exteriors of moons in our Solar System and possible worlds and moons of far away star systems.
It is exciting to think about possible life in a ocean under a mile or two of ice on some far away moon or planet. It is important to make a distinction between moons and planets because they are likely to have vastly different conditions that would effect the likelihood of sustainable life. A frozen moon with a subsurface ocean in likely to have little if any atmosphere above its frozen surface. The gas giants are known to shoot off a considerable amount of radiation so a moon like Jupiter’s Europa would be exposed to that radiation. But we know that ice provides a good layer of protection from cosmic radiation so that should help with protect any life that may be there. Radiation is just one example of the major differences between moons and planets that might have liquid oceans. If we are talking about an exoplanet around some other star system, then it is likely that a ocean world would likely have atmospheric layers of protection similar to those offered by Earth. This and other factors are why, when discussing this subject, you must make a very clear distinction between moons with oceans below a frozen layer of surface ice that is otherwise exposed to space and watery worlds like Earth but with much less if any land masses.
Sub-Surface Oceans on Moons
So how would intelligent life evolve underwater? We have only our own planet as an example (so far), but that does give us some information with which to extrapolate. Notice that the aquatic species most associated with having anything beyond basic animal intelligence are aquatic mammals like Cetacean dolphins and whales. There are rare examples such as certain types of cephalopods like octopus that show higher level problem solving capabilities but for the most part, non-mammal aquatic species from Earth that didn’t have at least part of their evolution on land are not known to have developed much intelligence. So using the limited amount of data provided by our only example (Earth), it doesn’t seem likely that intelligent life will evolve in the subsurface oceans of a moon.
It may be that subsurface oceans on moons have no gap in between the interior of the icy exterior and the warm water below. But it seems likely with gravity rotation and other factors that there is in fact a significant amount of open space between the water and the interior of the crust. These open areas under the crust of the exterior ice might be relatively small or they could potentially be huge (rotation causing continuous waves slapping up against the interior. They would be an excellent spot to build a base for exploration or maybe even colonization. Note that if we do not find existing life in the sub-surface oceans in our Solar System, they would make excellent homes for fish farms that could feed the Belt and Outer Planets.
Ocean Worlds Around Distant Stars
Again using the data available to use from our own planet, there is at least the possibility of life, and even intelligent life, developing on watery planets in other star systems. Since this article focuses on ocean worlds that have limited or no land masses, that raises the question about how intelligent life could develop on such a world. It seems that such conditions would not be conducive to the development of higher levels of intelligence. Such an environment would obviously limit the development of such basic things like the use of fire and tool creation let alone the development of more advanced scientific areas such as electronics, propulsion, etc. So even if an intelligent species did evolve on a water world, it is difficult to see how they would ever be able to develop transmitted communications, space travel, etc.
We don’t know what factors are optimal for the evolution of a species towards a level of intelligence beyond basic animal levels. There are many that come to mind that could influence a species development towards intelligence. Planet age, gravity, atmospheric challenges, proximity to radiation from space, placement on the local food chain, inter-species competition for limited food supplies, physical development of tool-capable appendages and so many more. With the many challenges that any species would find in developing intelligence, it seems that adding the additional challenges of evolving on a water planet or the sub-surface ocean of a frozen moon would make it that much less likely that we will find intelligent life on either.