Well, it now looks like Ganymede, one of Jupiter’s moons, has a subsurface ocean.
Ganymede’s great distinction among moons – apart from its size – is that it has its own magnetic field.
Hubble has managed to track that field’s behaviour by watching how it draws in and excites space particles, generating a glow of ultraviolet light around the satellite’s north and south poles.
But this intrinsic magnetic field also interweaves with Jupiter’s, and the aurora “rock” back and forth as a result of the interplay.
It is by modelling the expected rocking against what is observed by Hubble that scientists can infer something about the internal structure of the moon. And they now say a salty ocean at depth is the best explanation for what they see.
It seems like these subsurface oceans are all over the place in the outer solar system.
Ganymede is just one of a large list of objects in the Solar System now thought to hide an ocean deep below the surface. These include the dwarf planets Pluto and Ceres; other Jupiter moons – Europa and Calisto; Saturn’s moons Enceladus, Titan and Mimas; and possibly Neptune’s moon, Triton.
“The Solar System is now looking like a pretty soggy place,” joked Jim Green, the US space agency’s director of planetary science.
In some ways, this shouldn’t be too surprising. In the outer solar system, where temperatures are typically far below -100 °C, ice is basically a type of rock, and water is essentially a molten form of that ice rock, in the same way as magma is to the rock we’re familiar with.
But all of these subsurface oceans give us lots of environments that have the potential to host life similar to how we understand it, in that they have liquid water coupled with an energy source (whatever is keeping the water warm enough to be a liquid). One mystery of the origin of life is how likely that origin is to occur. If it’s more likely, then there’s more life in universe.
If, despite lots of places where conditions are favorable for it, we find don’t find life anywhere else in the solar system, then that may indicate that life is fairly rare in the universe. On the other hand, if we do find it, and we can establish that it’s not somehow related to life on Earth, then it seems like it indicate that life is pervasive throughout the universe.
How could life on Ganymede be related to life on Earth? It seems farfetched, but in an ancient solar system with asteroid strikes flinging debris into space, it’s very possible that microbes could be exchanged between planets. Martian rock has been detected in some of the meteorites that have crashed to Earth. That said, the outer solar system is much farther away, and any transfer between here and there seems much more tenuous.
None of this would have too much bearing on how prevalent intelligent life is in the universe, except perhaps to set a possible upper bound on it. Life existed on the Earth for billions of years before intelligence evolved, with intelligent tool using species being very rare. Even if life is pervasive in the universe, intelligent life is likely still exceedingly rare.