Many people think that the idea of extraterrestrial life in our system begins and ends with a search on Mars. While there is still some debate whether microbial life currently exists on the planet or that even greater forms of life resembling mammals could have lived once long ago in the planet's past, Mars is not the only good source for life, at least as how human beings understand it, within the solar system. I would say it's not even the top candidate.
Until recently, astronomers thought the only source of heat for a planet, satellite or asteroid was the Sun. So if a rocky body was too far away from the Sun, for instance, beyond the orbit of Mars, it was thought to be too cold to have liquid water or harbour life.
There turns out to be at least one other source of heat for a rocky body in our solar system and that is friction caused from gravitational force. As a satellite orbits its planet, it has to deal with varying degrees of gravitational pull both from its planet and neighbouring satellites. If the satellite has an eccentric orbit, meaning it comes closer then goes further away from if planet as it orbits, the friction is even more pronounced. Think of this effect kind of like a tidal effect seen on Earth where ocean water shifts from one side of the planet to the other. Except instead of on the surface, this movement happens internally inside the satellite.
What makes this effect interesting is that it can warm up the internal temperature of an icy satellite just enough so that there is actually a deep ocean of water below the icy surface.
Jupiter's satellite Europa is considered to be the top candidate for a large ocean that could potentially harbour life below its surface. It's smaller than its neighbouring moons but it's big enough that there is suspicion it could have a bigger ocean underneath its icy surface than the Earth has on its surface.
Jupiter's moon Ganymede also may have an ocean beneath its surface but due to its larger size and weaker frictional pull than Europa, this ocean may only be partially thawed out. Saturn's moon Enceladus also has the possibility of water beneath its surface but its size is much smaller so it is expected that much of the water freezes for substantial periods of time.
Each moon's icy surface is important because all of them lack an atmosphere. Much like how the Earth's atmosphere protects the planet from the Sun's radiation, the ice acts as a shield from the radiation as well as the cold temperatures from space.
There is a good chance that microbial life exists near hot springs on one or more of these satellites. I would not be surprised if we see marine life similar to our own on Europa once NASA and/or the ESA sends a probe to dig beneath the satellite's surface.
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