In the past twenty years it has become increasingly clear that Jupiter’s moon Europa is one of the most interesting destinations in the solar system, probably more interesting than the dry, airless deserts of Mars.
While very cold (basically, about 10 degrees K) and bathed in frightful radiation and gravitational tides from Jupiter, Europa appears to have a liquid ocean! This ocean is under 20 km of ice, and is kept liquid by the aforementioned tidal forces.
A recent paper by K. P. Hand and R. W. Carlson of JPL suggests that the ocean is salty water  . As the NASA web site indicates, energy+water+salt = life, at least on Earth.
We certainly have to go find out.
Someday, we will want to try to land on Europa, and visit the ocean. This will be one of the coolest space missions ever: it will have to land on the surface, drill through miles of granite hard ice, and then swim like a submarine in the ocean. Whatever amazing stuff is there will be down in the dark water, but the probe will surely be out of contact with Earth, so it will need to be really smart. And somehow, the probe needs to send home data….
How are we going to do all that? I have no idea!
The first mission is planned to be a close up, long term reconnaissance from space. But the radiation is way to strong for an orbiter to survive for any length of time. So NASA plans a series of flybys, swooping in again and again to record Europa from many angles and distances, and perhaps sample erupting gases.
- K. P. Hand and R. W. Carlson, Europa’s surface color suggests an ocean rich with sodium chloride. Geophysical Research Letters, 42 (9):3174-3178, 2015. http://dx.doi.org/10.1002/2015GL063559