Europa and Ganymede
These two moons of Jupiter are believed to have oceans deep below their surfaces. The European Space Agency has approved a Jupiter Icy moons Explorer, or Juice, to visit these strange worlds. However, the probe will not reach its targets until 2033 or later.
The largest moon of Saturn possesses a thick atmosphere and is also thought to have a large underground ocean as well as complex organic chemicals on its surface. Several probes have been proposed to land on Titan and explore it, but none have been approved so far.
A relative minor moon of Saturn, it has been found — by the US probe Cassini — to be venting water into outer space from an underground reservoir. It has also complex organic chemicals on its surface. A mission to take samples and return them to Earth around 2030 is being studied by US scientists.