Water is vital to life
on Earth and probably to life elsewhere.
This has led astronomers to become interested in oceans.
And to the discovery that oceans can be very different
to those beneath the open skies of Earth.
On three of the moons of Jupiter,
and two of the moons of Saturn,
there was good evidence of liquid oceans.
The first of these oceans to be discovered
was within Jupiter's moon Europa.
Though Europa is small,
it may boast more liquid water than the Earth.
The ocean could be over 100 kilometers deep.
Heat generated by Jupiter's powerful tides
keeps it warm underneath the icy shell.
Europe's Jupiter Icy Moons Explorer mission, or JUICE,
will launch in 2022 to study Jupiter
and three of its largest moons.
Perhaps one of the most promising oceans
for study is on Enceladus, a tiny moon of Saturn.
This is because the ocean does not stay under the ice cap.
Geysers at the moon's south pole shoot ocean
water hundreds of kilometers out into space.
Studies by NASA's Cassini probe showed that
the water in these jets contain organic molecules,
the building blocks of life.
There is also evidence of hydrothermal vents
at the bottom of the ocean.
This is significant because it is thought
that life first arose at hydrothermal vents on Earth.
A spacecraft flying through the jets of Enceladus
could sample the water for traces of complex
biological molecules such as DNA or protein.
There is no guarantee of life in the waters of Enceladus.
Or in any of the solar system's other hidden oceans.
But the possibility that something might be there
will drive ambitious explorations for decades to come.
Coming up, how to discover new planets.