ScienceDaily
Subsurface ocean
Liquid water under an ice shell, kept warm by tides rather than by sunlight.
Several outer moons hold more liquid water than Earth does, buried under kilometres of ice and heated by the flexing their parent planets impose on them. Europa, Enceladus, Ganymede, Titan and probably Callisto all qualify on current evidence — magnetic induction signatures, plume chemistry, and surfaces that have obviously moved. They are the most promising places to look for life beyond Earth precisely because they have the two things that matter: liquid water and a persistent energy source that has nothing to do with the Sun.
Where it has come up lately
Related terms
Gravity assist
Stealing a fraction of a planet's orbital momentum to go faster for free.
Regolith
The layer of broken rock and dust covering an airless world — and a serious engineering problem.
Sample return
Bringing a piece of somewhere else back to a laboratory on Earth.
Opposition and conjunction
The two alignments that decide when a planet is worth looking at and when it is unreachable.
Questions
What is Subsurface ocean?
Liquid water under an ice shell, kept warm by tides rather than by sunlight.
Why does it matter?
Europa, Enceladus, Ganymede, Titan and probably Callisto all qualify on current evidence — magnetic induction signatures, plume chemistry, and surfaces that have obviously moved. They are the most promising places to look for life beyond Earth precisely because they have the two things that matter: liquid water and a persistent energy source that has nothing to do with the Sun.