Icy-moon oceans and ice-shell interiors

5 problems

• What supplies the heat that keeps Enceladus's ocean liquid?

  Open

  Enceladus is a small moon of Saturn that sprays water from cracks near its south pole and loses far more heat than a body its size should. Tidal flexing by Saturn is the likely source, but where inside the moon it is turned into heat, and whether the ocean is in balance, is unclear.

• How thick is Europa's ice shell and does it exchange material?

  Open

  Europa, a moon of Jupiter, has an ocean under a shell of ice whose thickness estimates range from a few to tens of kilometers. Whether surface material can reach the ocean, carrying chemical energy sources for possible life, depends on that thickness and on how the ice moves.

• How do ice-covered oceans transport heat and salt to their shells?

  Open

  In icy moons the ocean is heated from below and capped by ice, while the moon's spin deflects the flow. How the currents carry heat and salt to the ice determines where the shell thins and how the ocean's chemistry reaches the surface.

• How young is Mimas's hidden ocean, and how did it form?

  Open

  Mimas, a small cratered moon of Saturn, wobbles slightly as it spins, and its orbit slowly turns, in ways that point to an ocean under 20 to 30 km of ice, though its surface shows no sign of activity. The ocean must therefore be very young, and why it formed so recently is unknown.

• Does Titan have a global liquid ocean or a slushy interior?

  Contested

  Cassini measured how much Titan, Saturn's largest moon, is squeezed by Saturn's tides, and the large response was first read as a sign of a hidden global ocean. A 2025 reanalysis argues instead for a slushy layer of ice with pockets of water.