Surfaces, interiors and evolution of solar system moons

This Perspective reviews what we have learnt about the major moons of the solar system over the past 30 years. Major discoveries include subsurface oceans in at least five of these moons, geological activity on at least three (Io, Triton and Enceladus), and an active hydrological cycle (involving hydrocarbons) on Titan. Geological activity, either current or past, is in most cases driven by tidal heating; thus, the orbital evolution of these moons is intimately connected to their geological evolution in a way that is not true of planets in this solar system. The rate of evolution depends on tidal dissipation in the parent planet, which is very poorly understood but may in some cases be larger than previously thought. How the satellites formed can in principle be deduced based on observations such as the extent of volatile depletion or degree of differentiation, but unique answers are elusive. In the early 2030s, a flotilla of missions including Europa Clipper, JUICE, Dragonfly, Tianwen-4 and MMX will solve some of the existing major puzzles identified here, and introduce new ones.