Geology and Surface Properties of the Galilean Moons

Abstract Over five decades of space exploration have revealed that the Galilean moons—Io, Europa, Ganymede, and Callisto—exhibit a wide spectrum of geological and surface features shaped by the interplay of endogenous and exogenous processes. Each moon displays distinct characteristics: Callisto’s ancient, heavily cratered terrain; Ganymede’s contrasting dark and bright regions; Europa’s extensive fracture networks; and Io’s intense volcanic activity. Their surfaces are primarily composed of water ice mixed with salts, volatiles, and organic compounds, with the exception of Io, and reflect gradients resulting from complex interactions between impact processes, resurfacing mechanisms, and radiation exposure. Surface composition offers valuable clues about potential habitability of subsurface oceans, particularly on Europa, which shows evidence of recent geological activity, liquid water-rock interactions and energy sources. This paper examines how forthcoming data from the Juice and Europa Clipper missions will significantly advance our geological understanding of the moons’ surface environments and their links to the subsurface. By providing high-resolution data and long-term observations from orbit, these missions will help confirm the distribution of subsurface liquid water reservoirs, identify key chemical species—including organics—across surfaces of varying ages, and pinpoint geologically interesting, potentially habitable sites. This information, in addition to laboratory studies or field work, among other efforts will be crucial for designing future in situ exploration to one or more moons or sample return missions, enabling a deeper investigation into the origin and evolution of the Jovian system and the search for signs of life.