Ganymede

Ganymede is the Solar System’s largest moon. It is a differentiated world with a metallic core and a deep global ocean buried beneath a thick ice shell, likely layered with high-pressure ices. Its surface is a patchwork of ancient, dark, heavily cratered plains and younger, brighter “grooved” terrains created by widespread extensional tectonics (and possibly minor cryovolcanism), forming long ridges, furrows, and fault-bounded blocks.

Depiction of Ganymede centered over 45° W. longitude; dark areas are Perrine (upper) and Nicholson (lower) regions; prominent craters are Tros (upper right) and Cisti (lower left) - wikimedia

Multi-ring impact structures and subdued crater rims hint at a mechanically warm lithosphere in the past, while polar frost caps and sputtering darkenings mark ongoing space-weathering. If the ocean directly contacts a rocky seafloor, hydrothermal chemistry could be possible; if it’s insulated by high-pressure ice, habitability would be more limited — one of the key questions missions like JUICE aim to resolve.

image/svg+xml Ganymede Iron & iron sulfide core (liquid) Iron core (solid) Rocky mantle Tetragonal ice (VI) Saltwater ocean Hexagonal ice (1h) Polar frost Dark terrain Light terrain Grooves Crater layers drawn to scale

Artist's cut-away representation of the internal structure of Ganymede. Layers drawn to scale. - wikimedia

# Habitability There’s currently no direct evidence of life on Ganymede. What we *do* have are habitability clues: Hubble measurements of Ganymede’s auroral “rocking” imply a global, salty subsurface ocean that can conduct electricity; later Hubble work detected tenuous water vapor from surface ice sublimation; and the surface shows radiolysis products like molecular oxygen/ozone created by particle bombardment—not biology.

Whether the ocean contacts rock (key for energy/chemistry) is still unknown; ESA’s JUICE will probe that habitability question in detail during its 2031–2035 tour.

Ganymede Global Geologic Map and Global Image Mosaic, assembled from the best available imagery from NASA Voyager 1 and 2, and Galileo spacecraft. wikimedia

# Sources - The search for a subsurface ocean in Ganymede with Hubble - wiley.com - Hubble Finds First Evidence of Water Vapor - nasa - Hubble Finds Evidence of Persistent Water Vapour Atmosphere - esahubble.org - Detection of ozone on Ganymede - nih.gov - Science Objectives - esa.int