Discovered in 1610, Ganymede has a diameter of 5,268 km, around 8 percent larger than that of Mercury and much larger than Pluto. The history of this largest moon in our Solar System can be divided into three phases: an early phase dominated by impact cratering and mixing of non-ice materials in the icy crust; a phase marked by great tectonic upheaval; and a late phase characterized by a gradual drop in heat flow and further impact cratering. The surface of Ganymede has been formed through a variety of processes and is a mix of two types of material: very old, highly cratered dark regions, and younger lighter regions.
“The dark/light distinction is based on sharp relative albedo contrasts at terrain boundaries, rather than on absolute albedo, because several other types of surface modification change the absolute albedo within these terrain classes. Dark materials cover 35 percent of Ganymede’s surface; almost the entire remainder of the surface is covered by light materials,” the researchers said in the description of the new map.
“Dark materials are heavily cratered, though not as heavily cratered as the surface of the neighboring satellite Callisto, suggesting that dark materials cannot be a primordial surface. At high resolution, dark materials are dominated by the downslope movement of loose dark regolith within impact craters and on the sides of bright ridges and hummocks. Observations suggest that dark materials are covered by a thin lag deposit of dark regolith derived by sublimation of a more ice-rich crust below. Dark materials commonly exhibit sets of concentric arcuate structures known as furrows. Furrows may be the remnants of ancient multi-ring impact basins, similar to intact impact basins on Callisto, such as Valhalla and Asgard.”