Ganymede was the first natural satellite, apart from the Moon, to be explored using radar. Goldstein Morris observed the reflection from the satellite at a wavelength of 12.6 cm for six nights. The dispersion coefficient turned out to be 12%, i.e. it is at least as large as any terrestrial planet, at this wavelength the roughness of Ganymede is more significant than that of the rest of the terrestrial planets.
So, Ganymede is a planet-like satellite, mainly consisting of water (probably in a liquid state), containing a core of rocky substances and a crust of ice (frozen water). Its surface is covered with roughly equal patches of ice and rocky matter of unknown mineralogical composition, and probably has the appearance of loose regolith supported by meteorite impacts and evaporation of hoarfrost. Ice and rocky matter, most likely, are not mixed, but are distributed in separate spots, with a slightly higher concentration of ice in the forward hemisphere. It is possible that Ganymede has an atmosphere with a surface pressure between 10-3 and 1 mbar, in which NH3 and CH are non-primary gases. But the evidence for the existence of an atmosphere is not indisputable. In view of the low strength of the ice crust on Ganymede, one can hope to detect only very recently formed largescale relief elements.