The masses of Jupiter and Saturn are more poorly known. Jupiter and Saturn have bright icy rather than rocky rings. Jupiter and Saturn have no methane in their atmospheres. Jupiter and Saturn have metallic hydrogen zones. Among the terrestrial planets, the Earth is the largest and has the most internal heat, while among the Galilean moons of Jupiter, Io is the smallest of the four and yet has the hottest interior.
Smaller objects cool off more slowly than larger objects.
It is dark and absorbs more sunlight than the other Galilean moons. It is heated by tidal stresses from Jupiter. It has vast amounts of radioactive materials inside of it. Mercury is heavily cratered, like the Moon, but with important differences. Craters on Mercury are shallower and flatter in appearance.