Surface of Ganymede
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Ganymede is the third closest Galilean Moon, and the largest of all Jupiter's moons. In fact, it is actually larger than Mercury and our Moon, and only three quarters the size of Mars. It has a density of only 1.9 g/cm³, about two times the density of water, implying that Ganymede is composed mostly of water and ice, though it does contain some rocky material and a small iron core. The iron core is inferred from Ganymede's size. It is large enough that in the past, radioactivity in the interior made the iron and rock molten, allowing for the heavier iron to sink to the center of the moon. Its ice crust is about 500 km thick, but it does have a 5-km thick liquid water layer about 170 km below the surface. Despite it being larger than Europa, its ice crust is too thick to allow life to exist, so we believe.
Ganymede orbits at approximately 1.07 million km from Jupiter, giving it an orbital period of 7.15 days (twice that of Europa and four times that of Io). Its radius is 41.3% that of Earth, but its mass is only 0.025% of Earth, as Earth is mostly iron, nickel, and rocky material while Ganymede, as mentioned above, is mostly ice and water. Much like the Moon as it orbits the Earth, Ganymede is tidally locked to Jupiter. For that matter, so are Io, Europa, and Callisto. This means that for every orbit the Galilean moons make around Jupiter, each one rotates on its axis once. So much like the same face of the Moon is facing Earth, the same faces of Io, Europa, Ganymede, and Callisto are pointed to Jupiter.
Tidally locked moon orbiting a planet
Small line pointed to planet at all times
Ganymede has a surface that is not uniform. About a third of the surface is old, dark, and heavily cratered. We know that this makes the surface old as the early Solar System was heavily bombarded by asteroids and comets, though there are some impacts today, but not on the scale seen when the Sun was just being born. A way to look at this is to compare the surface of the Earth with that of the Moon. The Earth has a very active surface, from earthquakes, volcanoes, flowing water, and the weather, where the Moon is not active at all. A larger percentage of the surface of the Moon is cratered compared to the Earth. We know that we are not being impacted as much now as the Earth was in the beginning, as we do not see large meteors every day. However, Ganymede also has a large percentage of its surface that is much younger. Much like Io and Europa are affected by tidal forces from Jupiter and the other Galilean moons, Ganymede is as well. The tidal forces can cause fractures in the surface crust, allowing liquid water to come up to the surface and flow. This flowing water creates grooves on the surface, which flow through the craters. If the grooves formed before the craters, then the grooves would be broken, but we see the grooves as continuous.
"Old" surface vs. "New" surface
The left side of the image is the older surface of Ganymede, dark and cratered. The right side is younger, with less craters and grooves running along the surface
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