What is #36: Ganymede?

Today is the 36th edition of this blog's What Is series, a series that takes a deeper look at individual objects or concepts in the field of astronomy. The last three editions in this series took a look at three of the four Galilean moons orbiting Jupiter: Europa, Io, and Callisto. That leaves Ganymede left.

National Oceanic and Atmospheric Administration [Public domain]

Ganymede is the largest of the four Galilean moons and the largest moon in the Solar System. Ganymede is larger than both Mercury and Pluto. Pluto is not classified as a planet (it's a dwarf planet), but Mercury is, so Ganymede is larger than one of the official planets in the Solar System. If Ganymede orbited the Sun instead of Jupiter, it would be called a planet!

Ganymede is similar to Callisto in composition. It's a rocky moon with several impact craters, as seen in the image above, but also containing much ice below the surface. The bright regions in the image are craters that shows ice underneath the top layer of the surface. Given the quantities of water ice, it might be an option for human habitation centuries down the road.

That does it for the Galilean moons. Jupiter has 60+ other moons, but don't worry, I'm not going to do an edition in this series in each of them. Well, if I run out of ideas I could, but astronomy is so fascinating that I can't imagine running out of ideas!