Ganymede is the largest moon in our solar system. In fact, it's bigger than the planet Mercury and the dwarf planet Pluto, and it is almost three-quarters the size of Mars. NASA explains that if Ganymede orbited the sun instead of Jupiter, it would be a planet. This color-enhanced view from NASA shows the "frosty polar caps in addition to the two predominant terrains on Ganymede, bright, grooved terrain and older, dark furrowed areas." There is evidence of a thin oxygen atmosphere on Ganymede, but experts believe it is too thin to support life.