The largest of all the moons in the solar system is actually larger than the planet Mercury. The moon also shows evidence to tectonic plates (the same process that causes earthquakes on Earth) and there may even be some underground ocean as on Europa though it is less likely to be nearly as extensive. Recent observations from the Hubble Space Telescope suggest that there may be a tenuously thin atmosphere of pure oxygen at Ganymede due to the radiating of water ice into oxygen and hydrogen. In this scenario the hydrogen is lost to space while the oxygen (being a heavier element) is retained at the surface.