The Largest body not to get the planetary nod
Ganymede is the largest body in the solar system not to receive the distinction of being a planet ranking at number 9 in size for the solar system. Larger than even mercury, Ganymede is approximately 41.3% the radius of Earth yet strangely is comprised out of only 2.5% its mass. The ultra light moon lacks the dense core that our earth has meaning it has been geologically inert for quite some time now. This is the reason we see ganymede having such a densely cratered surface because there are no geological forces tearing at he surface and eroding their structure. Oddly enough the planet is believed to contain a liquid saltwater ocean over 200 km deep below the surface. The discovery of this ocean was announced in 2001 following the galileo space probe’s mission.
haydenwalling 
Ganymede is a pretty exciting moon for at least one other reason: it has an incredibly large magnetic field for it’s size and location near Jupiter. It’s so strong that it outweighs Jupiter’s magnetic field at its orbital distance. Pretty incredibly!!
You would think it would have retained a larger atmosphere considering the relative strength of its magnetic field combined with that of Jupiters. I read a little further and there is apparently O2 and O3 in the Ganymede atmosphere which is pretty atypical for such a light world.