Moons May Yield Clues to What Makes Planets Habitable for Life

NASA spacecraft snaps gorgeous new photo of Jupiter's moons Io and Europa

Ocean worlds in disguise: Could there be a secret ocean on the moon Mimas? Mimas is the smallest of Saturn’s regular moons. If Mimas does have an ocean, it could signal the existence of a whole new group of “stealth” ocean worlds, which give no signs on the surface of underground water.

Are there any moons that are made of gas?