A moon can sit next door to another and still feel like a different world. Around Jupiter, that contrast shows up fast. Io ...

While Io, the most volcanically active moon in the solar system, appears completely dry and devoid of water ice, its neighbor Europa is thought to harbor a vast global ocean of liquid water beneath ...

A comet from another star system will swing by Mars on Friday as a fleet of spacecraft trains its sights on the interstellar visitor. We already know a decent amount about how planets form, but moon ...