Callisto

Callisto is the second-largest moon of Jupiter and the third-largest moon in the Solar System. It's known for its heavily cratered surface, making it one of the most heavily cratered objects in the Solar System.  

Key features of Callisto:

• Heavily Cratered Surface: Callisto's surface is ancient and heavily cratered, indicating a lack of significant geological activity.  

• Icy Crust: It's primarily composed of water ice, with a rocky core.

• Subsurface Ocean: Scientists believe that Callisto may have a subsurface ocean, making it a potential target for astrobiological research.  

• Weak Magnetic Field: It has a weak magnetic field, which suggests the presence of a salty, liquid layer beneath its icy crust.  

Despite its ancient and heavily cratered surface, Callisto's subsurface ocean makes it a fascinating object for future exploration.