Callisto is one of the largest moons orbiting Jupiter. It was discovered in 1610 by the Italian astronomer Galileo Galilei, making it one of the four Galilean moons, along with Io, Europa, and Ganymede.
Callisto's surface is one of the oldest and most heavily cratered in the Solar System, with no signs of significant tectonic activity or resurfacing. The moon's appearance is dominated by multi-ring impact structures, with the largest being Valhalla, a basin about 3,000 kilometers in diameter. The lack of geological activity suggests that Callisto has not experienced the intense tidal heating that affects other Jovian moons like Io and Europa.
Studies suggest that beneath Callisto's icy crust, there might be a salty ocean, possibly extending to depths of 100 to 200 kilometers. This ocean could be encased in layers of ice and might contain more water than all of Earth's oceans combined. However, unlike Europa, there is no direct evidence of a liquid water ocean on Callisto; it remains a hypothesis based on its density and the presence of magnetic fields.
Callisto has a very thin atmosphere composed mainly of carbon dioxide, with traces of oxygen, molecular hydrogen, and methane. This atmosphere is primarily sourced from the sublimation of surface ice due to solar radiation.
Callisto's interest stems from its potential for harboring life in its deep subsurface ocean. Its relatively benign radiation environment compared to other Jovian moons makes it a potential target for future human exploration or settlement.