NASA's picture of the Day, September 1: Jupiter’s Moon Callisto, a dirty iceball, may hide ocean beneath its surface

Callisto: Dirty Battered Iceball (Image: Kevin M. Gill/NASA)

Gaze up at the night sky and picture a world bigger than Mercury but lost well out of sight. Jupiter's moon Callisto, cratered and icy, could conceal mysteries beneath its battered surface.

How is Callisto's surface special?

Callisto's surface is the Solar System's most heavily cratered. Voyager 2 took a dramatic photo of this cratered world in 1979. The image has now been reprocessed, offering a clearer look. Impacts over billions of years fractured the icy crust. Light-coloured ridges across the dark surface reveal layers of ancient ice.

Beneath its surface could lie a hidden liquid ocean. Scientists believe this underground sea may harbour conditions for life. Such a possibility links Callisto with sister moons Europa and Ganymede. Each of these worlds shows signs of water beneath their shells. Although Callisto is larger than Earth’s Moon, it is less massive. Its high ice content explains the lower density compared to Luna.

Which missions will explore Callisto further?
Callisto was last closely studied by NASA’s Galileo spacecraft. That mission visited Jupiter’s system in the 1990s and 2000s. Now, two new spacecraft are headed back to Jupiter. ESA’s JUICE and NASA’s Europa Clipper will explore the planet’s largest moons. Their instruments will study whether oceans like Callisto’s truly exist.