Mean diameter: approximately 4,821 km · Orbits Jupiter.
Callisto is Jupiter’s second-largest moon and the third-largest overall. Its dark surface is densely marked by impact craters, making the Galileo photograph a striking record of collisions.
Galileo data also suggest a possible salty ocean below the surface. Keep the distinction clear: a suspected buried ocean is not a visible sea, and Callisto’s inclusion here rests on its measured size.
Sources: NASA moon guide · JPL size data.
Image source: NASA/JPL/DLR; reuse terms. Resized and JPEG-compressed.