Astronomy Picture of the Day

Explanation: What does the surface of Saturn's mysterious moon Iapetus look like? To help find out, the robotic Cassini spacecraft now orbiting Saturn was sent soaring last week just 2,000 kilometres from the unique equatorial ridge of the unusual walnut-shaped two-toned moon. The above image from Cassini is from about 4,000 kilometres out and allows objects under 100-metres across to be resolved. Cassini found an ancient and battered landscape of craters, sloping hills, and mountains as high as 10 kilometres and so rival the 8.8-kilometre height of Mt. Everest on Earth. Just above the centre of this image is a small bright patch where an impacting rock might have uncovered deep clean water ice. Space scientists will be studying flyby images like this for clues to the origin of Iapetus' unusual shape and colouring with particular emphasis because no more close flybys of the enigmatic world are planned.