New attempts to crack Saturn's 'walnut' moon

The mysterious equatorial ridge on Saturn's moon Iapetus is either a fossil ring system that fell to the surface, or a pile up of crustal rocks formed as the satellite changed its shape. The ridge, revealed by the Cassini probe, is unlike anything else in the solar system. It is up to 20 kilometres high and stretches 1300 km along the moon's equator, resembling the ridge on a walnut.