Of the 62 known moons of Saturn, Dione is only the fourth largest and yet one of the most intriguing. It was discovered by Giovanni Cassini in 1684, after whom the Cassini spacecraft that is currently in orbit around the Saturnian system was named.
Dione is largely made of rock and ice, with its surface strewn with impact craters ranging in size from just a few hundred metres across to others over 100 kilometres (62 miles) wide.
It is tidally locked to Saturn, so the same face always points towards the gas giant, though most of its craters are found on its trailing hemisphere relative to its orbital path. This has led scientists to speculate that past impact events have spun the moon 180 degrees. Dione is also bombarded with dust from Saturn’s rings, creating a fine icy powder.
Dione was first visited in 1980 by Voyager 1, which returned unusual images of its surface. Across its trailing hemisphere are wispy features that were at first thought to be remnants of ice volcanism. Observations by the Cassini spacecraft in 2004, however, revealed these features to be ice cliffs formed from tectonic activity. Some of these cliffs rise hundreds of metres high. The walls of the cliffs are bright due to water-ice being exposed.
While it may have once been geologically active, Dione bears little sign of any noticeable atmosphere save for a thin layer of molecular oxygen ions. This is referred to as an exosphere rather than a proper atmosphere. Regardless, it is the icy cliffs and cracks of this barren body that are of most interest, revealing some key information about how moons form.
Cassini – The Cassini spacecraft caught view of Dione on Christmas Eve in 2005 from a distance of 151,000km (94,000mi).
Chasms – Long chasms, some with icy walls stretching hundreds of metres high, scar the surface.
Rotation – An impact causing a crater just 35km (22mi) wide might have been enough to spin Dione.
Flat – Aside from chasms and craters Dione is relatively flat, suggesting its icy crust is quite weak.
Craters – Dione’s leading hemisphere is bombarded more regularly than the trailing hemisphere, forming craters.