Saturn’s family portrait
The sixth planet from the Sun has a distinctive ring system and at least 82 moons
1 Titan
The giant of the Saturn family, it is bigger than our own Moon and a little bigger than the planet Mercury, having a diameter of 5,149.46 kilometres (3,200 miles). It takes roughly 16 days to orbit Saturn and permanently presents one side towards the planet.
There is the exciting possibility that primitive life forms might exist in the liquid water ocean that lies beneath the moon’s surface. Unlike any other moon in the Solar System it has a substantial atmosphere that is mostly composed of nitrogen and methane.
2 Rhea
The second-largest moon. The surface is mainly composed of water ice and it has an ice mantle. Its chemical composition and evolutionary history are very like that of Dione. They both have ice cliffs that are caused mainly by tectonic strains that fractured the moons’ surfaces. The side of Rhea that always faces away from Saturn has two large impact craters: the 500-kilometre (311-mile) diameter Mamaldi basin and the 360-kilometre (224-mile) diameter Tirawa basin. The impact scar of Tirawa overlaps Mamaldi, indicating that it is geologically younger. The unusual far-ultraviolet radiation from Rhea detected by Cassini, centred near 184 nanometres in the electromagnetic spectrum, was probably caused by hydrazine. How or where it comes from still remains a mystery.
3 Dione
Although it’s smaller than Rhea, it has a higher mass density, which is 1.48 times that of liquid water, indicating it has a silicate rock core surrounded by ice. It might even have a liquid salt water ocean beneath its surface. An outstanding feature of the moon is a bright pattern of icy cliffs that were seen as long, wispy streaks in the images from the Voyager probes.
It has a landscape of craters, tectonic fractures and a tenuous exosphere. Some of the craters are 100 kilometres (62 miles) across, and there is a variety of heavily and lightly cratered plains.
4 Tethys
It is an irregular ball of ice with a diameter of 1,066 kilometres (662 miles). It has a high level of reflectivity, indicating it is mainly composed of water ice. It is similar to Dione and Rhea, except it is less cratered. The crater floors reflect a lot of light, suggesting the presence of water ice, and further reflectivity is caused by water-ice particles from Saturn’s E-ring that erupt from geysers on Enceladus and end up showering Tethys. It has two noteworthy features: the Ithaca Chasma, a 1,930-kilometre (1,200-mile) long crack along its surface that covers 75 per cent of its circumference, and the 445-kilometre (276-mile) Odysseus crater that dominates the western hemisphere. It’s possible that its impact helped create the Ithaca Chasma.