The weird and wonderful inner moons of Saturn revealed by Cassini

Saturn has a smorgasbord of little moons, some of them orbiting within or just next to its rings. In the “Grand Finale” stage of the NASA Cassini spacecraft’s mission, which ended in 2017, it observed five of the planet’s small inner moons up close, which have now been analysed by researchers.

The Cassini observations showed that ring particles are constantly falling onto them, and they are all slightly different in shape and characteristics. The innermost moons are much redder, because the rings contain iron and red organic material, and the outermost ones are bluer because of ice and water vapour from the liquid plumes of the larger and further-out moon Enceladus. These small moons are just a taste of the diversity in Saturn’s system: it has 62 moons, each different from the next.

Pan: Saturn’s reddest inner moon

Pan is a shepherd moon, orbiting in a gap in the rings and keeping the area free of stray bits of ice and dust. This process also gives it its weird equatorial ridge as the strays stick to the moon’s equator. It is the reddest of the inner moons.

Daphnis: The moon that makes waves

Daphnis has the lowest density of the inner moons at about 274 kilograms per cubic metre – about a quarter the density of cool water – which means it is probably extremely porous and spongy. Like Pan, it orbits within the rings, creating a wavy gap as it passes by and building up a small bump around its middle.

Atlas: A flattened, fluffy pita

Atlas’s equatorial ridge makes up 25 per cent of its total volume, giving it a flattened, pita bread-like shape. Parts of its surface seem to be covered with at least 10 metres of soft, fluffy dust.

Pandora: Bumpy with a smooth patch

Pandora is more heavily cratered than its neighbours. Despite being only about 100 kilometres across at its widest point, it has at least two craters that are 30 kilometres wide. Pandora doesn’t have a ridge like the other moons that orbit within the rings, but it does have a remarkably smooth patch that probably comes from the rings.

Epimetheus: Strange grooves

Epimetheus is the densest of the small inner moons, and also one of the largest. It shares a nearly identical orbit with another moon, called Janus. Its entire surface is covered in strange grooves, which may have formed like stretch marks as Saturn’s gravity constantly stretched and warped Epimetheus.

Journal reference: Science 10.1126/science.aat2349