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Tethys and the rings of Saturn

A gorgeous pic snapped by Cassini shows the outer rings of Saturn in crisp detail, with ice moon Tethys bringing up the rear.

NASA/JPL-Caltech/Space Science Institute

A new picture snapped by Saturn probe Cassini gives a beautifully clear view of Tethys, peeking through the gaps in the planet's outer rings.

Tethys is the fifth largest of Saturn's moons at 1,066 km (662 miles) in diameter, and the 16th largest moon in the solar system. Although this is less than a third of the size of Earth's moon, which is 3,476 km (2,160 miles) in diameter, its mass is still greater than the combined mass of every known smaller moon in the solar system.

It is composed mostly of water ice, heavily cratered and scarred. The largest of these scars is the graben named the Ithaca Chasma, 100 km wide and 2,000 km long, visible stretching horizontally just below the centre of the moon in the image above.

Although most of Tethys' scars are believed to have been caused by impacts, the Ithaca Chasma is a little different: it is believe to have been caused as the water inside Tethys cooled and froze, expanding the surface of the planet and causing the rift.

In the image, snapped July 14, 2014, the moon peeks through a gap between two of Saturn's outer rings: the A ring, the outermost of the wide "main" rings, and the narrow F ring. You can also see a small gap in the A ring. This is approximately 42 km (26 miles) across, and is caused by Daphnis, a moon about 8 km (5 miles) in diameter that orbits inside the ring.

Daphnis, named for a shepherd in Greek mythology and not visible in the picture, is what is known as a shepherd moon: a small satellite whose gravity shapes the edges of the rings, either by deflecting the material that drifts towards it or by attracting it to its own surface.