Dione and Tethys pose for Cassini

Cassini has snapped some stunning shots on its meanderings through the Saturnian system, but this one is a corker, even by its high standards.

The image, courtesy of NASA's JPL, shows the moons Dione and Tethys facing each other across Saturn's rings, Dione in the foreground some 860,000km from Cassini. Tethys is a further 640,000km distant.

The picture was taken in natural light using Cassini's narrow angle camera on 22 September this year.

Dione and Tethys were discovered in 1684 by Giovanni Cassini. The larger of the two moons, Dione is 1,126km across and is the densest moon of Saturn after Titan. It is thought to be mainly water ice around a rocky core.

Its surface is heavily cratered in places, mostly on its trailing hemisphere - the side facing away from its direction of motion, contrary to scientists expectations. Researchers now think the moon used to spin in the other direction, but was hit by an impactor big enough to change that over.

Tethys is not much smaller, measuring 1,071km in diameter. Scientists think it is composed almost entirely of water ice, and that the cracks across its surface were caused by its liquid interior welling up through the frozen crust. ®