Cassini snaps rare pic of three of Saturn's moons

A rare picture snapped by Saturn probe Cassini shows three of the planet's moons -- all of which are very different from each other.

NASA/JPL-Caltech/Space Science Institute

Saturn has somewhere in the ballpark of 62 moons that we know about, 53 of which have names, not including the moonlets in its rings. Pictures of these moons are not, therefore, precisely hard to come by -- but this one captured by Cassini, the probe sent on a joint mission from NASA, the ESA and the ASI to monitor the ringed planet, is something special.

It shows three of Saturn's moons -- Tethys, Hyperion and Prometheus -- all with wildly varying properties.

Tethys, discovered in 1684 and seen in the centre of the image, is one of the largest of Saturn's moons, coming in behind Titan, Rhea, Iapetus and Dione, and the 16th largest moon in the solar system with a diameter of 1,062 kilometres. This means that it is large enough to be rounded (although ellipsoid in shape), but its two hemispheres are different: the leading hemisphere is older and rougher, and the trailing is younger and smoother -- dubbed the "smooth plains".

Tethys -- composed almost entirely of ice, if its density of 0.98g per cubed centimetre is any indication -- has one of the most reflective surfaces in the solar system, the result of sandblasting particles from Saturn's E-ring. On its leading hemisphere, directly opposite the smooth plains, it has a 450km-diameter, shallow crater called Odysseus, giving the moon a similarity to the Death Star; and a rift 100km long and 3km deep called the Ithaca Chasma approximately concentric with Odysseus is believed to be the result of the expansion as the moon's water solidified into ice -- a sort of lunar stretch mark. You can see it in the image above on Tethys' lower right.

Hyperion, to the upper left of Tethys in the image, is only 270km in diameter, has an irregular, potato-like shape and a sponge-like surface caused by deep, sharp-edged craters. Astronomers believe that Hyperion's high porosity and low density are partly what causes its spin -- extremely irregular and unpredictable, on a wobbly axis. Another reason could be its proximity to Titan, which creates an irregular orbital resonance.

Finally, just peeking out from below Saturn's ring in the lower left is Prometheus. If Hyperion is like a potato, Prometheus is like a Kipfler potato -- long and thin and lumpy, at 134.6km in length. It hangs about Saturn's F Ring -- the outermost of the planet's discrete rings -- interacting with it and "shaping" it. This happens when Prometheus' gravitational field tugs at material in the ring, causing kinks and ripples. This makes Prometheus what is known as a "shepherd moon" -- a moon that alters or controls nearby objects with its gravity.