Jul 6, 2007
MEASURING a mere 165 miles in diameter, Hyperion is easily one of the smallest of the 34 moons known to be circling Saturn, the second-biggest planet in the solar system.
But stunning images of the tiny moon have revealed that Hyperion is also one of the most bizarre looking natural satellites in outer space.
Detailed images, taken when the US-European space probe Cassini came within 310 miles of the icy moon that orbits Saturn every 21.3 days, show that Hyperion looks amazingly like a giant sponge.
Scientists have revealed that its strange appearance is down to the fact that it is speckled with fresh craters up to six miles wide.
They believe that the surface of the oddly shaped moon is also highly porous, enhancing the preservation of the craters in remarkably good condition. Dr Peter Thomas and his colleagues at Cornell University in New York have published their detailed analysis of the Cassini pictures in the latest edition of the journal Nature.
Dr Thomas said the most striking visual aspect of Hyperion "is its sponge-like appearance, which is unlike that of any other object imaged to date" and was not picked up in earlier, less detailed pictures captured by Voyager.
He said: "The high porosity may enhance preservation of craters by minimising the amount of ejecta [material that is thrown out by a volcano] produced or retained, and accordingly may be crucial in crafting this unusual surface."
Scientists estimate the moon is 60 per cent as dense as water, which means much of its interior must be empty and suggests it is little more than a pile of space rubble.
In a second paper published in the journal, Dr Dale Cruikshank, an astrophysicist, and colleagues at the NASA Ames Research Centre in Moffett Field, California, say Hyperion's surface is quite reflective, but the bottoms of the craters are much darker.
The dark material from the crater bottoms appears to be similar to material on two of Saturn's other moons - the dark side of Lapetus and the surface of Phoebe.
Scientists also believe that Hyperion might have had a major collision which blew part of the moon away.
Its eccentric orbit also makes it subject to gravitational forces from Saturn which have set it tumbling in a chaotic orbit.
The Cassini probe is the fourth spacecraft to have been sent to explore Saturn since Pioneer 11 was the first to fly past the planet in 1979.
Voyager 1 flew past a year later, followed by its twin, Voyager 2, in 1981.
The £1.58 billion Cassini-Huygens project is a joint venture between NASA, the US space agency, the European Space Agency, and the Italian Space Agency.
After a seven-year voyage, Cassini first entered Saturn's orbit in July 2004. It then began a four-year mission that includes more than 70 orbits around the ringed planet and its moons.