The Cassini spacecraft has given astronomers a rare look at a lunar time capsule: a moon cryogenically frozen more than 3 billion years ago, the Jet Propulsion Laboratory (JPL) reported Tuesday.
Saturn's satellite Iapetus is the solar system's only moon known to have kept the shape it had when it was "just a few hundred million years old," said JPL, which is based in Pasadena in the Greater Los Angeles area.
Through instruments on Cassini, which has been in orbit around Saturn since June 2004, scientists learned that Iapetus has "a walnut shape, bulging at its midsection," according to the JPL.
Cassini scientist Julie Castillo said: "Iapetus spun fast, froze young and left behind a body with lasting curves."
Iapetus' rotation in its younger days -- at least five hours per revolution -- is believed to have been much faster than it is today.
According to Castillo, the fast spin gave the moon an oblate shape that increased the surface area in the same way the surface area of a round balloon stretches when the balloon is pressed into an oblate shape. By the time the rotation slowed down to a period of 16 hours, the outer shell of the moon had frozen.
"Iapetus' development literally stopped in its tracks," Castillo said.
It currently takes the moon 80 days to make a complete rotation.
Iapetus is the only one of Saturn's nearly 60 moons frozen in the shape it had when it was a relative teenager.
Cassini flew by Iapetus in early 2005. Its next close encounter with the moon is set for Sept. 10, when it swoops to within 620 miles (about 992 km) of the surface.
Launched on Oct. 15, 1997, the plutonium-powered Cassini is the most highly instrumented and scientifically capable interplanetary spacecraft ever deployed, according to the JPL, which is managing the mission for NASA (the National Aeronautics and Space Administration) and the European and Italian space agencies.