Closest Pass Yet

NASA's Cassini spacecraft capped off 2004 with an encounter of Saturn's ying-yang moon Iapetus (eye-APP-eh-tuss) on New Year's Eve.

It was Cassini's closest pass yet by one of Saturn’s smaller icy satellites since its arrival around the ringed giant on June 30th. The next close flyby of Iapetus is not until 2007.

Iapetus is a world of sharp contrasts. The leading hemisphere is as dark as a freshly-tarred street, and the white, trailing hemisphere resembles freshly-fallen snow.

Cassini flew by the two-toned moon at a distance of approximately 76,700 miles Friday.

"I can think of no better way than this to wrap up what has been a whirlwind year," said Robert T. Mitchell, program manager for the Cassini mission at NASA's Jet Propulsion Laboratory, Pasadena, CA. "The new year offers new opportunities, and 2005 will be the year of the icy satellites."

In 2005 Cassini will have 13 targeted encounters with five of Saturn's moons. "We have 43 close flybys of Titan still ahead of us during the four-year tour. Next year, eight of our 13 close flybys will be of Titan. We will also have a number of more distant flybys of the icy satellites, and let's not forget Saturn and the rings each time we come around," said Mitchell.

With a diameter of about 1,400 kilometers (890 miles), Iapetus is Saturn's third largest moon. It was discovered by Jean-Dominique Cassini in 1672. It was Cassini, for whom the Cassini-Huygens mission is named, who correctly deduced that one side of Iapetus was dark, while the other was white.

Scientists still do not agree on whether the dark material originated from an outside source or was created from Iapetus' own interior. One scenario for the outside deposit of material would involve dark particles being ejected from Saturn’s little moon Phoebe and drifting inward to coat Iapetus. The major problem with this model is that the dark material on Iapetus is redder than Phoebe, although the material could have undergone chemical changes that made it redder after its expulsion from Phoebe. One observation lending credence to the theory of an internal origin is the concentration of material on crater floors, which implies that something is filling in the craters. In one model proposed by scientists, methane could erupt from the interior and then become darkened by ultraviolet radiation.

Iapetus is odd in other respects. It is the only large Saturn moon in a highly inclined orbit, one that takes it far above and below the plane in which the rings and most of the moons orbit. It is less dense than objects of similar brightness, which implies it has a higher fraction of ice or possibly methane or ammonia in its interior.

The last look at Iapetus was by NASA's Voyager 1 and 2 spacecraft in 1980 and 1981. The Cassini images will be the highest resolution images yet of this mysterious moon.

The Iapetus flyby by Cassini follows the successful release of the Huygens probe on December 24.

FMI: http://saturn.jpl.nasa.gov, www.nasa.gov/cassini