Recent data from the Cassini mission to Saturn have shed new light on the surface of the saturnian moon Iapetus, particularly the presence and movement of carbon dioxide.

University of Arizona researchers published a paper this month on their work with a computer model to simulate the movement of carbon dioxide ice and make estimates about its longevity on the moon. This has implications for understanding what is happening on the surface of Iapetus, a moon that in some ways remains a mystery.

Iapetus is the third-largest moon of Saturn and is known for its "two distinct colors, white as a snowball and dark as coal," said Eric Palmer a planetary-sciences graduate student at the UA and lead author of the recent paper.

The pattern of light and dark on Iapetus is related to the composition of its surface. The light areas are water ice, and the dark areas are associated with carbonaceous material.

The carbon "is not much different from the graphite in a pencil," Palmer said. It is in these dark areas that solid carbon dioxide, sometimes referred to as dry ice, has been observed.

Iapetus has a "transient atmosphere," said Robert Brown, a professor at the Lunar and Planetary Laboratory at the UA and co-author of the paper.

For most purposes, it can be considered not to have any atmosphere at all. This means that sunlight generally hits the surface of the moon without the softening presence of an atmosphere, and even though Iapetus is much farther away from the sun than Earth, the ultraviolet light strikes the water ice with enough energy to photolyze it, splitting its hydrogen from its oxygen.

Most of the hydrogen and oxygen escape to space, but some of the atoms bond with the carbon around them, forming such chemicals as methane and carbon dioxide.

The carbon dioxide freezes into dry ice when it is cold enough. But with peak temperatures on the equator of Iapetus reaching to about -225 degrees Farenheit, it still can be too warm in the low-pressure environment for carbon dioxide to stay frozen, Palmer said.

The ice sublimates into a gas and moves, eventually settling down and freezing again when it is cold enough, until its new area heats up, too.

Carbon dioxide ice thus makes vast migrations during the 29.5 Earth years that make up a Saturnian year. It moves from the north to south pole during summer and then migrates back to the north pole during the winter.

During each migration, about 6 percent of the carbon dioxide escapes to space, Palmer said.

In each hop, the carbon dioxide can move about 93 miles, but the direction is random, Palmer said.

The interesting thing about the carbon dioxide ice is that "it shouldn't be there for very long," Brown said. But computer models made by Palmer and Brown estimate that, once generated, about half the carbon dioxide will still reside on the surface after 200 years.

Iapetus and its strange coloring are the source of some controversy among those who study it. The origin of the carbon covering half the moon is still unresolved.

Brown and Palmer said it will take more data, or the actual handling of the carbon, to determine whether it comes from the moon and has been gradually exposed as the water ice has evaporated or was picked up by the moon as space dust over the course of millions of years.

"Iapetus is really kind of a strange object," Palmer said.

Cassini facts

--Cassini's power is generated by radioisotope thermoelectric generators and is only 885 watts.

--The mission cost about $3.27 billion.

--The Huygens probe dropped onto Titan descended at about 12,400 mph.

--On its trip to Saturn, Cassini helped provide data supporting Einstein's theory of general relativity.

--Cassini has discovered new moons, new rings and molecular oxygen ions in the atmosphere of Saturn.

--Contact NASA Space Grant intern Eric Schwartz at 807-8012 or at eschwartz@azstarnet.com