CASSINI CLOSES IN ON THE CENTURIES-OLD MYSTERY OF SATURN'S MOON IAPETUS
Extensive analyses and modeling of Cassini imaging and heat-mapping data have confirmed and extended previous ideas that migrating ice, triggered by infalling reddish dust which darkens and warms the surface, may explain the mysterious two-toned "yin-yang" appearance of Saturn's moon Iapetus. The results, published online Dec. 10 in a pair of papers in the journal Science, provide what may be the most plausible explanation to date for the moon's bizarre appearance, which has puzzled astronomers for more than 300 years.
Shortly after he discovered Iapetus in 1671, the French-Italian astronomer Giovanni Domenico Cassini noticed that the surface of Iapetus is much darker on its leading side, the side which faces forward in its orbit around Saturn, than on the opposite trailing hemisphere. Voyager and Cassini images have shown that the dark material on the leading side extends onto the trailing side near the equator. The bright material on the trailing side, which consists mostly of water ice and is ten times brighter than the dark material, extends across the north and south poles onto the leading side.
One of the papers, led by Tilmann Denk of the Freie Universitat in Berlin, Germany, describes findings made by Cassini's Imaging Science Subsystem (ISS) cameras during the spacecraft's close flyby of Iapetus on September 10, 2007, and on previous encounters. "ISS images show that both the bright and dark materials on Iapetus' leading side are redder than similar material on the trailing side," says Denk, suggesting that the leading side is colored (and slightly darkened) by reddish dust that Iapetus has swept up in its orbit around Saturn. This observation provides new confirmation of an old idea, that Iapetus' leading side has been darkened somewhat by infalling dark dust from an external source, perhaps from one or more of Saturn's outer moons. The dust may be related to the giant ring around Saturn recently discovered by NASA's Spitzer Space Telescope. However, the ISS images show that this infalling dust cannot be the sole cause of the extreme global brightness dichotomy.
"It is impossible that the very complicated and sharp boundary between the dark and the bright regions is formed by simple infall of material. Thus, we had to find another mechanism," said Denk.
Close-up ISS images provide a clue, showing evidence for "thermal segregation", in which water ice has migrated locally from sunward-facing, and therefore, warmer areas to nearby poleward-facing and therefore colder areas, darkening and warming the former and brightening and cooling the latter.
The other paper, by John Spencer of Southwest Research Institute in Boulder, Colo., and Denk, adds runaway global migration of water ice into the picture to explain the global appearance of Iapetus. Their model synthesizes ISS results with thermal observations from Cassini's Composite Infrared Spectrometer (CIRS) and computer models. CIRS observations in 2005 and 2007 found that the dark regions reach temperatures high enough (129 Kelvin or -227 F) to evaporate many meters of ice over billions of years. Iapetus' very long rotation period, 79 days, contributes to these warm temperatures, by giving the sun more time to warm the surface each day than on faster-rotating moons. Spencer and Denk propose that the infalling dust darkens the leading side of Iapetus, which therefore absorbs more sunlight and heats up enough to trigger evaporation of the ice near the equator. The evaporating ice recondenses on the colder and brighter poles and on the trailing hemisphere. The loss of ice leaves dark material behind, causing further darkening, warming, and ice evaporation on the leading side and near the equator. Simultaneously, the trailing side and poles continue to brighten and cool due to ice condensation, until Iapetus ends up with extreme contrasts in surface brightness in the pattern we see today. The relatively small size of Iapetus, which is just 1,500 kilometers (900 miles) across, and its correspondingly low gravity, allow the ice to move easily from one hemisphere to another. "Iapetus is the victim of a runaway feedback loop, operating on a global scale," says Spencer.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team consists of scientists from the U.S., England, France, and Germany. The imaging operations center and team leader (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo. The Composite Infrared Spectrometer team is based at NASA's Goddard Space Flight Center, Greenbelt, Md., where the instrument was built, with significant hardware contributions from England and France.