Spectral Mapping of Phoebe Reveals Water and Carbon Dioxide-Rich Composition on Surface

Robert M. Nelson, William D. Smythe, Lucas Kamp, Mark Boryta, JPL; Bruce Hapke, Jeff Byrnes, University of Pittsburgh; Robert Brown, University of Arizona; Dale Cruikshank, NASA, Ames Research Center; Roger Clark, USGS, Flagstaff. Funded by the Cassini-Huygens Mission to Saturn and Titan

Four cube pairs each taken together were selected at closest approach. The same five areas were studied in each cube. A typical cube pair is shown above.

On June 11th, 2004, the Cassini Huygens spacecraft flew by Saturn's moon Phoebe, and acquired the most detailed images ever taken. In addition to taking images of Phoebe, Cassini scientists Robert Nelson et al, analyzed VIMS spectral image cubes to conduct spectral mapping of the moon to determine its chemical composition.

Four cube pairs each taken together were selected at closest approach. The same five areas were studied in each cube. A typical cube pair is shown above. Reflective areas are shown in green and blue; absorbing areas are yellow and cyan.

The Results: The spectral mapping of Phoebe revealed that at one micron wavelength the VIMS images show three bright and two dark regions. The bright regions are found to have more water ice and less carbon dioxide and the dark regions have less H₂0 and more CO₂.

The existence of carbon dioxide strongly suggests that Phoebe did not originate in the asteroid belt, but rather in much colder regions of the Solar System such as the Kuiper Belt. The Kuiper Belt is a vast reservoir of small, primitive bodies beyond the orbit of Neptune. There are additional spectral features in both the light and dark areas that have not yet been associated with specific minerals.

Voyager 2 took these images of Phoebe, on Sept. 4, 1981, from 2.2 million kilometers (1.36 million miles) away.

Implications

This has been the first intensive opportunity to study a Kuiper Belt object, giving scientists the opportunity to study an object possibly dating back to the formation of the solar system. By conducting spectral mapping, scientists can also help determine the surface morphology as well as the chemical composition of Phoebe.

Significance to Solar System Exploration

The images, taken on June 11, 2004, show water-ice, ferric iron, and carbon dioxide on Phoebe's surface. Image taken by ISS with VIMS spectral information superimposed by R. Clarke.

The Voyager 2 spacecraft flew by Phoebe in 1981 from a distance 2.2 million kilometers (1.4 million miles). At that distance it was only a vague dark shape with some lighter patches and somewhat red in color. Scientists had assumed it was an asteroid from the early solar system, but now they seemed to be mistaken.

Cassini's June 11th flyby revealed the surface of Phoebe from a distance of 2,068 kilometers (1,285 miles) which is more than 1000 times closer than Voyager 2.

The Cassini findings with the spectral mapping conducted suggest that Phoebe may be a a frozen artifact of a bygone era, some four billion years ago. It may be a captured Centaur, a Kuiper Belt object that migrated into the inner solar system made up of a primordial mixture of ice, rock and carbon-containing compounds -- the building blocks to the outer solar system.

For further information about science highlights and having your research highlighted, please contact Samantha Harvey at NASA's Jet Propulsion Laboratory, Samantha.K.Harvey@jpl.nasa.gov.

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