NASA Scientist Hopes to Explain Saturn's Mysterious Black Moon

Press Release From: Ames Research Center
Posted: Thursday, June 10, 2004

image NASA Ames planetary scientist is part of the science team that will study the data and images returned this week from the closest-ever flyby of Saturn's moon Phoebe.

The spectral data and images obtained from the June 11 flyby will help scientists determine the icy moon's surface composition and properties.

"This is a unique opportunity," said Dr. Dale Cruikshank, co-investigator for the Cassini Visual and Infrared Mapping Spectrometer (VIMS), an instrument that will measure the chemical signatures of Phoebe's surface. "We've never had a close-up look at an irregular, low-reflective moon of any planet before, so we are prepared to be surprised," he said.

Cruikshank will study the VIMS high-resolution spectral data to determine the distribution of recently observed water ice on Phoebe's surface. He also will use the data to determine the ability of Phoebe's surface to reflect light (known as its 'albedo') and the source of Phoebe's mysterious dark color. "This odd moon of Saturn has a little ice and a lot of black material on its surface, but beyond that, we know very little," Cruikshank noted.

Phoebe's surface color appears almost black when observed by powerful telescopes, scientists say. The moon, which is about 130 miles in diameter, reflects only 6 percent of the sunlight it receives.

Because of its dark color, and because Phoebe's orbit is irregular (elliptical, outside the plane of Saturn's equator and retrograde), scientists think the moon is probably a captured object, possibly a comet, asteroid or Kuiper Belt Object (KBO).

KBOs are lumps of ice, rock and black material in the outer solar system that were never drawn together by gravity to form a planet. They are of great interest to scientists because they are believed to be primordial, which means they probably date back to the formation of the solar system some 4.6 billion years ago. About half of the comets that occasionally come near the Earth and sun are KBOs.

One theory of Phoebe's mysterious dark color, which also is shared by the forward face of Iapatus, another nearby Saturn moon, is that it is due to the abundance of an organic material called tholin. Tholin is a sticky, waxy, dark red residue whose tiny particles cause the brownish haze of Saturn's largest moon, Titan.

The tholin that may cover Phoebe is thought by Cruikshank and others to be abiotic, which means it is not made from living organisms. Scientists hypothesize the tholin is a natural by-product of the organic chemistry of the carbonaceous materials that make up Phoebe. Comet dust is an example of abiotic organic material.

Since its discovery in 1898, Phoebe has been of interest to astronomers because it is so different from Saturn's other large moons. If Cassini finds that its surface is really made of carbonaceous organic material, scientists can use that information to learn about our solar system's formation and early history. Phoebe's surface material may even include amino acids, the building blocks of life.

On June 11, the Cassini orbiter will fly within about 1,200 miles of Phoebe. Data and images will be returned on June 12.

Cruikshank specializes in icy bodies in the outer solar system and the composition of small satellites, including all the satellites of Saturn.

The principal investigator of the VIMS team is Dr. Robert H. Brown of the University of Arizona, Tucson, Ariz.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington. JPL designed, developed and assembled the Cassini orbiter.

For further information about Cassini and the Phoebe flyby, visit: and

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