[meteorite-list] NASA Scientist Hopes to Explain Saturn's Mysterious 'Black' Moon (Phoebe)

[Ron Baalke]

http://www.arc.nasa.gov/aboutames-pressrelease-print.cfm?id=15000128

Kathleen Burton
NASA Ames Research Center, Moffett Field, Calif.
Phone: 650-604-1731 or 650-604-9000
Email: Kathleen.M.Burton at nasa.gov

NASA SCIENTIST HOPES TO EXPLAIN SATURN'S MYSTERIOUS 'BLACK' MOON
June 9, 2004

A 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:

http://saturn.jpl.nasa.gov/index.cfm and

http://www.jpl.nasa.gov/