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:
