Phoebe's Surface Reveals Clues to Its Origin
June 14, 2004
Images collected during Cassini's close flyby of Saturn's moon,
Phoebe, have yielded strong evidence that the tiny object may
contain ice-rich material, overlain with a thin layer of darker
material perhaps 300 to 500 meters (980 to 1,600 feet) thick.
The surface of Phoebe is also heavily potholed with large and
small craters. Images reveal
bright streaks in the ramparts of the largest craters, bright
rays which emanate from smaller craters, and uninterrupted
grooves across the face of the body.
"The imaging team is in hot debate at the moment on the
interpretations of our findings," said Dr. Carolyn Porco, Cassini
imaging team leader at the Space Science Institute in Boulder,
Colo. "Based on our images, some of us are leaning towards the
view that has been promoted recently, that Phoebe is probably ice-
rich and may be an object originating in the outer solar system,
more related to comets and Kuiper Belt objects than to
asteroids."
In ascertaining Phoebe's origin, imaging scientists are noting
important differences between the surface of Phoebe and that of
rocky asteroids which have been seen at comparable resolution.
"Asteroids seen up close, like Ida, Mathilde, and Eros, and the
small martian satellites do not have the bright 'speckling'
associated with the small craters that are seen on Phoebe," said
Dr. Peter Thomas, an imaging team member from Cornell University,
Ithaca, N.Y.
The landforms observed in the highest resolution images also
contain clues to the internal structure of Phoebe. Dr. Alfred
McEwen, an imaging team member from the University of Arizona,
Tucson, said, "Phoebe is a world of dramatic landforms, with
craters everywhere, landslides, and linear structures such as
grooves, ridges, and chains of pits. These are clues to the
internal properties of Phoebe, which we'll be looking at very
closely in order to understand Phoebe's origin and evolution."
"I think these images are showing us an ancient remnant of the
bodies that formed over four billion years ago in the outer
reaches of the solar system," said Dr. Torrence Johnson, an
imaging team member from NASA's Jet Propulsion Laboratory,
Pasadena, Calif. "Battered and beat-up as it is, it is still
giving us clues to its origin and history."
Phoebe may be an icy interloper from the distant outer solar
system which found itself captured by giant Saturn in its
earliest, formative years. Final conclusions on Phoebe's origins
await a combination of the results on Phoebe's surface
structures, mass and composition gathered from all 11
instruments, which collected data during the flyby on June 11,
2004.
"This has been an impressive whirlwind flyby and it's only a
curtain raiser on the events about to begin," said Porco.
Cassini arrives in orbit around Saturn on the evening of June 30,
2004 (July 1 Universal Time).
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL, a
division of the California Institute of Technology in Pasadena,
manages the Cassini-Huygens mission for NASA's Office of Space
Science, Washington, D.C. The Cassini orbiter and its two onboard
cameras were designed, developed and assembled at JPL. The
imaging team is based at the Space Science Institute, Boulder,
Colo.
For the latest images and information about the Cassini-Huygens
mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home
page, http://ciclops.org. For more
information on NASA programs, visit www.nasa.gov.
