Study Indicates Ceres May Have Water-Ice Mantle

http://www.swri.org/9what/releases/2005/ceres.htm

Study indicates largest body in asteroid belt may have water-ice mantle
Southwest Research Institute (SwRI) News
September 8, 2005

Boulder, Colo. -- September 8, 2005 -- A new study of Ceres, the
largest
known object in the asteroid belt between Mars and Jupiter, shows
evidence of a planet-like round shape and a surprisingly complex
internal structure, with a rocky core possibly surrounded by a mantle
rich in water-ice.

The study, to be reported September 8 in a letter to the journal Nature
by investigators from Cornell University, Southwest Research Institute
(SwRI), the University of Maryland, University of California at Los
Angeles and the Planetary Science Institute in Tucson, Ariz., used
Hubble Space Telescope images made during observations in December 2003
and January 2004 to examine surface features of the asteroid in
reflected sunlight, study its rotational properties and search for any
small moons. The paper is led by Dr. Peter C. Thomas of the Center for
Radiophysics and Space Research at Cornell University. The project was
funded by NASA through the Space Telescope Science Institute.

"This is the first time we have seen Ceres in such detail and can even
say something about its interior," said the project team's leader, Dr.
Joel Parker, assistant director of SwRI's Space Studies Department.
"You
can watch it rotate in our observations, and you get the feeling of it
being a whole new world, not just a bit of rocky debris."

Differences in surface features among asteroids preserve a record of
how
the asteroid belt evolved between the earliest days of the solar system
and about 4.6 billion years ago, when Jupiter's growing gravitational
pull halted the accretion of bodies in the asteroid belt.

Some asteroids, like 4 Vesta, the second-most massive body in the
asteroid belt, have a crust, mantle and core, indicating that they
experienced sufficient thermal evolution to differentiate into layered
structures.

Other asteroids appear more homogeneous, similar to carbonaceous
meteorites that have undergone only minimal thermal processing. In the
past, Ceres -- about the size of Texas at 580 miles across -- was
placed
in this latter category, in part because of its low density, its low
albedo or heat signature, and its relatively featureless visible
reflectance.

However, the new study indicates that Ceres' round shape and smoothness
resemble more that of a gravitationally relaxed object, or one whose
shape is determined by hydrostatic equilibrium. If so, it would be the
only asteroid thus far to be characterized in that way. Also, its shape
is more flattened than would be expected of a homogeneous object, but
consistent with a central mass concentration indicating a layered
makeup.

The relaxed state, differentiated structure and mean density observed
on
Ceres strongly suggest water-ice as the primary mantle constituent.
Unlike the icy moons of Jupiter and Saturn, however, the higher heating
available at Ceres' distance from the sun makes water-ice unstable at
the surface. If water-ice has been at the surface of Ceres, it may
currently hide just below a thin residual layer of clay and dark
carbonaceous materials.

Editors: A movie showing the rotation of Ceres is available at
http://www.swri.org/press/2005/ceres.htm .