[meteorite-list] Ceres: The Wet Look

[Ron Baalke]

http://skytonight.com/news/4765721.html

Ceres: The Wet Look
by J. Kelly Beatty
Sky & Telescope
November 28, 2006

Last summer's vote by members of the International Astronomical Union
elevated Ceres from being merely the largest member of the asteroid belt
to a prime candidate for "dwarf planet" status. And from what
astronomers have learned about it recently, Ceres is making a good case
for that promotion.

The most recent insights come from a trio of astronomers led by Andrew
S. Rivkin (Applied Physics Laboratory), who examined the big asteroid's
near-infrared spectral signature last year. Previous work had already
established the presence of clay-like minerals that include water as
part of their molecular structure. Rivkin's team has used the body's
infrared fingerprint to refine the kinds of materials that might lie on
its surface. The best candidates, he reported at a recent meeting of
planetary scientists, are iron-rich clays that contain roughly 5%
carbonates - just the kind of minerals that would form on what was once
a wet surface.

The new results follow on the heels of observations with the Hubble
Space Telescope, conducted last year, that imply Ceres is more than just
a rocky jumble. Peter C. Thomas (Cornell University) and others
determined that the its slightly squashed shape and spin rate match what
would be expected for a body that had differentiated (segretated) into a
rocky core and a water-ice exterior. Only a thin rind of rock and dust
may be hiding an icy layer 60 to 120 kilometers (40 to 80 miles) thick.

And Hubble images are no longer the last word on Ceres' appearance. Last
month a team of European researchers led by Benoit Carry (LESIA, France)
released images of the asteroid taken four years ago with the Keck
Observatory's powerful adaptive-optics camera. They find that Ceres has
a smooth shape overall, slightly fatter across its midsection (481 km)
than through its poles (447 km).

According to Carry, the surface of Ceres displays a wealth of bright and
dark markings, some of which might be due to regional differences in
composition.

All of this is whetting the appetite of scientists involved in Dawn
<http://dawn.jpl.nasa.gov/>, a NASA spacecraft that narrowly avoided
outright cancellation earlier this year. Now scheduled for launch next
summer, Dawn will spend time orbiting both Ceres and Vesta, an equally
intriguing asteroid.