[meteorite-list] Ceres: Asteroid or Miniplanet?

[Ron Baalke]

http://www.news.cornell.edu/stories/Sept05/Ceres.to.html

Asteroid or miniplanet? Cornell astronomer finds Ceres appears to 
have shape and interior similar to terrestrial planets

By Thomas Oberst (cunews at cornell.edu)
Cornell University News Service
September 15, 2005

ITHACA, N.Y. -- When is a space rock more than just a space rock?

Ceres 1 was already holding the title of the solar system's largest 
asteroid. Now new observations show the space rock may be more worthy 
of the appellation "miniplanet."

On Sept. 7 NASA released photographs of Ceres that show the rock is a 
smooth ellipsoid, or oblong sphere, with an average diameter of 
approximately 590 miles -- about the size of Texas. A scientific 
paper on the findings, by a group led by Peter C. Thomas, senior 
research associate at Cornell University's Center for Radiophysics 
and Space Research, appeared in the Sept. 9 issue of the journal 
Nature.

Co-author Joel Parker, an astronomer at the Southwest Research 
Institute in Boulder, Colo., used the Hubble Space Telescope's 
Advanced Camera for Surveys to snap 267 images of Ceres on Dec. 28, 
2003, during a nine-hour period -- one Ceres "day."

Being ellipsoid and smooth is special for a rock. It indicates that 
the body is heavy enough to possess gravity strong enough to suck its 
own surface smooth -- a process called "gravitational relaxing." 
Because the process typically requires a mass of many trillions of 
tons, depending on the temperature, the average pebble is not going 
to be gravitationally relaxed; even most asteroids aren't.

By combining the new information on Ceres' roundness with previous 
independent measurements of its mass, Thomas and his colleagues 
inferred that Ceres must have a "differentiated interior" similar to 
the terrestrial planets. Although this possibility had been 
previously predicted, it was not widely accepted. "We used the best 
telescope available to apply a basic geophysical test of other 
people's predictions," Thomas said.

Based on their own models and observations, Thomas and his colleagues 
believe Ceres contains a rocky silicate core and icy mantel covered 
by a crust of carbon-rich compounds and clays. Furthermore, they 
predict that the icy mantel may contain more frozen water than all of 
the fresh water on Earth.

Whenever water is mentioned, people ask about life. But Thomas says 
the possibility is "very remote," noting that even if the interior of 
Ceres were warm enough for some of the water to liquefy, Ceres 
probably lacked a sufficient energy source for life to develop.

Ceres was discovered in 1801 by Sicilian astronomer Giuseppe Piazzi 
and declared to be the "missing planet" predicted between Mars and 
Jupiter. However, the title was revoked in 1802 when Ceres was found 
to be a member of the hundreds of thousands of other rocks and debris 
of the Asteroid Belt.

Since the discovery of 2003 UB313 -- which some have hailed as the 
tenth planet -- in July, some astronomers (and many non-astronomers) 
have begun to question whether objects such as Ceres should also be 
enshrined as planets.

Thomas professes a lack of concern about Ceres' place in the solar 
system. "There are plenty of other interesting things and processes 
in Ceres to contemplate rather than whether or not it should be 
called a planet," he said. But for those who prefer a more definitive 
answer, Thomas offers: "You can call Ceres a 'minor planet' or 
'miniplanet' if you'd like, but I would not call it a 'full-fledged 
planet.'"

The other authors of the Nature paper are L.A. McFadden of the 
University of Maryland, S.A. Stern and E.F. Young of the Southwest 
Research Institute, C.T. Russell of the University of California-Los 
Angeles and M.V. Sykes of the Planetary Science Institute, Tuscon, 
Ariz. Funding for the project was provided by NASA through the Space 
Telescope Science Institute.

Thomas Oberst is a writing intern at the Cornell News Service.

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