[meteorite-list] Ceres May Be An Asteroid Impersonator

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 17 Jul 2008 16:25:54 -0700 (PDT)
Message-ID: <200807172325.QAA06734_at_zagami.jpl.nasa.gov>

http://www.sciencenews.org/view/generic/id/34157/title/Ceres_may_be_an_asteroid_impersonator

Ceres may be an asteroid impersonator
By Ron Cowen
Science News
July 15, 2008

The largest member of the asteroid belt could have emigrated from the
solar system's fringe

If planetary scientist Bill McKinnon's hunch is right, the largest
asteroid in the solar system isn't an asteroid at all. Ceres, as the
470-kilometer-wide object is called, may be a relative of Pluto that
formed at the solar system's fringes but came in from the cold several
billion years ago.

McKinnon, based at Washington University in St. Louis, said he was first
struck by Ceres' unusually low density - more similar to icy comets from
the outer solar system than the rocky bodies found in the asteroid belt
that lies between the orbits of Mars and Jupiter. The density of Ceres,
referred to as a dwarf planet, is only slightly higher than that of
Pluto. Models suggest Ceres "looks remarkably Pluto-like," McKinnon says.

But it was a recently developed model of the early solar system that
prompted McKinnon to formally propose that Ceres might be an escapee
from the Kuiper belt, an outer solar system reservoir of frozen bodies
that includes Pluto. He presented his proposal July 15 in Baltimore at
the Asteroids, Comets, Meteors conference.

According to the model, developed by researchers including Hal Levison
and Bill Bottke of the Southwest Research Institute in Boulder, Colo.,
and Alessandro Morbidelli of Observatory of the C??te d'Azur in Nice,
France, the orbits of the outer four planets - Jupiter, Saturn, Uranus
and Neptune - were initially packed much closer together than they are
today.

Beyond these planets resided a band of dust, ice and gas particles. Over
time, as some of these particles leaked inward, their gravitational tug
lengthened the distance between the orbs. For instance, Jupiter migrated
inward, while Saturn moved outward.

At some point, according to the theory, Saturn reached a gravitational
sweet spot: The time it took to go around the sun became exactly twice
that of Jupiter's. That interplay strengthened the planets' mutual tug,
and ultimately hurled Uranus and Neptune into the outlying band of dust,
ice and gas. The entry of Uranus and Neptune scattered debris from the
chilly band, sending some of its denizens into the inner solar system.

That's how Ceres might have migrated from the outer solar system into
the asteroid belt, McKinnon suggests.

"We are saying that many objects from the outer solar system - what we
call the primordial disk of comets that went on to produce the Kuiper
belt - are captured in the outer part of the asteroid belt as a
byproduct of the model," Bottke says. He and Levison presented updated
versions of the theory at the meeting just before McKinnon's presentation.

"I consider McKinnon's idea as something of a thought balloon to
stimulate thinking," Bottke says. "It is indeed possible that he is
correct, but I would not bet for it at this point."

Additional information on Ceres' composition, to be gathered by NASA's
Dawn spacecraft when it visits Ceres in 2015, could clarify the body's
origin. But proof may require measuring the ratio of hydrogen to its
heavier isotope, deuterium, in the ices or water vapor venting from the
body, which would require a mission beyond Dawn, McKinnon says. If the
ratio matches that observed in comets, "the case is closed" for Ceres
being an emigre to the asteroid belt, he says.
Received on Thu 17 Jul 2008 07:25:54 PM PDT