[meteorite-list] SwRI Researchers Ask: Where Have All The Comets Gone?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:35 2004
Message-ID: <200206241455.HAA06935_at_zagami.jpl.nasa.gov>

http://www.swri.edu/9what/releases/levinson.htm

Southwest Research Institute (SwRI) News

For more information contact:

Maria Martinez, Communications Department
(210) 522-3305, Fax (210) 522-3547

Dr. Hal Levison
(303) 546-0292, hal_at_boulder.swri.edu

June 20, 2002

SwRI researchers ask: Where have all the comets gone?

Boulder, Colorado -- Most comets disintegrate after their first few
passages through the inner solar system, say scientists at Southwest
Research Institute (SwRI). A new study has revealed that 99 percent
of the objects from the cloud of comets at the edge of the solar
system, known as the Oort cloud, break apart sometime after they
enter the inner solar system. The findings appear in the current
issue of the journal Science.

For several decades, astronomers have wondered about the fate of
comets once they stop producing their beautiful and easily seen
tails. Some have speculated that these objects are still there but
have become dormant -- that is, they have lost the material that
allows them to generate tails -- making them much harder to detect.
Others have suggested that comets disintegrate, leaving no visible
trace.

Led by Dr. Harold F. Levison, a staff scientist at the SwRI Boulder
office, the team compared computer models with observations to
determine the fate of the missing comets. The team created thousands
of fictitious new comets, tracked the comets as they entered the
solar system from the Oort cloud, and calculated their evolution
based on the gravitational influences of the sun, planets, and
Milky Way. By following comet trajectories until they were ejected
from the solar system, hit a planet, or struck the sun, team
members estimated the number of dormant comets that should have
been observed if all active comets had become dormant. This number
is 100 times larger than what is actually seen. From this, team
members deduced that 99 percent of these objects vanish.

"These objects are simply not where we expect them to be," says
Levison. "The only explanation I can think of is that they go
'poof.'"

Interestingly, comets originating in the Kuiper Belt, a cometary
source just beyond Neptune, do not disrupt nearly as often as
those originating in the Oort cloud. Both comet classes are
believed to be composed of similar mixtures of ice and rock, but
their different disruption behaviors could reflect the chemical
or physical characteristics of their formation areas. Another
theory is that the inconsistency between the classes could be
related to evolutionary processes. Most Oort cloud comets move
quickly from distant orbits to orbits that closely approach the
sun, while Kuiper Belt objects move slowly through the planetary
regions. This could suggest that different thermal histories lead
to different disruption rates.

"It is possible that Oort cloud comets disrupt because of strong
thermal gradients or volatile pressure buildup, while Kuiper Belt
objects survive because they are warmed more slowly," says Levison.

NASA provided funding for the program. The paper "The Mass
Disruption of Oort Cloud Comets," by Levison, Dr. Alessandro
Morbidelli (Observatoire de la Côte d'Azur), Dr. Luke Dones
(SwRI), Dr. Robert Jedicke (University of Arizona), Dr. Paul A.
Wiegert (Queen's University, Ontario), and Dr. William F. Bottke
(SwRI) appears in the June 21 issue of Science.
Received on Mon 24 Jun 2002 10:55:17 AM PDT