[meteorite-list] View of Comets As Pristine Relics Of Solar System Formation Evolves

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:29 2004
Message-ID: <200308061745.KAA15633_at_zagami.jpl.nasa.gov>

View of comets as pristine relics of solar system formation evolves
Southwest Research Institute
August 6, 2003

The long-held perspective that comets are pristine remnants from the
formation of the solar system has evolved from the prevailing views
of 30 years ago, finds planetary scientist Dr. S. Alan Stern in a
paper published in the journal Nature.

"It's fair to say that a sea change has taken place," says Stern,
director of the Space Studies Department in the SwRI Space Science
and Engineering Division. "We used to consider comets as wholly
unchanged relics that had been stored ever since the era of solar
system formation in a distant, cold, timeless deep freeze called
the Oort cloud. We now appreciate that a variety of processes
slowly modify comets during their storage there," he says. "As a
result, it's become clear that the Oort cloud and its cousin the
Kuiper Belt are not such perfect deep freezes."

The first evolutionary process to be recognized as affecting comets
during their long storage was radiation damage, followed by the
discovery that sandblasting from dust grains in the interstellar
medium plays an important role. Next, researchers theorized that
comets in the Oort cloud are heated to scientifically significant
temperatures by passing stars and supernovae, says Stern. More
recently, researchers are finding that comets in the Kuiper Belt
are heavily damaged by collisions.

"It also now seems inevitable that most comets from the Kuiper Belt,
though constructed of ancient material, cannot themselves be ancient --
instead they must be 'recently' created chips off larger Kuiper
Belt Objects, formed as a result of violent impacts," says Stern.
"This is truly a paradigm shift. Many of the short-period comets we
see aren't even ancient!"

The classical view that comets do not evolve while they are stored far
from the sun in the Oort cloud and Kuiper Belt began to change as far
back as the 1970s, but the pace of discoveries about the way comets
evolve picked up considerably in the 1980s and 1990s.

As a result of these findings, astronomers now better appreciate that
comets, though still the most pristine bodies known, have been
modified in several important ways since their birth, says Stern.

The realization that comets evolve during their long storage in the
Oort cloud and Kuiper Belt provides insight and context to more
confidently evaluate the results of astronomical and space mission
observations of comets. So, too, it suggests that cometary sample
return missions now on the drawing board for NASA should employ
relatively deep subsurface sampling if truly pristine, ancient
material is to be collected.
Received on Wed 06 Aug 2003 01:45:30 PM PDT