[meteorite-list] How Do Comets Evolve?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sat Dec 24 19:51:28 2005
Message-ID: <200512242342.jBONgSK25165_at_zagami.jpl.nasa.gov>

http://deepimpact.jpl.nasa.gov/mission/update-200512.html

Deep Impact Mission Update - December 2005

How Do Comets Evolve?
By Ray Brown

Introduction. - Tempel 1 is like a marshmallow being roasted on a stick.
Rotating it takes nearly 41 hours. Each five and a half years it is
moved nearer the fire and withdrawn to a cooler place. With each
approach to the sun a bit more of its constituent dust and volatile
material is driven off into interplanetary space; so it evolves.

Evolution of volatiles. - The volatiles, as we will note later in this
article, are in the form of ice when they are at temperatures below some
200 Kelvins. Examples are the ices of water and carbon dioxide.

Tempel 1 is thought to heat up and cool down rapidly. This thermal
cycling bakes the ices out of the material near the surface contributing
somewhat to its powdery consistency. It's like the marshmallow had been
dipped in powdered sugar. Because large amounts of certain molecules
appear in the gas ejected by the impactor's crash, it is suggested that
the impact has excavated ices not normally boiled off into the coma and
tail.

Evolution of solids. - Solids entrained in the interior ice are swept
out of Tempel 1 by the ices which are turning directly into gas. Some of
these dust grains have enough momentum to carry them into the coma where
they are swept away into the tail by the action of sunlight. Other
grains with less energy fall back onto the surface of Tempel 1. This
also contributes to the build up of powder on the surface.

But Tempel 1 is not alone in powdering its face. Over time, collisions
with other solar system bodies cause craters and thus we can conjecture
that debris from those collisions may also provide a contribution.

Be it noted that much more solid ejecta was expelled from the crater
than astronomers expected. It is suspected that the dust is a silicate
because the ejecta's cooling rate resembles that of liquid silica droplets.

A Rugged surface. - The surface of Tempel 1 reveals an aging comet.
There are a least three recognizable levels of topography that we assume
were not present when the comet formed. At the point of the arrow marked
on the comet nucleus, you can see a
relatively flat area. Four small unlabeled arrows at the right edge of
the flat area point to bright lines that are escarpments lit edge-on by
the sun. The scarps are about 20 meters high and at their foot there
stretches away to the right a wishbone shaped lowland.

In the lower part of the picture, you can see a couple of "circular
features". They are slightly darker than the surrounding material and,
since they have a circular ridge, they are presumed to be impact
craters. Impact craters are important from an evolutionary standpoint.
They are present on moons, planets and even asteroids. Although impact
craters are still forming, witness the Arizona meteor crater, a mere 20
to 50 thousand years old, there is an interval in solar system history
called the massive early bombardment when collisions between
interplanetary bodies were relatively frequent. We wonder whether
Tempel 1 is a bombardee.

Between the two flat areas is a band of lesser brightness. It is
suspected of constituting multiple exposed layers of material rising
gradually toward the top of the picture. But note that the comet's
surface is, overall, homogeneous in brightness and color.

Generally there are rough spots such as the area at the bottom of this
close up of part of the nucleus. It is suspected that surface
irregularities at the impact point caused rays to appear in the ejecta.
The rays are best seen in this image, which has been enhanced to show
them better.

To summarize. - The evidences of Tempel 1's evolution are: the lack of
volatiles near the surface, a powdery surface, different levels of
terrain and the presence of impact craters.
Received on Sat 24 Dec 2005 06:42:27 PM PST