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 Each five and a half years it is moved nearer the fire and withdrawn to a cooler place.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.