Comets: Complex Worlds

Press Release From: American Astronomical Society
Posted: Wednesday, October 9, 2002

Funny things happen on the surfaces of comets. We are 'somewhat' used to picutres of planets, moons and asteroids, but when the NASA/JPL Deep Space 1 spacecraft flew by the comet 19/P Borrelly, the DS1 science team was amazed. These pictures gave us the clearest images to date of a comet and what we saw was a complex, active and unexpected view of a very different small world. Asteroid surfaces are dominated by effects of impact cratering. When we look at asteroids what we see are either the craters themselves or the regolith, the crushed rock ejecta from previous craters.

Comets are different. They are composed of a mix of dust, ice and frozen gases. What dominates comet surfaces are features produced by the sublimation of gases and ices and the Sun warms the comet during the passage through the inner solar system. Sublimation produces a variety of features wthat we have called ridges, hills, depressions and mesas. What we do not see on Borrelly are any impact craters.

Lets take a few examples: On Earth a mesa is a flat-topped, steep-sided hill that is capped by a strong or resistant layer of rock over a much weaker layer or rock. The mesa is formed by erosion of the less resistant lower layer material. The spectacular scenery of Monument Valley in Arizona is an example of mesas.

What we see on Borrelly are a series of flat-topped, steep-sided hills in the central area of the comet near the most active regions. We call these mesas and they are probably formed much like the terrestrial mesas. The top of the mesa has a thick insulating layer of dust, but the steep sides expose the underlying ice-rich comet material. Ices sublimate out the sides of mesas, undercutting the thick, insulating layer and causing sections of it to collapse on the valley floor.

While Borrelly is only 8 km long and 4 km wide, it seems to be broken into two pieces. The lower part of the comet is canted about 15 degrees with respect to the upper portion. These sections appear to 'chaff' against each other, raising what look like compressional ridges at the boundary of the two sections. These ridges are all roughly parallel to each other, raising what look like compressional ridges at the boundary of the two sections. These ridges are all rooughly parallel to each other and also parallel to the section boundary.

The imagery from the DS1 flyby of the comet Borelly opens a new page in comet exploration, showing that comets are not only complex and geologically interesting objects in their own right, but are also significantly different from other small bodies.

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