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.
