It is known that comets are aging very rapidly on cosmic scales, because they rapidly shedmass. The processes involved are
(i) normal activity-sublimation of ices and expulsion of dust from discrete emission sources on and/or below the surface
of a comet's nucleus, and (ii) nuclear fragmentation. Both modes are episodic in nature, the latter includes major steps in
the comet's life cycle. The role and history of dynamical techniques used are described and results on mass losses due to
sublimation and dust expulsion are reviewed. Studies of split comets, Holmes-like exploding comets, and cataclysmically
fragmenting comets show that masses of 10 to 100 million tons are involved in the fragmentation process. This and other
information is used to investigate the nature of comets' episodic aging. Based on recent advances in understanding the
surface morphology of cometary nuclei by close-up imaging, a possible mechanism for large-scale fragmentation events
is proposed and shown to be consistent with evidence available from observations. Strongly flattened, pancake-like shapes
appear to be required for comet fragments by conceptual constraints. Possible end states are briefly examined.
I review three types of related cometary phenomena, which, in the order listed, are increasingly important in studies of
cometary evolution: dust jet-like morphological features in comet heads; outbursts; and nucleus fragmentation. Gas-driven
collimated jets consisting predominantly of microscopic dust particles are a standard mode of comet activity, as has amply
been documented by numerous ground-based observations and even more convincingly by closeup images taken with the
cameras aboard the space missions that flew by four periodic comets between 1986 and 2005. Gas expanding from discrete
emission sources on or below the nucleus surface drags dust with it into the atmosphere in quantities that vary with time.
Briefly occurring sudden great enhancements of activity are known as outbursts. Dust ejecta in most outbursts do not
include larger objects than boulder-sized ones, thus limiting the total mass delivered in such episodes. At times, however,
outbursts accompany a nucleus fragmentation event, which signals a major episode. Fragmentation events have a tendency
to recur, their products offering a complex hierarchy of large fragments. A rapid sequence of fragmentation events may end
up with a sudden, complete disintegration of a significant fraction of the original comet or, more rarely, the entire comet.
Indications are that cascading fragmentation is the most efficient process of comet aging and ultimate demise.
Conference Committee Involvement (2)
Instruments, Methods, and Missions for Astrobiology XI
12 August 2008 | San Diego, California, United States
Instruments, Methods, and Missions for Astrobiology X
28 August 2007 | San Diego, California, United States
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