Dust jets, outbursts, and fragmentation of comets

摘要

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.