Comet Impact on McMaster

n November 1, Dr. Jane Luu of Harvard University presented a talk for the McMaster Physics and Astronomy colloquium series entitled "Comet Nuclei and Their Origin". In her lecture, Dr. Luu touched upon a number of aspects including the composition, structure, and origin of comets. The current hot topic in comet circles (ellipses?) has been the recent confirmation of the Kuiper belt's existence. This subject played a feature role in the concluding portions of Dr. Luu's presentation.

By way of introduction, the audience was informed that the composition of comets is dominated by water-ices. In spite of this icy nature, however, a typical comet presents a fairly low albedo, reflecting less than 5% of incident light. This compares with values of about 90% for snow and 10% for the Moon, making the moniker "dirty snowball" a fitting one to apply to comets. Although the nucleus of an average comet may extend to a diameter of perhaps ten kilometres, an obscuring coma of gas can surround it for hundreds of kilometres. The escape velocity from a typical comet is only on the order of 1 m/s, and as a result the coma is not gravitationally locked into place.

Dr. Luu explained that comets may be monitored for periodic variations in brightness which indicate a rotation of their nuclei. A nucleus which is not nicely spherical will present different amounts of surface area at different times (somewhat like a gigantic potato rotating in space!). A rotation will lead to regular luminosity fluctuations, which are observed at both optical and infrared wavelengths. The amplitude of these variations may be used to discern the ratio of the long to short axes of the nucleus. It appears likely the aspherical nature of comet nuclei is the result of some shape modification processes. Jets which emanate from comet nuclei are themselves observed to be distinctly asymmetric in their location.

The origin of comets is a subject that has recently undergone some significant revisions in scientific thinking. The Oort cloud, a spherical halo consisting of billions of comets located approximately 100 000 AU from the inner solar system, has often been thought of as the original home for all comets. In reality, while the Oort cloud seems to be supplying us with long-period comets (T<200 years), gravitational capture from this distant cloud has almost certainly been too inefficient to explain the observed flux of short-period comets. In addition, Dr. Luu noted that if the orbital inclination of these short-period comets was conserved during capture, it would be hard to explain the fact that short- period comets are by nature low inclination objects. This argument was originally advanced by a Canadian team of researchers, and led to renewed interest in the existence of a "belt" of comets located just beyond the orbit of Neptune.

This region of high comet density was dubbed the Kuiper belt after an American astronomer who suggested its existence in the early 1950's. (An English astronomer named Edgeworth had a similar idea but seems to have missed out in the publicity sweepstakes). Just over five years ago, Dr. Luu and other researchers began looking for objects at Kuiper belt distances in order to test the rejuvenated hypothesis. Eventually, they succeeded in finding a rather large body (150 km in diameter) some 40 AU from the sun, and have since discovered many smaller ones at similar distances. Estimates now place the population of the Kuiper belt at between 100 million to 1 billion comets, constituting a total mass just under l% that of the Earth. Indeed, the planet Pluto and its moon Charon may in fact simply be the largest members of the Kuiper comet belt. As Terrance Dickinson recently noted, were it not for decades of history, the title of "planet" might now be removed from the frozen outpost known as Pluto. The Kuiper belt itself, and comets in general, will continue to be a source of intrigue for Dr. Jane Luu. With comets playing a dual role as survivors from the early solar system and potential links to understanding distant stellar systems, it is not too hard to share her interest.

David Fleming


Hamilton Amateur Astronomers
Maintained by Grant Dixon

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