Cometary brightness variation and nucleus structure

The Bobrovnikoff and Beyer photometric data for more than 100 comets have been analyzed for intrinsic brightness variations,before andafter perihelion, according to ther ^–n law, wherer is solar distance. The Oort and Schmidt classification of comet age has been extended and applied with Marsden's new determinations of inverse semi-major axis, 1/a, original. All classes of comets withP>25 yr show statistically the same value ofn after perihelion. New comets approach perihelion with smaller values ofn and older comets with increasingly larger values (Table II). For comets ofP<25 yr,n is larger and erratic.A physical interpretation involves the quick loss of a frosting of super-volatile materials from new comets; then, for all comets, the development of an insulating crust after perihelion. The crust also includes globs of meteoroidal and icy material. The crust tends to be purged near perihelion but generally to grow in a spotty fashion with cometary age. The orientation of the axes of rotating comets is shown to be an important unknown factor in cometary brightness variations. A speculation is made concerning the axis of rotation for C/Kohoutek, 1973 XII.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978.This research has been supported by Grant No. NSG 7082 from the U.S. National Aeronautics and Space Administration.