Has comet Halley retained its primordial angular momentum?

Abstract

In 1967, Hartmann & Larson1 and Fish2 pointed out a relationship between the 'angular momentum density' (angular momentum per unit mass) and mass of the planets and asteroids, which was interpreted as resulting from the primordial accretion of these objects. These authors noted that this relationship holds if the planets rotate with approximately the same period, without taking into account density variations, and Alfven3 has provided a possible mechanism for this occurrence. Here I show that the nuclei of several comets, including Halley, obey the same relationship, suggesting that these objects too retain their primordial angular momentum. The angular momentum versus mass relation depends on the nuclear dimensions, dynamical characteristics and density; if primordial accretion is assumed, knowing two of these parameters allows the determination of the third. Applying this method to comet Halley yields a bulk density of 0.30+0.22−0.13g cm−3.