Shape Evolution of Cometary Nuclei
Astronomical Journal, 2003, Astron. J., 125, 3366-3377.
In a Nutshell
The Kuiper belt is widely thought to be the source of the short-period (specifically "Jupiter Family") comets. If so, one might expect the physical properties of the latter to be like of impact fragments, as produced in the laboratory by splintering impacts. To test this expectation, we measured and compared the shapes of a set of cometary nuclei with the shapes of impact produced fragments, measured in the same way. The result, shown in the Figure below, is that the two types of object have shape distributions which are NOT the same. The most important difference is that the nuclei are, on the average, more elongated than impact fragments. In case this difference is an artifact of the difference in scale between the comets and the laboratory impact samples, we also made a comparison with the shapes of small main-belt asteroids. Again, the comet nuclei are much more elongated than the asteroids.
How Could This Be?
The most likely explanation, and the one that we prefer, is that the observed cometary nuclei are highly modified with respect to their Kuiper belt counterparts. Shape is naturally modified by mass loss from the cometary nucleus, whether the mass loss proceeds anisotropically or not. Our data are consistent with the suggestion that the known cometary nuclei are but shrivelled remnants of once larger nuclei. They have lost much of their initial mass through sublimation induced by heating by the sun. They could have started out with the modest axis ratios shown by the impact fragments but their shapes have long since lost any memory of this early phase.
If the comets have lost a big enough fraction of their mass as to measurably change their shapes, then it is reasonable to expect that the mass loss has been strong enough to also substantially modify the rotations. In fact, rotation may have played a role in the shape modification through centripetally induced break-up.
What we see in the nuclei of short-period comets are objects highly modified from their state when formed.
The paper itself as a PDF file.
Last updated April 2003