REDISTRIBUTION OF PARTICLES ACROSS THE NUCLEUS OF COMET 67P/CHURYUMOV-GERASIMENKO

The European Space Agency’s Rosetta spacecraft entered orbit around the nucleus of the Jupiter-family comet, 67P/Churyumov-Gerasimenko on 6 August 2014. The scientific imaging system onboard is called OSIRIS and comprises a dual camera system with a high-resolution narrow-angle camera (NAC) and a lower resolution wide-angle camera (WAC).

Sublimation-driven ejection of material from an active area is the initiator of material motion in a cometary system. The non-volatile material (usually referred to as dust) is accelerated by gas, and much of it reaches escape velocity, thereby permanently leaving the cometary environment. However, it is now clear that not all non-volatile material acquires sufficient energy to escape. There is evidence in the images for the accumulation of non-escaping dust particles emitted from areas of activity in a form of “airfall”. Furthermore, there are observations of features that appear similar to aeolian ripples, dune-like structures, possible wind-tails, and near-boulder deposits, indicating that other processes may be at work in transporting non-volatile material across the surface.

We show by numerical modelling that a type of saltation is plausible even under the rarified gas densities seen at the surface of the nucleus. However, interparticle cohesive forces present difficulties for this model, and an alternative mechanism for the initiation of reptation and creep may result from the airfall mechanism. The requirements on gas density and other parameters of this alternative make it a more attractive explanation for the observations. The uncertainties and implications will be discussed, based on Thomas et al. Astron. Astrophys. 583, A17, dx.doi:10.1051/0004-6361/201526049, 2015.