How Rough are Comets?

Abstract:

Comets and primitive asteroids are windows into the formation of the Solar System, offering clues to the physical state and architecture of a planetary system at its birth. A comet sample return has been among NASA’s priorities for a New Frontiers class mission to understand the composition of primitive bodies as proxies for the temperature and other physical conditions during planetary formation.

Reconnaissance missions reveal the surfaces of asteroids 433 Eros, 101955 Bennu, 162173 Ryugu, and 25143 Itokawa to be boulder-strewn and seemingly inhospitable to a possible landing and surface sample return. The two year close investigation through a perihelion passage of 67P/ Churyumov-Gerasimenko by the Rosetta spacecraft has resulted in a wealth of data that from which the macroscopic characteristics of the surface can be derived. We make use of the roughness model of Buratti and Veverka (1985, Icarus 64, 320) to derive quantitative values of the rough facets on the surface ranging from clumps of particles to boulders and ridges. We find that the roughness is surprising low, similar to the low-albedo side of Iapetus (Lee et al. 2010, Icarus 206, 623) rather than to that of asteroids and icy moons. We attribute this finding to infilling and ponding by cometary dust that is reaccreted onto the comet.

Our results show that there are smooth, receptive locations on cometary surfaces, but that prior quantitative analysis of candidate sample-return regions will need to occur. In addition, the accessible surface material may be largely processed dust that is not representative of the primitive material which is sought in a comet sample return.

Copyright 2020. Government funding acknowledged.