Surface geomorphology of Jupiter Family Comets: A geologic process perspective

Abstract

Recent spacecraft encounters with Jupiter Family Comets have revealed markedly diverse surface morphologies: Wild 2 is dominated by steep-walled and flat-floored depressions, Tempel 1 is relatively smooth and exhibits evidence for flows and layering, while Hartley 2 is bi-lobed with knobby terrain at its ends and a much smoother terrain in its middle. This diversity of surface morphologies has been interpreted as an evolutionary sequence (Belton, M.J.S. [2010]. Icarus 210, 881–897) where Jupiter Family Comets evolve from Wild 2 morphology, then to Tempel 1 morphology, and finally to Hartley 2 morphology. We propose instead that the diversity of surface morphology reflects geologic processing with diverse outcomes. In addition to impact cratering, we consider surface modification driven by the cometary activity which is responsible for gas and dust production. We consider eolian erosion that may be driven by the outflow of cometary vapor, making use of information from wind tunnel experiments and in situ studies of eolian erosion on Mars. We adopt the model of van der Waals cohesion recently proposed by Scheeres et al. (Scheeres, D., Hartzell, C., Sanchez, P., Swift, M. [2010]. Icarus 210, 968–984) and find that the average CO2 vapor outflow flux at Hartley 2 of 0.95×1017cm−2s−1 implies wind speeds sufficient to mobilize particles of 10cm size even close to the icy reservoirs where the vapor is evolved, below the surface. We suggest that particles are mobilized and entrained in flows within sub-surface outflow channels, emerging to be readily lifted into the coma, and fragmenting in the process. Although water production from Hartley 2 is greater, most of it is evolved in the coma from icy particles and does not contribute to eolian erosion of the nucleus. On the other hand, the much lower vapor outflow flux at Tempel 1 of 4×1014cm−2s−1 is insufficient, in the present model, to mobilize particles but is consistent with generating repeated fluidization episodes. The Wild 2 case is intermediate, with average outflow flux 7×1016cm−2s−1 which would be, in our model, sufficient to support eolian erosion. The steep-walled, flat-floor depressions on Wild 2 may have originated as impact features but have been subsequently modified by eolian erosion causing slope recession. We do not have definitive evidence to discriminate between an evolutionary sequence and diverse geologic outcomes, but we suggest future in situ observations at comets that may do so.