CO-driven activity constrains the origin of comets

Abstract

Context. An open question in the study of comets is the so-called cohesion bottleneck, that is, how dust particles detach from the nucleus.
Aims: We test whether the CO pressure buildup inside the pebbles of which cometary nuclei consist can overcome this cohesion bottleneck.
Methods: A recently developed pebble-diffusion model was applied here to comet C/2017K2 PANSTARRS, assuming a CO-driven activity.
Results: (i) The CO-gas pressure inside the pebbles erodes the nucleus into the observed dust, which is composed of refractories, H₂O ice and CO₂ ice. (ii) The CO-driven activity onset occurs up to heliocentric distances of 85 au, depending on the spin orientation of the comet nucleus. (iii) The activity onset observed at ≈26 au suggests a low obliquity of the nucleus spin axis with activity in a polar summer. (iv) At 14 au, the smallest size of the ejected dust is ≈0.1 mm, consistent with observations. (v) The observed dust-loss rate of ≈200 kg s^-1 implies a fallout ≥30%, a nucleus surface active area ≥10 km², a CO-gas loss rate ≥10 kg s^-1, and a dust-to-gas ratio ≤20. (vi) The CO-driven activity never stops if the average refractory-to-all-ices mass ratio in the nucleus is ≤4.5 for a nucleus all-ices-to-CO mass ratio ≈4, as observed in comets Hale-Bopp and Hyakutake. These results make comet C/2017K2 similar to the Rosetta target comet 67P/Churyumov-Gerasimenko. (vii) The erosion lifetime of cometary planetesimals is a factor 10³ shorter than the timescale of catastrophic collisions. This means that the comets we observe today cannot be products of catastrophic collisions.