The Sublimative Torques of Jupiter Family Comets and Mass Wasting Events on Their Nuclei

TL;DR

This study compares models of sublimative torques on comet nuclei, revealing that nucleus size and heliocentric distance primarily influence torque, and introduces a new comet classification based on mass-wasting activity frequency.

Contribution

It directly compares two sublimative torque models to clarify the role of gas-surface interactions and proposes a novel comet classification scheme based on mass-wasting activity.

Findings

Torque depends mainly on nucleus size and heliocentric distance.

Mass wasting exposes fresh volatiles, influencing activity.

New classification scheme for comets based on activity recurrence.

Abstract

Sublimative outgassing of comets produces torques that alter the rotation state of their nuclei. Recently, parameterized sublimative torque models have been developed to study rotation state changes of individual comet nuclei and populations of cometary bodies. However, these models simplify the interactions between the escaping gas and cometary surface into only a few parameters that hide the details of these complex interactions. Here we directly compare the X-parameter model (Samarasinha & Mueller, 2013) with the SYORP model (Steckloff & Jacobson, 2016) to tease out insights into the details of the gas-surface interactions driving sublimative torques. We find that, for both of these models to accurately model sublimative torques, the number of sublimating molecules that contribute to the net torque is largely independent of the detailed shape and activity of the nucleus, but rather depends primarily on the size of the nucleus and the effective heliocentric distance of the comet. We suggest that cometary activity must be largely restricted to regions of steep gravitational surface slopes (above the angle of repose), where mass wasting can refresh activity by shedding mantles of refractory materials and exposing fresh volatiles. We propose a new classification scheme for comets based on the frequency of this mass-wasting process (relative to the timescale of activity fading): quasi-equilibrium, episodic, quasi-dormant, and extinct.