Systematics and Consequences of Comet Nucleus Outgassing Torques

Abstract

Anisotropic outgassing from comets exerts a torque sufficient to rapidly change the angular momentum of the nucleus, potentially leading to rotational instability. Here, we use empirical measures of spin changes in a sample of comets to characterize the torques, and to compare them with expectations from a simple model. Both the data and the model show that the characteristic spin-up timescale, τ_s, is a strong function of nucleus radius, r_n. Empirically, we find that the timescale for comets (most with perihelion 1-2 au and eccentricity ∼0.5) varies as ${\tau }_{s}\sim 100{r}_{{\rm{n}}}^{2}$ , where r_n is expressed in kilometers, and τ_s is in years. The fraction of the nucleus surface that is active varies as ${f}_{{\rm{A}}}\sim 0.1{r}_{{\rm{n}}}^{-2}$ . We find that the median value of the dimensionless moment arm of the torque is k_T = 0.007 (i.e., ∼0.7% of the escaping momentum torques the nucleus), with weak (<3σ) evidence for a size dependence ${k}_{T}\sim {10}^{-3}{r}_{{\rm{n}}}^{2}$ . Sub-kilometer nuclei have spin-up timescales comparable to their orbital periods, confirming that outgassing torques are quickly capable of driving small nuclei toward rotational disruption. Torque-induced rotational instability likely accounts for the paucity of sub-kilometer short-period cometary nuclei, and for the pre-perihelion destruction of sungrazing comets. Torques from sustained outgassing on small active asteroids can rival YORP torques, even for very small (≲1 g s^-1) mass-loss rates. Finally, we highlight the important role played by observational biases in the measured distributions of τ_s, f_A, and k_T.