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 to 2 AU and eccentricity ∼0.5) varies as τ s ∼ 100r 2 n , 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 A ∼ 0.1r −2 n . 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 ∼ 10 −3 r 2 n . Sub-kilometer nuclei have spinup timescales comparable to their orbital periods, confirming that outgassing torques are quickly capable of driving small nuclei towards rotational disruption. Torque-induced rotational instability likely accounts for the paucity of subkilometer 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 .