Systematics and Consequences of Comet Nucleus Outgassing Torques

TL;DR

This study investigates how anisotropic outgassing affects comet nucleus rotation, revealing size-dependent spin-up timescales and implications for comet stability and destruction, supported by empirical data and simple modeling.

Contribution

It provides the first empirical characterization of outgassing torques on comets and compares these with a simple model, highlighting size-dependent effects and implications for comet evolution.

Findings

Spin-up timescale scales as 100 r_n^2 years

Active surface fraction decreases with size as 0.1 r_n^{-2}

Small nuclei are prone to rotational disruption

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, $\tau_s$, 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 $\sim$0.5) varies as $\tau_s \sim 100 r_n^{2}$, where $r_n$ is expressed in kilometers and $\tau_s$ is in years. The fraction of the nucleus surface that is active varies as $f_A \sim 0.1 r_n^{-2}$. We find that the median value of the dimensionless moment arm of the torque is $k_T$ = 0.007 (i.e. $\sim$0.7\% of the escaping momentum torques the nucleus), with weak ($<$3$\sigma$) evidence for a size dependence $k_T \sim 10^{-3} r_n^2$. Sub-kilometer nuclei have spin-up timescales comparable to their orbital periods, confirming that outgassing torques are quickly capable of driving small nuclei towards 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 ($\lesssim$1 g s$^{-1}$) mass loss rates. Finally, we highlight the important role played by observational biases in the measured distributions of $\tau_s$, $f_A$ and $k_T$.