Systematics and Consequences of Comet Nucleus Outgassing Torques

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, is a strong function of nucleus radius, r.    Empirically, we find that the timescale  varies as τ ~ 100 r², where r is expressed in kilometers and τ is in years.  The  fraction of the nucleus surface that is active varies as f_A ~ 0.1/r², giving f_A = 1 at r = 0.3 km.   We find that the dimensionless moment arm of the torque is widely scattered, with a median value $k_T$ = 0.004 (i.e. about 0.4% of the escaping momentum torques the nucleus), and weak (<3σ) evidence for a size dependence, k_T ~ 0.001 r².  Sub-kilometer nuclei have spin-up timescales comparable to or less than their orbital periods, confirming that outgassing torques are  quickly capable of driving small nuclei towards rotational disruption.  Torque-induced rotational instability likely contributes to the paucity of sub-kilometer short-period cometary nuclei, and can account for the pre-perihelion destruction of sungrazing comets.  Outgassing torques on small active asteroids can rival YORP torques, even for unobservably small (<1 g/s) mass loss rates.  Finally, we discuss the important roles played by detection, spectroscopic and survival biases in the measured distributions of τ, f_A and k_T.

A full description is published at Jewitt, D. Astronomical Journal, 161:262 (12pp) (2021).

Figure 1: Published determinations of nucleus radius vs. spin-up timescale for short-period comets, most with perihelia in the 1 to 2 AU range. A strong size dependence is evident.  The solid line indicates a power law with index 2.  From Jewitt (2021).