Boulder-induced spin variability in the YORP effect

Abstract

Boulders have been observed as a dominant feature on all rubble-pile asteroids, as seen by recent survey missions to small bodies. Thermal re-radiation induced torque, known as the YORP effect, is a dominant influence on the rotational dynamics of a body in the rubble-pile size regime below 1 km in diameter. YORP is also extremely sensitive to small-scale topography such as regolith, boulders, and craters. We analyze the effects of boulders on the YORP spin torque of an asteroid, where simulated populations are based on the observed size and placement distributions on real bodies, to show how influential these features are when making YORP estimates. Models are produced by adding many geometric boulder shapes to an existing asteroid shape model. This analysis applies previous polyhedral YORP approximations which calculate the spin and obliquity coefficients through summation over faceted shapes. YORP is seen to be highly variable with boulder size, placement, and dominant orientation on both Bennu and Itokawa shape models. We find that the maximum influence due to a boulder can be up to 135% of the total YORP when large boulders affect the dynamics or several small ones have a combined effect. For a single feature, this is a large and highly sensitive and variable global influence and represents the maximum of the observed size range. These results also pertain to a static case, while there is expected material redistribution and breakup through the experience of YORP spin changes which may be a factor in limiting unbounded YORP spin-up or spin-down over time. We compare the magnitude and uncertainty of boulder-induced YORP torque to other analytical models.