Asteroid Obliquity Evolution due to Boulder-Induced YORP

The YORP effect, caused by asteroid surfaces re-radiating thermal energy, is a major source of dynamical evolution for small bodies with diameters less than one kilometer. This additional torque is induced by asymmetry on the surface and can act on the spin pole to change the pole obliquity in small amounts per year and large amounts over the YORP timescale, inciting new dynamical states such as chaotic tumbling (Breiter and Murawiecka, 2019). It is also well known that small asteroids have boulders littering their surface that contribute to YORP spin and obliquity shifting. The effect of these boulders has thus far been characterized by tangential YORP which describes the intra-radiative effect that acts secularly over time to spin-up an asteroid (Golubov and Krugly, 2012). However, the geometric addition of these unresolvable features is not characterized for small bodies despite the recent collection of in-situ data of boulder sizes, shapes, and number densities on the surfaces of visited asteroids. We aim to characterize YORP torque due to unseen boulders and apply this knowledge to ground-based observations of asteroid shapes and estimates of YORP dynamics. By conducting Monte Carlo simulations of realistic surface populations made up of many boulders, we find how the factors of their shapes, locations, and dominant radiation directions can change the dynamics of obliquity through the YORP effect. We do this by analyzing the first-order YORP obliquity coefficients that translate geometric factors into attitude dynamics (Scheeres, 2007). We have already shown how this large-scale surface roughness can alter YORP spin torque up to 135% for a single boulder (Baker and McMahon, 2024). However, the large population size and varied directions can dampen individual contributions. There is also additional YORP torque from the small-scale roughness of regolith due to the thermal-infrared beaming effect which is known to add significant uncertainty to relatively weak YORP estimates, possibly dampening the rotational acceleration up to 50% (Rozitis and Green, 2012). Overall, many sources of YORP can serve to amplify or cancel out the effects of another. By isolating the boulder-induced geometric YORP effect, we can apply these theories to unresolved bodies in the rubble-pile size regime and estimate how much YORP acceleration is due solely to the additional surface area and radiation directions added by boulders.