Analytic theory for the tangential YORP produced by the asteroid regolith

TL;DR

This paper develops an analytic model for the tangential YORP effect caused by asteroid surface roughness, showing it can significantly influence asteroid spin dynamics, especially for small-scale surface features like regolith and boulders.

Contribution

The authors derive an approximate analytic formula for the tangential YORP effect due to surface roughness, extending previous models to include regolith and boulder roughness effects.

Findings

TYORP can be 5-70 times greater than normal YORP for Ryugu.

Surface roughness significantly impacts asteroid spin evolution.

The model applies to both regolith and boulder surface features.

Abstract

The tangential YORP effect is a radiation pressure torque produced by asymmetric thermal emission by structures on the asteroid surface. As such structures, previous works considered boulders of different shapes lying on the surface of the asteroid. We study the tangential YORP produced by the rough interface of the asteroid's regolith. We create an approximate analytic theory of heat conduction on a slightly non-flat sinusoidal surface. We analyze the published data on the small-scale shape of the asteroid (162173) Ryugu and estimate its tangential YORP due to the surface roughness. We derive an analytic formula that expresses the TYORP of a sinusoidal surface in terms of its geometric and thermal properties. TYORP is maximal at the thermal parameter of the order of unity and for the shape irregularities of the order of the thermal wavelength. Application of this equation to Ryugu predicts TYORP, which is 5-70 times greater than its normal YORP effect. The contribution of the small-scale regolith roughness to the YORP effect of the asteroid can be comparable to the normal YORP and the tangential YORP produced by boulders. The same theory can describe roughness on the asteroid boulders, thus adding a new term to the previously considered TYORP created by boulders.