The crater-induced YORP effect

Abstract

Context. The Yarkovsky−O'Keefe−Radzievskii−Paddack (YORP) effect plays an important role in the rotational properties and evolution of asteroids. While the YORP effect induced by the macroscopic shape of the asteroid and by the presence of surface boulders has been well studied, no investigation has been performed yet regarding how craters with given properties influence this effect. Aims. We introduce and estimate the crater-induced YORP effect (CYORP), which arises from the concave structure of the crater, to investigate the magnitude of the resulting torques as a function of varying properties of the crater and the asteroid by a semi-analytical method. Methods. By using a simple spherical shape model of the crater and assuming zero thermal inertia, we calculated the total YORP torque due to the crater, which was averaged over the spin and orbital motions of the asteroid, accounting for self-sheltering and self-sheltering effects.Results. The general form of the CYORP torque can be expressed in terms of the crater radius R 0 and the asteroid radius R ast :, where W is an efficiency factor. We find that the typical values of W are about 0.04 and 0.025 for the spin and obliquity component, respectively, which indicates that the CYORP can be comparable to the normal YORP torque when the size of the crater is about one-tenth of the size of the asteroid, or equivalently when the crater/roughness covers one-tenth of the asteroid surface. Although the torque decreases with the crater size R 0 as ∼ R 2 0 , the combined contribution of all small craters can become non-negligible due to their large number when the commonly used power-law crater size distribution is considered. The CYORP torque of small concave structures, usually considered as surface roughness, is essential to the accurate calculation of the complete YORP torque. Under the CYORP effect that is produced by collisions, asteroids go through a random walk in spin rate and obliquity, with a YORP reset timescale typically of 0.4 Myr. This has strong implications for the rotational evolution and orbital evolution of asteroids.Conclusions. Craters and roughness on asteroid surfaces, which correspond to concave structures, can influence the YORP torques and therefore the rotational properties and evolution of asteroids. We suggest that the CYORP effect should be considered in the future investigation of the YORP effect on asteroids.

Direct N-body Simulations of Satellite Formation around Small Asteroids: Insights from DART’s Encounter with the Didymos System

“…First, it has been shown that the magnitude and direction of the YORP torque are highly sensitive to small changes in the primary's shape, meaning that each landslide, mass shedding event, and natural impact could change the strength and direction of the YORP effect. This could make YORP spin-up effectively a random walk process and may significantly increase the amount of time required to build a secondary in orbit (Statler 2009;Cotto-Figueroa et al 2015;Zhou et al 2022). This effect may significantly decrease the efficiency of such a formation process, making a scenario in which the secondary forms from a single rotational disruption event more attractive.…”

The Yarkovsky Effect on the Long-term Evolution of Binary Asteroids

“…As a result, predicting even the instantaneous YORP value may be very difficult (this is the reason why the presently achieved YORP detections are often smaller than theoretically expected; see the latest compilation in the discussion section of Ďurech et al 2024). From a longterm perspective, the movement of boulders (Golubov & Krugly 2012) and formation of impact craters (Zhou et al 2022;Zhou & Michel 2024) could modify or even reverse the direction of the YORP torque. Building on the YORP's shape sensitivity, Bottke et al (2015) introduced the "stochastic YORP" concept and showed that it overall weakens the longterm effects of nominal YORP.…”

A semi-analytical thermal model for craters with application to the crater-induced YORP effect

“…However, accurately calculating the YORP effect on a real asteroid remains a challenge, as it has been demonstrated to be highly sensitive to surface topology (Statler 2009;Breiter et al 2009), such as uniform small-scale roughness (Rozitis & Green 2012), boulders (Golubov & Krugly 2012;Golubov et al 2014Golubov et al , 2021Ševeček et al 2015;Golubov 2017;Golubov & Scheeres 2019;Golubov & Lipatova 2022), and craters (Zhou et al 2022). Although the YORP torque caused by boulders and the tangential radiative force has been well studied, the YORP effect caused by concave structures has not yet been fully explored.…”

“…To account explicitly for the YORP torque caused by roughness and craters, Zhou et al (2022) developed a semi-analytical model that is computationally efficient for the crater-induced YORP (CYORP) torque. The CYORP torque is defined as the torque difference between the crater and the flat ground:…”

“…As a first step, this model assumed a zero thermal conductivity. Zhou et al (2022) found that roughness or craters that cover 10% of the asteroid's surface area could produce a CYORP torque comparable to the normal YORP (NYORP) torque, which arises from the macroscopic shape, with ignorance of the fine surface structure. Based on this number, which assumes that all craters have a depth-to-diameter ratio of 0.16, the reset of the YORP torque by the CYORP torque could be as short as 0.4 Myr.…”