Cohesive regolith on fast rotating asteroids

Abstract

The migration of cohesive regolith on the surface of an otherwise monolithic or strong asteroid is studied using theoretical and simulation models. The theory and simulations show that under an increasing spin rate (such as due to the YORP effect), the regolith covering is preferentially lost across certain regions of the body. For regolith with little or no cohesive strength, failure occurs by landsliding from the mid latitudes of the body at high enough spin rates.As the cohesive strength of the regolith increases, failure occurs by fission of grains (or coherent chunks of grains) across a greater extent of latitudes and eventually will first occur at the equator. As the spin rate is further increased, failure regions migrate from the first failure point to higher and lower latitudes. Eventually failure will encompass the equatorial region, however there always remains a region of high latitudes (around the poles) that will not undergo failure for arbitrarily high spin rates (unless disturbed by some other phenomenon). With these results a scaling law is derived that can be used to determine whether observed asteroids could retain surface regolith grains of a given size. The implications of this for the interpretation of spectral observations of small asteroids and boulder migration on large asteroids are discussed.

Global Patterns of Recent Mass Movement on Asteroid (101955) Bennu

“…For example, a decrease in spin period (where the asteroid rotates more quickly, referred to as “spin‐up”) as a result of the YORP effect will alter the effective slope on the surface on timescales of millions of years or less (e.g., Bottke et al, 2006; Rubincam, 2000). If the critical angle of repose is exceeded owing to spin‐up, slopes are likely to fail, resulting in downslope motion and potentially altering the shape of the asteroid (e.g., Keller et al, 2010; Sanchez & Scheeres, 2020; Scheeres, 2015; Walsh, 2018). YORP spin‐up is dependent on the detailed shape of a body; by changing that shape with each successive landslide or bulge, it may be a self‐limiting process (Statler, 2009).…”

Reconstructing the formation history of top-shaped asteroids from the surface boulder distribution

“…The distribution of these boulders, that was precisely measured in unprecedented detail by the two spacecrafts 1,2 , constitutes a record of the geological evolution of the surface regolith since the origin of these asteroids. Here, we show that during the regolith migration driven by YORP spin-up [6][7][8][9] , the surface boulders coevolve with the underlying regolith and exhibit diverse dynamical behaviors: they can remain undisturbed, sink into the regolith layer and become tilted, or totally be buried by the downslope deposition, depending on their latitudes. The predominant geological features commonly observed on top-shaped asteroids, including the boulder-rich region near the pole 1,10 , the deficiency of large boulders at the equatorial area 10,11 and partially buried, oblique boulders exposed on the regolith surface 12,13 , are commensurate with this coevolution scenario.…”

Granular flow on a rotating and gravitating elliptical body

“…Both these aspects will, as we see in later sections, add to the complexity of the motion of the regolith. Finally, the analysis so far parallels the recent work of Sanchez & Scheeres (2020) who included cohesion, but predicated their development on the force balance of a single boulder on a spherical body. In contrast, ours is a continuum analysis and we have not included the effect of cohesion in this work to retain focus on the flow of grains over a non-spherical body in the presence of rotation and varying gravity.…”

“…grains lost contact with the surface. This was followed by a similar study on the fate of cohesive regolith on fast-spinning asteroids (Sanchez & Scheeres 2020). However, the dynamics of the moving regolith was not considered.…”

“…Using the soft-sphere DEM developed by Sánchez & Scheeres (2011), Sanchez & Scheeres (2020) recently conducted local DE simulations of cohesive regolith on asteroids, restricting themselves to a lune on the surface of a sphere. Sanchez & Scheeres (2020) employed 5000 spherical grains with a density of and diameters of 2–3 cm, which allowed them a regolith depth of approximately 12 cm. Our DE simulations, in contrast, span the entire body, for which we took larger diameter grains to limit computational requirements.…”