Equatorial cavities on asteroids, an evidence of fission events

Abstract

This paper investigates the equatorial cavities found on asteroids 2008 EV5 and 2000 DP107 Alpha. As the likelihood of these cavities being impact craters is demonstrated to be low, the paper presents a fission mechanism that explains their existence as a scar of past fission events. The dynamical environment of bicone (or "top"-shaped) asteroids is such that, at high spin rates, an identifiable equatorial region enters into tension before the rest of the body. We propose hypothetical past shapes for 2008 EV5 and 2000 DP107, with mass added within the cavity to recreate a smoother equatorial ridge. The dynamical environment of these hypothetical parent bodies reveal that this modified region is indeed set in tension when spin is increased. The fission process requires tensile strength at the interface between the ejecta and the remaining body, at the moment of fission, between 0 to 2 Pa for 2008 EV5 and between 0 to 15 Pa for 2000 DP107, depending on the precise fission scenario considered. Going back to the spin-up deformation phase of the asteroids, the paper examines how kinetic sieving can form predominantly rocky equators, whose tensile strength could be much lower than that of the rest of the body. This process could explain the low cohesion values implied for this fission mechanism.

Cohesive regolith on fast rotating asteroids

“…Results relating to the angle of friction alone have been shown previously for asteroids, however the inclusion of the bond number into the failure conditions is new in this paper, and highlights important transitions that may occur. One exciting area of application is to the failure conditions for larger boulders on rubble pile bodies, such as analyzed in Tardivel et al (2018). Future analysis will test the models developed here for their applicability to the loss of larger boulders on rapidly rotating asteroids.…”

The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements

“…Such asteroids are commonly found within the near-Earth asteroid population, and are the most frequently found morphology for binary asteroid primaries (which constitute about 15% of the near-Earth asteroid population) [16]. Binary primaries actually spin even faster than Bennu in general, implying that they have an even narrower lobe about the equator, which increases the likelihood that material can enter orbit and leave the lobe, potentially forming binaries [17,18,19]. Thus, our observation that the surface morphology follows the rotational Roche lobe may also be an important clue linking binary formation to fastspinning, top-shaped asteroids.…”

“…To probe the effect of an internal deformation on the surface regolith, the granular mechanics model outlined in ref. [18] was applied to a representative longitude lune, starting at a spherical shape and distorting it into an equatorial bulge to mimic the Bennu ridge. For both cohesionless and cohesive grains we did not observe significant distortion of the surface material on the equator, consistent with features on the surface potentially being retained during a period of shape deformation due to internal failure.…”

Impact excitation of a seismic pulse and vibrational normal modes on asteroid Bennu and associated slumping of regolith

“…The equatorial ridge is redder than the poles, suggesting that the ridge material contains smaller grains that preferentially migrates to the geo-potential low (Binzel et al, 2015). Notably (Tardivel et al, 2018) predict the opposite trend, that deformation during spin-up would generate a rocky equator and sandy tropics. We lack constraints on the depth of Bennu's regolith layer, however the asteroid porosity and near spherical shape (implying an absence of embedded monoliths) could be consistent with thick layers of regolith, subsurface rubble or porous and cracked rock.…”