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

Abstract

The exploration of near‐Earth asteroids has revealed dynamic surfaces characterized by mobile, unconsolidated material that responds to local geophysical gradients, resulting in distinct morphologies and boulder distributions. The OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer) mission confirmed that asteroid (101955) Bennu is a rubble pile with an unconsolidated surface dominated by boulders. In this work, we documented morphologies indicative of mass movement on Bennu and assessed the relationship to slope and other geologic features on the surface. We found globally distributed morphologic evidence of mass movement on Bennu up to ~70° latitude and on spatial scales ranging from individual boulders (meter scale) to a single debris flow ~100 m long and several meters thick. The apparent direction of mass movement is consistent with the local downslope direction and dominantly moves from the midlatitudes toward the equator. Mass movement appears to have altered the surface expression of large (≥30m diameter) boulders, excavating them in the midlatitudes and burying them in the equatorial region. Up to a 10 ± 1 m depth of material may have been transported away from the midlatitudes, which would have deposited a layer ~5 ± 1 m thick in the equatorial region assuming a stagnated flow model. This mass movement could explain the observed paucity of small (<50‐m diameter) craters and may have contributed material to Bennu's equatorial ridge. Models of changes in slope suggest that the midlatitude mass movement occurred in the past several hundred thousand years in regions that became steeper by several degrees.

Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu

“…This suggests that the trend in predominant fracture orientations should shift from N‐S near the equator toward NW‐SE and NE‐SW at higher and lower latitudes, respectively. Statistical mapping of the orientation of through‐going fractures observed on Bennu (Molaro, Walsh, et al, 2020; Walsh et al, 2019) is needed to assess whether this trend is present, though it may be influenced by mass movement (Barnouin et al, 2019; Jawin et al, 2020; Walsh et al, 2019). Such work may provide additional constraints on fatigue stress thresholds and crack propagation rates for carbonaceous chondrite materials.…”

“…Even if the ejection events are ultimately driven by impact processes, it is likely that the exfoliation layers play a strong role in the particle size distribution of impact ejecta both leaving and remaining on Bennu's surface. The popping up of an exfoliation layer's edge over a curved surface could also loft loose particles resting on the boulder (Jawin et al, 2020) from impact and/or other thermal mechanisms, even if the exfoliation flake itself does not disaggregate.…”

The Morphometry of Impact Craters on Bennu

“…The extent of degradation should be a function of both crater age (older craters should be more degraded than younger ones) and the level of geologic activity in an area (craters in regions prone to resurfacing should be more degraded than similarly aged craters in less active regions). Based on maps of mass movement events on Bennu, Jawin et al (2020) inferred that between ~5 and 10 m of material could have been removed from the lower midlatitudes and transported equatorward, potentially contributing to crater infilling and erasure along the way.…”

“…On Bennu, the steepest slopes are currently located between 30° and 70° latitude in both the northern and southern hemispheres, and these regions are the most prone to mass movement (Barnouin, Daly, Palmer, et al, 2019). The primary direction of mass movement on Bennu is from the polar and midlatitude regions toward the equatorial region, here defined as ±20° latitude (Jawin et al, 2020). The equatorial ridge is the ultimate sink for downslope movement of material (Daly et al, 2020).…”

“…These large craters are presumably some of the oldest on Bennu, which gives mass wasting and infilling processes a long time to operate. However, Walsh et al (2019) and Jawin et al (2020) found that the only large crater with signs of infilling, which they attributed to mass wasting, is the one shown in Figure 1f. Degradation and infilling thus are unlikely to be the sole causes of this narrow range.…”