Regolith migration on asteroid 101955 Bennu inferred from boulder alignment

J.R. Marshall, C. Beddingfield, L.K. Fenton, K. Walsh, D.S. Lauretta

Planetary and Space Science
In Press, Journal Pre-proof, Available online 21 February 2023


“The alignment of elongated-boulder long axes was measured for the surface regolith of asteroid Bennu in order to quantify patterns of regolith fabric that might shed light on the geophysical and geomorphological history of the asteroid. We found a preferred north-south alignment of boulder long-axis orientations in mid-latitudes (±∼20°–60°) suggesting that regolith is or has been migrating towards the equator following latitudinal lines. Our baseline premise is that migration is likely a manifestation of centrifugal forces resulting from Bennu’s relatively high spin rate. From a comparison with terrestrial regolith analogs (colluvial materials) it is inferred that this alignment is indicative of a horizontally extensional stress state in mid latitudes. In contrast, we found a chaotic alignment of boulders in the equatorial latitudes ± ∼0°–20° implicating a compressional stress regime that has caused piling and upward accumulation of regolith to form part of, if not the whole of Bennu’s equatorial ridge. Our observations of boulder alignments and analyses of regolith stresses lead us to the proposition that horizontal drift of regolith in latitudes ±20-90° is primarily driven by the localized balance between gravity and horizontal shear – whereas regolith drift in equatorial regions ± ∼0-20° is driven primarily by shear forces transmittted longitudinally through boulder-to-boulder compressional contact.”