Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

Michel, Patrick; Ballouz, Ronald-Louis; Barnouin, Olivier S.; Jutzi, Martin; Walsh, K. J.; May, Brian Harold; Manzoni, C.; Richardson, D. C.; Schwartz, S. R.; Sugita, Seiji; Watanabe, Sei‐ichiro; Miyamoto, H.; Hirabayashi, Masatoshi; Bottke, W. F.; Connolly, H. C.; Yoshikawa, Makoto; Лауретта, Д. С.

doi:10.1038/s41467-020-16433-z

Cited by 108 publications

(114 citation statements)

References 58 publications

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“…It is also possible that the ridge is an artifact of the reaccumulation process following the catastrophic breakup of a parent asteroid in the main asteroid belt, which preferentially leads to a spheroidal or top‐like shape (Michel et al, 2020). The ridge on Bennu appears to be a geologically old feature (Walsh et al, 2019) based on the number of large (~50‐ to 100‐m diameter) craters superposing it, which supports the reaccumulation model of ridge formation.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2020

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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.

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Section: Discussionmentioning

confidence: 99%

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

et al. 2020

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“…An alternate possibility is an early formation of the inner underdense region and the sculpting of the original equatorial bulge in the aftermath of the catastrophic disruption and reaccretion that created Bennu ( 35 , 48 , 54 ). It is well known that the angular momentum of an accreting mass distribution will exert control over how the material settles and the morphology of its final shape.…”

Section: Discussionmentioning

confidence: 99%

“…The current observations do not clearly show such a signature, leading us to conclude that the underdense center may be a much older geophysical feature of the body, predating the current distribution of surface material toward the equatorial bulge. This indicates that the underdense center formed either at an earlier epoch or when Bennu was originally formed by accretion following the catastrophic disruption of its parent body (29,48). This failure mode would also naturally cause a reshaping of the body at the equator, where internal motion and the lower gravitational and rotational acceleration would lead to an oblate shape similar to the averaged profile (Fig.…”

Section: Characteristics Of Bennu's Centermentioning

confidence: 94%

Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu

et al. 2020

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The gravity field of a small body provides insight into its internal mass distribution. We used two approaches to measure the gravity field of the rubble-pile asteroid (101955) Bennu: (i) tracking and modeling the spacecraft in orbit about the asteroid and (ii) tracking and modeling pebble-sized particles naturally ejected from Bennu’s surface into sustained orbits. These approaches yield statistically consistent results up to degree and order 3, with the particle-based field being statistically significant up to degree and order 9. Comparisons with a constant-density shape model show that Bennu has a heterogeneous mass distribution. These deviations can be modeled with lower densities at Bennu’s equatorial bulge and center. The lower-density equator is consistent with recent migration and redistribution of material. The lower-density center is consistent with a past period of rapid rotation, either from a previous Yarkovsky-O’Keefe-Radzievskii-Paddack cycle or arising during Bennu’s accretion following the disruption of its parent body.

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“…The unique features of Bennu and Ryugu include the oblate shapes with equatorial ridges, the so-called top-shapes ( Figure 1). Re-accumulation processes after catastrophic disruptions may play a key role in the formation of top shapes and their precursor shapes [Michel et al, 2020]. These shapes may have further evolved into the complete form of top shapes at later stages.…”

Section: Introductionmentioning

confidence: 99%

Spin-driven evolution of asteroids' top-shapes at fast and slow spins seen from (101955) Bennu and (162173) Ryugu

Hirabayashi

Nakano

Tatsumi

et al. 2020

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Proximity observations by OSIRIS-REx and Hayabusa2 provided clues on the shape evolution processes of the target asteroids, (101955) Bennu and (162173) Ryugu. Their oblate shapes with equatorial ridges, or the so-called top shapes, may have evolved due to their rotational conditions at present and in the past. Different shape evolution scenarios were previously proposed; Bennu's top shape may have been driven by surface processing, while Ryugu's may have been developed due to large deformation. These two scenarios seem to be inconsistent. Here, we revisit the structural analyses in earlier works and fill a gap to connect these explanations. We also apply a semi-analytical technique for computing the cohesive strength distribution in a uniformly rotating triaxial ellipsoid to characterize the global failure of top-shaped bodies. Assuming that the structure is uniform, our semi-analytical approach describes the spatial variations in failed regions at different spin periods; surface regions are the most sensitive at longer spin periods, while interiors fail structurally at shorter spin periods. This finding suggests that the shape evolution of a top shape may vary due to rotation and internal structure, which can explain the different evolution scenarios of Bennu's and Ryugu's top shapes. We interpret our results as the indications of top shapes' various evolution processes. Highlights • We reanalyzed results from FEM analyses for asteroids Bennu and Ryugu, the targets of OSIRIS-REx and Hayabusa2, respectively. • We also applied a semi-analytical approach to quantify how the failure modes of top shapes vary with spin. • Top shapes may evolve by two deformation modes: surface processing at low spin and internal failure at high spin.

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Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

Cited by 108 publications

References 58 publications

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

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

Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu

Spin-driven evolution of asteroids' top-shapes at fast and slow spins seen from (101955) Bennu and (162173) Ryugu

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