Shape, spin, and stability of Bennu
Abstract
Images of asteroid (101955) Bennu acquired by the OSIRIS-REx mission reveal a rocky world covered in rubble, including numerous boulders with diameters up to tens of meters. The geologic evolution of Bennu is driven in large part by downslope migration of surface material and rubble, which may be dislodged from an initial state by number of processes, such as YORP-induced spin-up, re-accumulation, impact-induced seismic shaking, thermal stresses, or tidal disruption by close encounters with larger bodies.
Shape models of Bennu have been developed from images using stereophotoclinometry and from lidar data collected by the OSIRIS-REx Laser Altimeter. A spherical harmonic decomposition of the shape reveals a strong contribution at degree 4. The zonal component is largely due to the equatorial ridge typical of top-shaped asteroids, but there is also a strong sectoral component that reflects high-standing longitudinal ridges, which are most obvious as a "squarish" outline as viewed from the poles. At higher degrees (>10), the power in the topography drops off more slowly than would be predicted from a Kaula-type rule. This flatter curve may indicate that the long-wavelength (> 80 m) shape is dominated by internal structure and terracing, and that the short-wavelength topography is dominated by a relatively scale-invariant rock distribution. Although unconsolidated, Bennu is not completely strengthless because the shape deviates from a hydrostatic surface. Previous analysis of the shapes and spins of small bodies indicate that these properties will gradually evolve towards a condition of maximum topographic stability; that is, a state of low topographic variation, low slopes, and low surface erosion (mass-wasting) rates. However, Bennu is already rotating faster than the optimum rate for this stability condition, and the YORP effect is further increasing the rotation rate. Rotational stability analysis demonstrates that an internal friction angle of at least 18° is necessary to prevent Bennu from failure via mass wasting in the absence of cohesion. However, if the observed increase in rotation rate of 2.7×10-5 s-1 / Myr persists, in less than 0.6 Myr, no amount of internal friction would be sufficient to prevent despinning and disruption of Bennu, and significant cohesion would be required at faster rates.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P51A..01R
- Keywords:
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- 6218 Jovian satellites;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6299 General or miscellaneous;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5480 Volcanism;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 5499 General or miscellaneous;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS