Geology of Bennu's Equatorial Region

The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission is currently studying the surface of near-Earth asteroid (101955) Bennu in preparation for collecting a sample from its surface and returning it to Earth. Radar observations previously indicated that Bennu has a top-like shape (an oblate spheroid), and images obtained by the OSIRIS-REx spacecraft after arrival at Bennu confirmed this finding.

Approximately 1/3rd of near-Earth asteroids (NEAs) larger than 100m appear to have equatorial bulges or ridges based on radar observations. These features have been hypothesized to be related to rotational spin-up by the thermal YORP effect, as they are commonly found among some of the most rapidly rotating NEAs. Recent models have also postulated that a top-shape could be produced during rubble pile reaccumulation as part of their formation following a catastrophic asteroid disruption in the Main Belt.

On Bennu, some of the largest candidate craters, those over 100 m in diameter, are found in equatorial latitudes. These candidate craters are stratigraphically younger than the equatorial ridge, suggesting that the ridge is likely one of the oldest features on the surface of Bennu. However, there are relatively fewer craters found throughout the mid-latitudes, which suggests recent resurfacing that is supported by morphologic evidence of relatively recent equatorial mass movement. Similarly, the equatorial regions have, on average, significantly lower slopes than the mid-latitudes.

An old equatorial feature was likely present during Bennu's time in the Main Asteroid Belt. Either YORP spin-up drove the formation of the bulk shape long ago, or a different process, such as its immediate reaccumulation after disruption of the parent body, contributed or was responsible. Meanwhile, current processes continue to move material towards and trap material in the equatorial region, contributing to its distinction from the rest of Bennu's surface.

Acknowledgements: This material is based upon work supported by NASA under Contracts NNM10AA11C and NNG12FD66C issued through the New Frontiers Program. Marco Delbo, Patrick Michel and Stephen R. Schwartz acknowledge support from the Centre National d'Études Spatiales, as well as the Academies of Excellence on Complex Systems and Space, Environment, Risk and Resilience of the Initiative d'EXcellence 'Joint, Excellent, and Dynamic Initiative' (IDEX JEDI) of the Université Côte d'Azur.