[P-PS04] Regolith Science

[PPS04-09] Global Geology of Bennu from NASA's OSIRIS-REx Space Mission

*Kevin John Walsh¹、Erica R Jawin²、Timothy McCoy²、Harold Connolly³、Dante S Lauretta⁴、Ronald-Louis Ballouz⁴、Olivier S Barnouin⁵、Chloe Beddingfield⁶、Carina Bennet⁴、Edward Bierhaus⁷、Keara Burke⁴、Benton Clark⁸、Michael Daly⁹、Marco Delbo¹⁰、Daniella DellaGiustina⁴、Jason Dworkin¹¹、Christine Hartzell¹²、John Marshall⁶、Patrick Michel¹⁰、Jamie Molaro¹³、Michael Nolan⁴、Maurizio Pajola¹⁴、Michael Perry⁵、Bashar Rizk⁴、Scott Sandford¹⁵、Daniel Scheeres¹⁶、Steven Schwartz⁴、David Trang¹⁷ (1.Southwest Research Institute Boulder、2.Smithsonian National Institute、3.Dept. of Geology, Rowan University、4.Lunar and Planetary Laboratory, University of Arizona、5.Johns Hopkins University Applied Physics Laboratory、6.SETI Institute、7.Lockheed Martin Space、8.Space Science Institute、9.York University、10.Laboratoire Lagrange, Université Côte d’Azur、11.NASA Goddard Space Flight Center、12.Department of Aerospace Engineering, University of Maryland、13.Planetary Science Institute、14.INAF - Astronomical Observatory of Padova、15.NASA Ames Research Center、16.Smead Dept. of Aerospace Engineering Sciences, University of Colorado、17.HIGP/University of Hawaii at Manoa)

キーワード：Rubble Pile Asteroids, Asteroids, Craters

NASA’s OSIRIS-REx sample return mission has been studying near-Earth Asteroid (101955) Bennu since December 2018 to understand the nature of this body and to support site selection and sample acquisition. This effort has produced a wealth of data with which to constrain its geologic history. Here we report on the global geology of Bennu based on analyses of images combined with shape, mass, and slope measurements. Bennu is a 500-m-diameter object with an oblate spheroidal shape and an equatorial bulge that is similar to other near-Earth asteroids, including Ryugu, as observed by the Hayabusa2 sample return space mission. Candidate impact craters have been observed at a range of diameters, reaching up to 150 m, across the surface of Bennu. An apparent concentration of large craters at low latitudes suggest that the equatorial ridge is stratigraphically old.

The population of boulders is diverse, with many of the largest boulders being darker than the average surface albedo and exhibiting a hummocky texture. A smoother and brighter class of boulders occurs at smaller sizes, and the populations are mixed throughout the surface. Notably there is a distinct lack of very large boulders in the equatorial region (+/– 25 deg.). Only 2 of the 16 boulders larger than 30 m are located in this band. Patterns of apparent mass movement across the surface suggest movement from the mid-latitudes towards the equator. There is evidence of imbrication in the local downslope direction, as well as evidence of mass movement at smaller particle sizes in the form of partial burial of boulders and pileup of boulders at locations downslope of scarps. Many of these signatures are found specifically in regions with high surface slope and in regions where the surface slope would have increased substantially as the asteroid experienced its observed increase in spin rate. The spatial distributions of craters, boulders, observed particle ejection events and signatures of mass movement suggest a dynamic recent history in some regions of Bennu’s surface, and we use these data to divine information about the most mobile elements of its surface and their? relation to the history of Bennu’s equatorial ridge.