[P-PS03] Solar System Small Bodies: A New Frontier Arising Hayabusa 2, OSIRIS-REx and Other Projects

[PPS03-21] Modeling surface mobility mechanisms on a top-shaped near-Earth asteroid

★Invited Papers

*Ronald Ballouz¹、Nicola Baresi²、Olivier S Barnouin³、Carina A Bennet¹、Edward B Bierhaus⁴、Keara N Burke¹、Harold C Connolly⁵、Sarah T Crites²、Daniella DellaGiustitna¹、Erica Jawin⁶、Dante S Lauretta¹、Patrick Michel⁷、Derek C Richardson⁸、Dan J Scheeres⁹、Stephen Schwartz¹、Seiji Sugita¹⁰、Eri Tatsumi¹⁰、Florian Thuillet⁷、Kevin Walsh¹¹ (1.Lunar and Planetary Lab, University of Arizona, Tucson, AZ, USA、2.Institute of Space and Astronautical Science, Sagamihara, Japan、3.The Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA、4.Lockheed Martin, Boulder, CO, USA、5.Department of Geology, Rowan University, Glassboro, NJ, USA、6.Smithsonian National Museum of Natural History, Washington, DC, USA、7.UCA-OCA-CNRS, Lagrange Laboratory, Nice, France、8.Dept. of Astronomy, University of Maryland, College Park, MD, USA、9.Dept. of Aerospace Engineering Sciences, University of Colorado, Boulder, CO, USA、10.University of Tokyo, Tokyo, Japan、11.Southwest Research Institute, Boulder, CO, USA)

キーワード：Near Earth Asteroids, OSIRIS-REx, Hayabusa2

The advent of near-Earth asteroid (NEA) encounters with spacecraft has provided a view of planetary surfaces evolving on the smallest objects yet explored. The arrival of OSIRIS-REx and Hayabusa2 at their target asteroids (101955) Bennu, with average diameter (d) = 0.49 km, and (162173) Ryugu (d = 0.90 km), revealed cratered top-shaped worlds with signs of surface mobility [1, 2]. Compared to larger planetary bodies, Bennu and Ryugu’s small sizes and gravities challenge the conventional understanding of surface modification mechanisms, such as the redistribution of boulders on the surface or the erasure of craters.

On the larger NEA (433) Eros (d = 16.8 km), boulders were spatially correlated with impact craters, which degrade through impact-induced seismic shaking [3]. However, our poor understanding of rubble-pile interiors [4], combined with processes that operate more efficiently on smaller NEAs (such as thermal re-radiation torques, i.e., the YORP effect [5]), introduces new complexities to understanding surface mobility on Bennu and Ryugu.

Here, we first present evidence of a dynamic surface on Bennu and Ryugu. Then, we present an analysis of the dynamical processes that may dominate the surface modification of small top-shaped NEAs. We model the redistribution of material on the surface due to different input energy sources: i) impact-induced seismic shaking, ii) YORP spin-up, iii) impact cratering, and iv) planetary encounters. We use a combination of techniques that allows us to approach the problem at multiple length and time scales. The first step models the global effect of the energy sources using analytical techniques. In the second step, we model the fine grain scale interactions in a microgravity environment in short timescales using a particle dynamics code [6,7]. Finally, we propagate the accumulated effect of the fine-scale interactions by integrating individual surface modifications over the typical timescale of each individual process. The results are used to assess which of the above listed mechanisms are the most effective at transforming small NEA surfaces.

References:

1. Walsh et al. 2019, Nature Geoscience, Accepted.

2. Sugita et al. 2019, Science, Accepted.

3. Richardson, J.E. et al. (2005) Icarus, 179, 324–349.

4. Scheeres, D.J. et al. (2015) in Asteroids IV (eds. Michel, DeMeo, & Bottke), pp. 745–766.

5. Bottke, W.F. et al. (2006) Annual Reviews of E&PS, 34, 157–191.

6. Richardson, D.C. et al. (2011) Icarus, 212, 427–437.

7. Schwartz, S.R. et al. (2012) Granular Matter, 14, 363–380.



Acknowledgements:

This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program and from Grant no. 80NSSC18K0226 as part of The OSIRIS-REx Participating Scientist Program. P.M. and F.T. P.M. acknowledge funding support from the French space agency CNES and from Academies of Excellence: Complex systems and Space, environment, risk, and resilience, part of the IDEX JEDI of the Université Côte d’Azur.