Fine-grained regolith, a surface layer of unconsolidated granular material, has been considered to cover the surfaces of airless bodies and determine their spectral and thermophysical properties. However, surfaces of asteroids visited by recent sample return missions are dominated by centimetre- to metre-sized boulders, indicating an active fine-grained regolith removal process at work. To understand asteroid regolith evolution, we apply recent space and laboratory experimental results to simulate regolith fragmentation processes, loss by impact ejecta, and electrostatic removal of fine-grained regolith. Here we show that coarse, boulder-rich scenery probably emerges on kilometre-sized and smaller asteroids within a few million years, as the electrostatic removal of fine-grained regolith dominates production by fragmentation. Surface thermal inertia enhancement associated with fine-grained regolith loss on small main-belt asteroids increases the Yarkovsky drift and the probability of orbital excursion to the near-Earth environment through resonances with giant planets. We suggest that the competition between electrostatic erosion and space weathering shapes the appearance of reflectance spectra of small asteroids, related to the size, spatial and spectral distributions of S-complex asteroids.
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Source data are provided with this paper. Modelling results presented in this work can be found in the source data files.
Parameters and equations needed to perform the calculation are described in Methods. The codes used in this study are available from the corresponding author upon reasonable request.
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This work is supported by the NASA/ROSES NNH15ZDA001N-SSW Solar System Workings programme (grant number NNX16AO81G) and the NASA/SSERVI Institute for Modeling Plasma, Atmospheres and Cosmic Dust. H.-W.H. thanks M. Delbo and W.-H. Ip for fruitful discussions and T.-W. Liu, C.-Y. Wang, Li. Hsu and Lu. Hsu for their support.
The authors declare no competing interests.
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A schematic illustrates the three processes considered in shaping the regolith size evolution on airless bodies.
Extended Data Fig. 2 Size distribution of electrostatically lofted particles from laboratory experiments.
The cumulative grain size distribution of lofted particles derived from laboratory experiments based on the results shown in Figure S2B in the Supporting Information of Wang et al. (2016). Two power-law size distributions are fitted to grain radius ranges 4 to 25 μm and 25 to 70 μm. The experiment was carried out using irregular shaped Mars simulants, 38 - 48 μm in diameter, under 120 eV electron beam irradiation. The fact that there are larger grains suggests the lofted grains are in form of aggregates.
The maximum lofting speed as a function of grain size and the relation with the escape probability of an electrostatic-lofted grain from a rotating body. The particle size range considered is from 1 to 100 μm.
Extended Data Fig. 4 Observable and modeled power-law size distribution indices considering partial coverage.
The size indices derived from Ryugu and Bennu images (Michikami et al., 2019, Burke et al., 2021) and the modeling results are also shown. The dash line marks a reference for an 1-to-1 relationship between the two.
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Hsu, HW., Wang, X., Carroll, A. et al. Fine-grained regolith loss on sub-km asteroids. Nat Astron 6, 1043–1050 (2022). https://doi.org/10.1038/s41550-022-01717-9