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A record of post-accretion asteroid surface mixing preserved in the Aguas Zarcas meteorite

Abstract

Particle ejection and redeposition events on the surface of asteroid 101955 Bennu, which led to transport, mixing and loss of material, have been observed frequently by NASA’s OSIRIS-REx mission. Besides large-scale impacts, this may be one of the most important post-accretional processes on small carbonaceous asteroids. Here we looked for relics of such activity in a Bennu analogue, the carbonaceous chondrite Aguas Zarcas. We discovered compact fragments that were strongly shocked, redistributed and deposited onto an unshocked lithology, consistent with surficial re-accretion on Aguas Zarcas’s parent body. Such re-accretion could be driven by large-scale impacts or by frequent pebble transport from endogenous asteroidal activity such as observed at Bennu. The latter hypothesis is supported by the matching size distribution of the Aguas Zarcas compact fragments with that of the Bennu ejecta. Such mixing has hitherto been unexplored in other regolith breccias, and further analysis will determine how common such processes are.

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Fig. 1: Slices from μCT image stacks of four types of rocks and BSE image of a polished compact AZ fragment.
Fig. 2: Chondrules fitted with ellipsoid shapes.
Fig. 3: Relationship between aspect ratio and shock pressure.
Fig. 4: Distribution of volume‐equivalent spherical diameters for particles ejected from asteroid Bennu and for the compact AZ fragments.
Fig. 5: Characteristics of modelled ejecta.

Data availability

All data needed to evaluate the conclusions in the paper are present in the paper and the Supplementary Information. The original μCT data are deposited in MorphoSource, a data repository specialized for 3D representations of physical objects. The DOIs for the μCT data from the University of Chicago’s PaleoCT Lab are as follows: RF-1 (10.17602/M2/M450464), RF-2 (10.17602/M2/M450467), RF-3 (10.17602/M2/M450470), CF-1 (10.17602/M2/M450431), CF-2 (10.17602/M2/M450434), CF-3 (10.17602/M2/M450437), CF-4 (10.17602/M2/M450440), CF-5 (10.17602/M2/M450443), CF-6 (10.17602/M2/M450446), CF-7 (10.17602/M2/M450449), CF-8 (10.17602/M2/M450452), CF-9 (10.17602/M2/M450455), CF-10 (10.17602/M2/M450458), and CF-11 (10.17602/M2/M450461). The DOIs for the μCT Data from the University of Texas High-Resolution X-ray Computed Tomography Facility (UTCT) are as follows: Leoville (10.17602/M2/M446809), Murchison (10.17602/M2/M446787), RF-1 (10.17602/M2/M446768), and CF-10 (10.17602/M2/M446740). Source data are provided with this paper.

Code availability

Monte Carlo simulations with MATLAB code are deposited in Knowledge@UChicago47, a repository hosted by the University of Chicago.

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Acknowledgements

P.R.H. thanks the Boudreaux family for donating Aguas Zarcas to the Field Museum’s Robert A. Pritzker Center. Funding from NASA’s Emerging Worlds program (80NSSC21K0389 to P.R.H. and 80NSSC21K0374 to A.M.D.) and from National Science Foundation (grant EAR-1762458 to UTCT Facility) is gratefully acknowledged. P.R.H., X.Y. acknowledge support for this project from the Field Museum’s Science Innovation Award and the TAWANI Foundation. We thank J. Holstein and K. Keating for help with sample preparation, J. Maisano for µCT data acquisition of samples acquired at the High-Resolution X-ray Computed Tomography Facility of the University of Texas at Austin (UTCT), G. Olack for maintaining the FIB-SEM facility at the University of Chicago and J. Greer for discussions regarding components of AZ and scientific illustration. X.Y. acknowledges support from UTCT for attending the UTCT Short Course: Quantitative Analysis with XCT.

Author information

Authors and Affiliations

Authors

Contributions

X.Y. and P.R.H. conceived the study and wrote the paper with input from all authors. R.D.H provided expertise on the data processing, interpretation and visualization. A.M.D contributed to the investigation and Monte Carlo model. A.I.N. conducted the initial μCT scanning of samples. X.Y. and P.R.H prepared the samples for SEM/EDS analysis and A.M.D helped explain the data.

Corresponding author

Correspondence to Xin Yang.

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Extended data

Extended Data Fig. 1 3D aspect ratios of fitted ellipsoids of chondrules in AZ fragments versus C parameters.

Two groups, regular AZ and compact AZ, are clearly separated, with RF-3 in the compact group. Error bars are one standard deviation (n = 22–85).

Source data

Extended Data Fig. 2 Back-scattered electron (BSE) images of regular AZ (left panel; Kerraouch et al. 7) and compact AZ (right panel; this study).

Both lithologies show the same texture, that is, chondrules at low abundance embedded in an aqueously altered matrix enriched with phyllosilicates (irregular patchy light grey material in the matrix between the chondrules). Right panel adapted from ref. 7 under a Creative Commons license CC BY 4.0.

Extended Data Fig. 3 BSE images of chondrules from the compact fragment CF-10.

a, Chondrule mainly consisting of forsterite (Fo), containing round sulfide grains. b, Radial pyroxene (Py) chondrule. c, Porphyritic olivine–pyroxene chondrule containing sulfide (Sul) grains. d, Porphyritic olivine–pyroxene chondrule containing sulfide veins and phyllosilicate (Phy).

Extended Data Fig. 4 Distributions of absolute locations (upper panel) and of displacement angles (lower panel) for redeposition onto a Bennu-like asteroid.

The modeled body has the same size and bulk density as that of Bennu (490 m in diameter and 1.26 g cm–3 in bulk density).

Source data

Extended Data Fig. 5 Schematic portrayal of the history of the formation of the Aguas Zarcas chondrite.

Fractures were generated in chondrules before or during the accretion of the parent body and were filled simultaneously by shock mobilization or later by thermal/aqueous alteration. Then a hypervelocity impact caused chondrule flattening and fracturing in the matrix, and the compact AZ lithology was formed. The compact AZ was fragmented by meteoroid impacts and thermal fracturing. Then particle ejection and reaccretion events redistributed rock fragments with distinct lithologies, mixing compact AZ into regular AZ. Later impacts consolidated the mixed lithologies and resulted in the final ejection.

Supplementary information

Supplementary Information

Supplementary discussion, Figs. 1–4 and Table 1.

Source data

Source Data Fig. 2

Statistical source data.

Source Data Fig. 3

Statistical source data.

Source Data Fig. 4

Statistical source data.

Source Data Fig. 5

Statistical source data.

Source Data Extended Data Fig. 1

Statistical source data.

Source Data Extended Data Fig. 4

Statistical source data.

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Yang, X., Hanna, R.D., Davis, A.M. et al. A record of post-accretion asteroid surface mixing preserved in the Aguas Zarcas meteorite. Nat Astron 6, 1051–1058 (2022). https://doi.org/10.1038/s41550-022-01746-4

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