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Prolonged rock exhumation at the rims of kilometer-scale lunar craters
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  • Cole A Nypaver,
  • Bradley J Thomson,
  • Caleb I. Fassett,
  • Gerald Patterson,
  • Edgard G Rivera-Valentín
Cole A Nypaver
University of Tennessee

Corresponding Author:cnypaver@vols.utk.edu

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Bradley J Thomson
University of Tennessee
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Caleb I. Fassett
Marshall Space Flight Center (NASA)
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Gerald Patterson
Johns Hopkins University Applied Physics Laboratory
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Edgard G Rivera-Valentín
USRA/Lunar and Planetary Institute
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Fresh impact ejecta deposits on the lunar surface can be characterized as heterogeneous mixtures of boulders, cobbles, and fine-grained regolith that are deposited on the lunar surface during the impact crater formation process. Over time, the boulders associated with ejecta deposits break down into fine-grained regolith due to a combination of bombardment and thermal fatigue. Several qualitative observations of old (>2.0 Ga) kilometer-scale lunar impact ejecta deposits made here in high-resolution images reveal tens of large (>1 m) boulders associated with kilometer-scale crater rims and near-proximal ejecta deposits on the lunar maria. These observations went undescribed in prior measurements of lunar boulder breakdown which suggested that lunar boulders should be destroyed in <300 Myr due to micrometeoroid impacts and other processes (e.g., Basilevsky et al., 2015). Here, we use a combination of radar and thermal-infrared data from the Lunar Reconnaissance Orbiter spacecraft to show that kilometer-scale impact crater rims exhibit elevated rock abundances for the lifetime of the lunar maria. We interpret these results as indicating that boulders are continually being uncovered at crater rims due to downslope movement of the overlying regolith. Moreover, rocks found at crater rims that have been exhumed from depth in geologically recent times are locally derived and unlikely to have come from other areas of the Moon. Future collection of lunar samples at crater rims will serve to mitigate the potential for sample contamination from distal sources, helping to ensure accurate geologic interpretations from the collected samples.
Jul 2021Published in Journal of Geophysical Research: Planets volume 126 issue 7. 10.1029/2021JE006897