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- Title
Prolonged Rock Exhumation at the Rims of Kilometer‐Scale Lunar Craters.
- Authors
Nypaver, Cole A.; Thomson, Bradley J.; Fassett, Caleb I.; Rivera‐Valentín, Edgard G.; Patterson, Gerald W.
- Abstract
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 surface boulders associated with ejecta deposits break down into fine‐grained regolith due to a combination of later impacts and thermal fatigue. Nonetheless, observations of old (>2.0 Ga) kilometer‐scale (0.8–2.0 km) lunar impact craters in high‐resolution images reveal >1 m boulders along their rims and in their near‐proximal ejecta deposits on the lunar maria. Here, we use a combination of radar and thermal‐infrared data from the Lunar Reconnaissance Orbiter spacecraft to show that the rims of kilometer‐sized impact craters 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. Plain Language Summary: Any asteroid or comet that strikes the surface of the Moon will produce and deposit a mixture of large rocks and fine‐grained soil, known as ejecta, on the lunar surface. The result of the numerous impacts of all scales that have occurred on the Moon is that the entire surface is covered by a regolith: a layer made up of dust, sand, and pulverized rocks. Exposed rocks on the lunar surface are broken down over time and reduced in size, likely due to impact from other meteoroids and thermal expansion and contraction. Prior studies have observed that rock breakdown typically takes no longer than ∼300 million years for >2 m boulders at the surface. Here, we observe a population of boulders present at the rims of 2–3 billion year old impact craters. Reconciling these observations is possible if rocks at the rims of lunar impact craters are being continually uncovered due to the downslope movement of overlying lunar regolith. Moreover, because they are uncovered from the subsurface, rocks at crater rims are less likely to have undergone transport from another part of the Moon, which makes them an attractive potential source for future lunar samples. Key Points: Rocks at kilometer‐scale impact crater rims are continually being uncovered due to the downslope movement of the overlying regolithTopographic rims associated with 0.5–2.0 km diameter lunar impact craters exhibit recently exhumed boulders for >3.0 GaLunar sample collection at impact crater rims may yield material that is unlikely to have undergone transport from other areas of the Moon
- Subjects
EXHUMATION; LUNAR craters; IMPACT craters; LUNAR surface; LUNAR exploration
- Publication
Journal of Geophysical Research. Planets, 2021, Vol 126, Issue 7, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2021JE006897