1Steven A.Binnie, 1Kunihiko Nishiizumi, 1Kees C.Welten, 2,3Marc W.Caffee, 4Dirk Hoffmeister
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.09.036]
1Space Sciences Laboratory, University of California, Berkeley, 7 Gauss Way, Berkeley, CA 94720-7450, USA
2Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907-1306, USA
3Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907-1306, USA
4Institute for Geography, University of Cologne, Otto-Fischer-Str. 4, 50674, Cologne, Germany
Measurements of cosmogenic 10Be, 26Al and 36Cl in Apollo 16 double drive core 68002/68001 are combined with a high resolution digital surface model of the sampling site to investigate the surface processes on the Moon. We find both a significant deficit of solar cosmic ray (SCR)-produced 26Al and a lack of SCR-produced 36Cl in the top 3-5 g/cm2 of the lunar regolith. The topographic model shows the core was taken from just inside a crater with a rim diameter of 25-30 cm. These observations are consistent with regolith removal and displacement by a shallow impact that occurred on the order of 100 kyr ago, or less. Our findings are also compatible with shallow mixing, or gardening, of the lunar regolith to depths of a few cm, a value often found in other lunar cores over the ∼106 yr averaging times of 26Al and 53Mn measurements. More definitive regolith mixing depth estimates are not possible due to the likelihood of disturbance in the top of the core as a result of sampling and/or handling. Our results support the hypothesis that the lunar surface experiences more frequent disturbances by small primary and secondary impacts than has previously been assumed. Additionally, we find no evidence that fine-grained ejecta from the 2.0 Myr old South Ray Crater impact reached this site. If the layer of fine-grained ejecta that reached the sampling site from the South Ray Crater was no more than a few cm thick, this absence can be explained by the erosion that formed the small, relatively recent crater at the coring location.