The nature of volatiles in the lunar regolith

Layer-by-layer analysis of the fine fraction of Luna-16 sample L1635 by X-ray photoelectron spectroscopy (XPS) revealed a zonal structure of its constituent particles. The outer zone, to a depth of 1200 Angstrom, is enriched with Si and Fe and depleted in refractory elements Mg, Ca, and Al as compared to the bulk composition of the sample. The second zone, in the depth range from 1200 to 4500 Angstrom, is of very unusual composition: it is enriched with carbon (up to 60 at. %) and zinc (up to 4 at. %) and contains some other volatiles. The lower zone, at depths from 4500 to 7000 Angstrom, is composed of a silicate material; however, the concentrations of refractory elements are noticeably higher than those in the near-surface layer. There is a difference in structural and chemical characteristics of most of the elements between different zones. These findings point to the exogenous origin of the enrichment of the studied sample with volatiles and are related to the fall of a volatile-rich body (comet or carbonaceous chondrite) on the Moon. The hypothesis of the cometary (carbonaceous-chondrite) impact explains the main patterns of the volatile distribution in the lunar regolith, including the formation of green and orange glasses from Apollo 15 and 17 collections