High enrichment of carbon and volatile elements in the surface layers of Luna 16 soil sample 1635: Result of comet or meteorite impact?

Surface analysis by X-ray photoelectron spectroscopy (XPS) in combination with Ar etching was performed on a lunar regolith sample from the Luna 16 drill-core. The sample contains only ∼10% of glass spherules; these are of highly variable composition, which distinguishes them from the pristine (volcanic) lunar glasses with their restricted compositional range. The XPS analyses revealed that practically all grains in this sample are coated by layers of different chemical composition. The outer 1200 Å have a normal Fe,Mg-silicate composition. The middle zone between 1200 and 4400 Å is strongly enriched in carbon (up to 60 at%) and in Zn (4 at%). From the electron binding energy of the carbon atoms, it can be concluded that most of the carbon is present in the form of nanodiamonds and graphite. Other volatile elements (Cd, Pb, Ga, P, and Cl) were detected at the 0.1–0.2 at% level in this zone. The inner zone below 4400 Å has again a silicate composition, but is more refractory than the outer zone, i.e. higher in Ca and Al. We propose that this layering was formed by condensation from a carbon-rich cloud, originating from the impact of a comet or a carbonaceous chondrite-like body. XPS analysis of the surface layers showed variations in the chemical state of the main elements (Mg, Si, Al, Ca, Fe) in these layers, corresponding to different molecular structures. Similar variations were found in condensate films produced in laser-pulse impact simulation experiments.