New insights into the regional and local geological context of the Luna 16 landing site

Abstract

The results of our regional and local studies of the Fecunditatis basin and Mare Fecunditatis, and areas adjacent to the Luna 16 landing site suggest the following. If the Fecunditatis basin is an impact structure, which is consistent with the crustal thickness models and gravity data, it predates the Nectaris basins (∼4.17 Ga). The unusually flat topographic profile of the Fecunditatis basin can be explained by viscous relaxation of the original topography of the basin and the subsequent filling of the basin by ejecta from the Nectaris and Crisium basins. The results of our crater size-frequency measurements within the mare domain of the Fecunditatis basin yield an absolute model age (AMA) of ∼3.81 Ga. Thus, the population of all recognizable craters within Mare Fecunditatis denotes the existence of an extensive episode/episodes of resurfacing that has erased the ancient crater population accumulated after the Fecunditatis impact event but before the mare emplacement. Our determinations of AMAs of the surface of Mare Fecunditatis (∼3.45 Ga) are consistent with most radiometric age determinations from the Luna 16 samples that are clustered at ages of ∼3.5–3.4 Ga. The current Mare Fecunditatis surface is likely to represent the latter episodes of volcanism, the main phase of which was active during this time interval. Although older radiometric ages have not yet been reported, the presence of older basalts in the Luna 16 sample should not be ruled out, taking into account the likely multiple sources of its materials and the complex nature of the emplacement of lavas in Mare Fecunditatis. The thickness of mare basalts in Mare Fecunditatis are highly variable. At the regional scale, thicknesses are most likely at least ∼1.5 km, in order to be able to hide a significant number of ancient larger craters. In the areas of partly or completely flooded craters, the mare thickness increases up to ∼2–2.7 km. In the vicinity of the Luna 16 landing site, however, the thickness of the most than a few hundred meters.