Orbital gamma-ray data and large-scale lunar problems

Abstract

The orbital gamma-ray data obtained during Apollo 15 and 16 have been used to infer new information about the moon and to generalize some of the information provided by the study of lunar samples. These orbital data show that Mg, and to a lesser extent Fe, are major variables in the chemical composition of the early lunar crust. However, Fe does not attain high enrichments until the genesis of mare basalts later in lunar history. The orbital data show that the lunar highlands are not composed of a homogeneous material of anorthositic chemical composition. Rather, the petrogenetic processes associated with the lunar highlands have produced a wide range in Mg/Fe ratios. The variations in Mg/Fe ratios between large regions of the non-mare and highland areas of the lunar surface are nearly equal in magnitude to those seen in the non-mare and highland lunar samples. The chemical variations in lunar samples require extensive chemical differentiation to produce. Thus regional variations in the mode and/or extent of chemical differentiation exist and the early intense bombardment of the lunar surface has not destroyed this record. The central highlands and the high-Th regions immediately to the west (Lalande and Fra Mauro) differ markedly from other pre-mare and highland regions by having very high Mg/Fe ratios. Only Mendeleev has an equally high ratio but it has much lower Mg and Fe concentrations. The central highlands also differ from the farside highlands by having higher Mg concentrations and lower K/Th ratios. Some of the Apollo samples with high Mg/Fe ratios may be representative of the predominant materials in those areas that show the highest Mg/Fe ratios. The large depression on the lunar farside (Big Backside Basin) contains at least a two-component mixture of rock types: one type is probably the local highland material and the other is a basaltic material whose bulk composition approaches that of the frontside mare regions. The surface elevation in the Big Backside Basin is similar to that of the frontside mare and the high-Th regions. The high-Th areas and the mare areas (Cognitum and Imbrium) that are colinear on Fe vs. Th and K vs. Th plots all have about the same surface elevations. This suggests that the high-Th material (“KREEP”) was emplaced as lava flows on a pre-existing low surface. Mare Serenitatis has the lowest surface elevation of any mare included in this study and the chemical composition of its surface is nearer to that of pure mare basalts than that of other maria studied. Taken together with the thicker fill in Mare Serenitatis, this suggests that the thicker fill has resulted in a larger mare-basalt component in the regolith, i.e., vertical mixing has been the major means of mixing non-mare material with mare basalts to produce the mare regoliths.