More Surprises from the Moon

Even with the naked eye, the dark, extensive plains of the lunar maria can be clearly seen on the surface of the Moon. The maria formed after meteorite impacts created large craters that later filled with lava flows. Mare volcanism is the dominant type of volcanic activity on the Moon and the lavas are made up of basaltic rocks. However, non-mare volcanic deposits, though rare, have been observed on the lunar nearside. The deposits are distinguished from the maria because they are compositionally more evolved rich in silica, potassium and thorium. The deposits are limited in surface extent and it was unknown whether similar non-mare volcanism occurred at all on the Moon s farside. Writing in Nature Geoscience, Jolliff et al. report using Lunar Reconnaissance Orbiter images and compositional data to identify the rare occurrence of more compositionally evolved volcanic deposits in an isolated area on the Moon s farside. In the 1960s and 1970s, rock and soil samples were collected by the Apollo and Luna missions, by the USA and USSR respectively. This material represents a geologic treasure trove that continues to provide a wealth of information about the Moon and its evolution, and it was a very small fraction of these samples that gave the first hint that non-mare volcanic activity might have occurred. The samples contained fragments of complex volcanic rocks that were unrelated to the maria basalts. Violent bombardment of the Moon by meteorite impacts has caused significant mixing of the rocks at its surface, so the fragments could have had a source hundreds or thousands of kilometres away. The origin of the fragments was unknown. Several decades later, the Lunar Prospector mission used a gamma-ray spectrometer to map the distribution and abundance of various elements, including thorium, on the Moon s surface. The maps identified a distinct and large area of high thorium concentration, as well as several smaller, but equally peculiar areas of high thorium concentration on the nearside of the Moon (Fig. 1). The rocks that contained high thorium contents also exhibited geomorphological and spectral features that were typical of volcanic deposits, and so the thorium hotspots were thought to represent non-mare volcanism on the nearside of the Moon. A relatively large region with extremely high thorium concentrations - the Compton-Belkovich thorium anomaly - was also identified on the Moon s farside. This thorium hotspot was particularly unusual because it was completely isolated, alone on the farside of the Moon, far from the nearest maria. No high-resolution image data were available for this region, so a definitive interpretation of the source of this isolated anomaly has been impossible.