Gamma Rays, Meteorites, Lunar Samples, and the Composition of the

orbiter in 1997-1998 allowed scientists to measure the concentrations of several elements on the entire lunar surface. The data have been widely used by planetary scientists to determine the chemical composition of the Moon and infer something about the processes operating when it formed. However, specialists in the study of lunar samples have been a bit uneasy about the details of the elemental compositions and have offered modest, but significant, corrections to the gamma ray data to make them more in line with what we know from samples. The latest of these approaches to correcting the gamma-ray data has been done by Paul Warren (University of California, Los Angeles), a renowned lunar sample specialist. He concentrated on correcting the analysis for the element thorium (Th), whose natural radioactive decay releases characteristic gamma rays. Thorium is an important element because we understand its behavior during the formation and subsequent evolution of magma, and because it is a refractory element-that is, it condenses at a high temperature from a gas. This means that if you know the thorium concentration, you also know the concentrations of all other refractory elements with similar geochemical behavior, which includes the rare earth elements, uranium, zirconium, titanium, calcium, and aluminum. Using his revised global thorium concentration as a springboard, Warren then estimated the concentration of numerous elements in the entire rocky portion of the Moon, which makes up more than 95 % of the orb that graces the night sky. His estimates do not agree with those produced by others, which will lead to continued debate and refinement of the Moon's chemical composition. References