Figure 3. The K/Th ratio of the terrestrial planets and the Moon. Mercury’s ratio, determined from MESSENGER Gamma-Ray Spectrometer (GRS) measurements (Peplowski et al., 2011, 2012a) is similar to that of the other planets, and an order of magnitude higher than that of the volatile-depleted Moon. Adapted from the original image, provided by NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.