Remote Chemical Sensing Using Nuclear Spectroscopy 781
FIGURE 10 (Continued)A large portion of the western near side is enriched in
radioactive elements such as Th (Fig. 10c). K, Th, and U are
incompatible with major lunar minerals and were likely con-
centrated in the residual melt during lunar differentiation.
Consequently, their distribution on the surface and with
depth has important implications to lunar evolution. The as-
sociation of high Th concentrations with the mare suggests
that heating by radioactive elements may have significantly
influenced lunar thermal evolution and mare volcanism.
The distribution of TiO 2 is shown in Fig. 10d as a 5◦
equal area map. The low spatial resolution of the TiO 2 map
compared to FeO and Th is a consequence of the rela-
tively low intensity of the Ti gamma rays and their posi-
tion in the gamma ray spectrum near strong peaks from O
and Fe (Fig. 9). The abundance of TiO 2 can be used to
classify mare basalts. Strong spatial variations in the abun-
dance of TiO 2 , for example, indicate that different source
regions and processes were involved in creating the basalts