The Mars Odyssey Gamma Subsystem (GS) underpins most of this narrative, sensing the planetary subsurface gamma photons. Midlatitudinal maps of Ca, Cl, Fe, K, H2O, Si, and Th mass fractions have been generated with the GS. We first determine whether GS estimates represent chemical composition on the ground by computing representative in situ compositions for Pathfinder, Spirit, Opportunity, and Viking 1 landing sites. GS estimates compare favorably with the in situ estimates for Cl and K. However, the GS-determined Fe content at each landing site is consistently higher than the in situ value. Nevertheless, these comparisons reassure us that the GS data are indeed representative of the actual surface of Mars.

Next, we describe statistical analysis methods with particular emphasis on comparing distributions, computing multivariate correlations, and modeling optimal predictor sets hierarchically. Examples of their application using the GS data set clarify their use in the geochemically oriented chapters that follow. Multivariate analyses indicate a remarkable correlation among K, Th, and the areal fraction of the mineralogically distinct surface type 2 (ST2).

With our chemical insight into ST2, we consider the likelihood of different genetic scenarios that have been proposed previously. Consistent with the multivariate results, we observe significant enrichment of both K and Th in regions representative of ST2. In addition, Si does not appear to be significantly enriched in ST2. These results are more consistent with ST2 originating from a compositionally distinct mantle source than the aqueous alteration of basalts.

Lastly, we delineate chemically striking regions to be analyzed with data from other missions. We also examine a Tharsis region marked by the enrichment of Cl and depletion of Fe and Si, and find it to overlap significantly with a radar stealth region. Surface dust observed at the two rover sites mixed with and indurated by Ca-bearing sulfate salts would be a reasonable chemical and physical analog to meter-scale depths. The sulfates may have been produced by regional-scale activity of ground-ice driven brines.