Quantifying secular variations in the chemical composition of the Martian crust provides unique insights into the processes that have guided the evolution of the Martian crust‐mantle system. Using global abundances for a suite of elements determined by the Gamma Ray Spectrometer (GRS) on board the Mars Odyssey spacecraft and global mapping of apparent surface age adapted from existing geologic maps in the USGS Martian Geologic Investigation series, we report the average abundance of K, Th, Fe, Cl, H, and Si for the major Martian geologic epochs (Noachian, Hesperian, and Amazonian). Average GRS‐determined K and Th abundances generally decrease by 9% and 7%, respectively, between the Hesperian and the Amazonian, possibly implying evolving magma chemistry throughout major resurfacing events (although the effects of surficial alteration processes cannot be entirely discounted). GRS‐determined Fe and Cl averages increase by 12% and 19%, respectively, with younger apparent relative surface age, suggesting the possible mobilization and transport of these elements through aqueous processes (although an igneous origin for the variation in Fe also cannot be excluded). While H abundance does vary with surface age, the relationship is likely not governed by geologic processes. No statistically reliable apparent surface age relation was found for Si.

Gamma ray spectra measured by the Mars Odyssey gamma ray spectrometer while in orbit around Mars were analyzed to identify the sources of 334 observed peaks and features. Most peaks were of a standard Gaussian shape with a low‐energy tail. However, Doppler‐broadened and sawtooth‐shaped peaks were also observed in the spectra. The sources of most peaks were identified. Many peaks come from the gamma ray detector material or materials around the detector, particularly Ti and Mg. Identifications were often confirmed by analyzing other spectra, such as those measured during the cruise to Mars, before and after the boom deployment, at the end of solar particle events, and over thick frozen CO2 polar caps during winters.

Analysis of data acquired by the Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument led to the identification of two distinct surface types.“Surface type 1,” lying mostly within southern low‐albedo terrain, is likely to be composed of basalt. However, there are several competing mineralogical models for “surface type 2,” which is found primarily in northern low‐albedo terrain. We attempt to identify which models better agree with element concentrations determined through the Gamma Subsystem (GS) of the Gamma Ray Spectrometer (GRS) instrument suite aboard the Mars Odyssey spacecraft. We have binned GS data over threshold regions derived from maps of surface types 1 and 2 and examined the spatial correlation of element concentrations with surface type 2. The elements we have examined are Cl, Fe, H, K, Si, and Th. Our results show that K concentration, Th concentration, and the areal abundance of surface type 2 are strongly correlated, with significant enrichment of both K and Th in regions representative of surface type 2. In addition, Si does not appear to be significantly enriched in surface type 2. These results are more consistent with surface type 2 originating from a compositionally distinct mantle source than the aqueous alteration of basalts.