This chapter reviews key findings from analyses of spectral reflectance measurements of Mercury taken by the MESSENGER mission. Mercury’s crust lacks the 1-µm crystal field absorption due to ferrous iron that is common on other silicate bodies, yet is unusually low in reflectance. The most likely darkening phase is carbon as graphite. Variations in reflectance and color reveal that volcanic plains averaging >5 km in thickness overlie graphite-rich low-reflectance material, which may have originated as a graphite flotation crust from a magma ocean. The one unambiguous absorption due to an oxidized transition metal, an ultraviolet oxygen–metal charge transfer band in bright, pyroclastic deposits, may originate by oxidation of carbon and sulfides, reducing 0.3–1 wt.% ferrous iron in silicates to a metallic state, unsaturating the very strong oxygen–metal charge transfer band.

The Alpha-Particle X-ray Spectrometer (APXS) is part of the scientific payload of all four Mars rovers to date. It determines the chemical composition of rocks and soils using X-ray spectroscopy during irradiation with alpha particles and X-rays from 244 cm. All elements heavier than fluorine can be detected by their characteristic X-ray lines. Typically, 16 elements are quantified for each martian sample. An additional 10 trace elements can be quantified for unusual high abundances. The APXS has provided compositional data at 4 landing sites, analyzing more than 1000 samples along a combined traverse of ~70 km. The diverse composition of soils and rocks has provided insights about martian geology and environmental conditions. Soils at all landing sites are similar and basaltic, but enriched in S, Cl, and Zn, likely from volcanic exhalations. A variety of igneous rocks have been documented. High sulfur concentrations in Ca sulfate veins, ferric sulfate subsurface soil deposits, and the extensive Burns formation with ~30% sulfate indicate extensive interactions with acidic fluids in the past. APXS bulk geochemistry complements mineralogy data and images and delivers crucial constraints for the interpretation of other investigations, like ground truth for orbital remote sensing instruments or comparison with martian meteorites.

The advent of multiple orbital and in situ missions to planetary bodies beyond Earth has enabled characterization of extraterrestrial shallow crustal processes. We describe examples of interpreting geochemical, isotopic, and radar properties from multiple remote datasets, supplemented with in situ observations from rovers and landers, meteorites, and lunar samples. Given the availability of distinct data types and the relevance to bulk-silicate bodies in the Solar System, we present five case studies for the Moon and Mars. The first involves lunar magmatic processes in relation to TiO2 and radargram-derived physical properties. Next, O and Fe isotope variations relative to the Mg number provide insight into the degree of fractional crystallization in lunar lava flows. Physical mixing of endmembers and chemical weathering processes in Gusev crater soil on Mars are discussed. Effective use of the Chemical Index of Alteration (CIA) is also considered by comparing mineralogic observations across Mars with terrestrial references. Lastly, the nature of bulk soil hydration on Mars is described by assessing chemical variations with Principal Component Analysis (PCA). This chapter describes in situ analyses and mapping across local and regional scales. Data synthesis also involves contrasting depth scales from tens of microns to multiple kilometers.