Advanced spectroscopic sensors recently flown to the Moon have revealed unexpected discoveries about Earth’s nearest neighbor as well as provided detailed insights and constraints about how early crust evolves on an airless planetary body. Discussed here are (a) global assessment of the variety and distribution of major lunar mineral components and lithologies; (b) some of the remarkable new findings, such as the pervasive presence of OH across the surface and new rock types identified (Mg-spinel anorthosite) that are not identified in current lunar samples; and (c) expectations for the future as additional modern sensors provide a stronger foundation for remote compositional analysis of the Moon. Spectroscopic data continue to provide the cornerstone for identifying and understanding the regional and global character of lunar compositional variations and document key products and processes of crustal evolution.

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.

Stand-off Raman spectroscopy is emerging as a critical new tool for planetary exploration. Mounted on a rover, a stand-off Raman system can be used to rapidly identify areas of interest for subsequent, synergistic investigations with other stand-off or close-up (arm-mounted) instruments; survey broad areas and perform reconnaissance tasks from a fixed location; and increase the efficiency of mission operations where targets of interest are in areas that are too hard to access for a rover. Not surprisingly, NASA’s next Mars mission will fly a stand-off Raman system as part of the SuperCam instrument package. This chapter reviews two stand-off Raman systems that paved the way for the development of new technologies and instrument architectures for robotic stand-off planetary exploration using Raman spectroscopy, including the SuperCam instrument suite.

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 first Laser-Induced Breakdown Spectroscopy (LIBS) instrument for extraterrestrial applications is part of the ChemCam instrument suite onboard the Curiosity Mars rover. ChemCam may be used in a number of operational modes depending on the science questions of interest, including active (with laser) and passive (spectrometers only) modes, and there is important synergy between ChemCam and other payload instruments. Notable discoveries made with ChemCam LIBS data include the characterization of hydrogen in rocks and soils, discovery of boron on Mars, and characterization of other trace elements (Li, F, Rb, Sr, Ba) that were previously never or rarely quantified on Mars, depth-dependent chemical trends on rock surfaces, and a much broader range of bulk-rock chemical compositions than was previously recognized, including highly evolved igneous rocks. In addition to ChemCam, another LIBS instrument is slated to fly to Mars on the Mars 2020 rover mission as part of the combined Raman-LIBS SuperCam instrument.

A variety of features in the visible and near-infrared regions that are observed in remote sensing applications are the result of electronic transitions, typically involving cations of transition metals, most commonly Fe and Ti, or the molecular species S. The position and intensity of these features are sensitive not only to the particular cation, but also to its oxidation state, the particular phase in which it occurs, the geometric structure of the site that it occupies, and interactions between and among neighboring cations. Often these features are diagnostic for the host mineral.

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.

Thermal infrared data collected by the Thermal Emission Spectrometer (TES) and Thermal Emission Imaging System (THEMIS) instruments have significantly impacted the understanding of martian surface mineralogy. Spatial/temporal variations in igneous lithologies; the discovery of quartz, carbonates, and chlorides; and the widespread identification of amorphous, silica-enriched materials reveal a planet that has experienced a diversity of primary and secondary geo-logic processes including igneous crustal evolution, regional sedimentation, aqueous alteration, and glacial/periglacial activity.

Multispectral imaging – the acquisition of spatially contiguous imaging data in a modest number (~3–16) of spectral bandpasses – has proven to be a powerful technique for augmenting panchromatic imaging observations on Mars focused on geologic and/or atmospheric context. Specifically, multispectral imaging using modern digital CCD photodetectors and narrowband filters in the 400–1100 nm wavelength region on the Mars Pathfinder, Mars Exploration Rover, Phoenix, and Mars Science Laboratory missions has provided new information on the composition and mineralogy of fine-grained regolith components (dust, soils, sand, spherules, coatings), rocky surface regions (cobbles, pebbles, boulders, outcrops, and fracture-filling veins), meteorites, and airborne dust and other aerosols. Here we review recent scientific results from Mars surface-based multispectral imaging investigations, including the ways that these observations have been used in concert with other kinds of measurements to enhance the overall scientific return from Mars surface missions.

Spectral modeling techniques have been developed for the analysis of planetary surfaces using large thermal infrared (TIR) spacecraft datasets. These techniques can be applied to three main spectral analysis problems: (1) correction for atmospheric effects for the recovery of surface emissivity; (2) isolation and separation of surface spectral endmembers for the characterization of surface mineralogy; and (3) determination of surface anisothermality for the retrieval of surface physical properties and correction for thermal emission in near-infrared spectral data. These modeling techniques have been extensively applied to martian and lunar spacecraft datasets, forming a basis for the retrieval of surface physical and compositional properties.

This chapter provides a brief review of missions using X-ray, gamma-ray, and neutron spectroscopy to determine the chemical composition of planetary surfaces. This chapter presents the history of planetary radiation measurements, including significant discoveries. Summary tables with links to the archived data provide a resource for readers interested in working in this field. Upcoming missions and possible future directions are described.

An ever-increasing number of laboratory facilities are enabling in situ spectral reflectance measurements of materials under conditions relevant to all the bodies in the Solar System, from Mercury to Pluto and beyond. Results derived from these facilities demonstrate that exposure of different materials to various planetary surface conditions can provide insights into the endogenic and exogenic processes that operate to modify their surface spectra, and their relative importance. Temperature, surface atmospheric pressure, atmospheric composition, radiation environment, and exposure to the space environment have all been shown to measurably affect reflectance and emittance spectra of a wide range of materials. Planetary surfaces are dynamic environments, and as our ability to reproduce a wider range of planetary surface conditions improves, so will our ability to better determine the surface composition of these bodies, and by extension, their geologic history.

A Miniature Thermal Emission Spectrometer (Mini-TES), based on a Michelson interferometer and Cassegrain telescope, was carried by the Spirit rover in Gusev crater and Opportunity rover at Meridiani Planum to determine the bulk mineralogy of surface materials. Spectra from the plains of Gusev demonstrate the ubiquity of olivine-rich basaltic rocks, with additional examples lofted into the adjacent Columbia Hills by meteoroid impacts. Hundreds of rocks observed with mini-TES in the Columbia Hills display spectral characteristics of variable alteration intensity, but likely with very little water involved. Rare exceptions include a tephra deposit cemented by Mg–Fe carbonates and nodular opaline silica rocks, likely indicative of a hot spring/geyser environment. Opportunity’s mini-TES confirmed orbital identification of crystalline hematite at Meridiani Planum and spectral characteristics indicative of a transition from a precursor goethite phase. The sedimentary bedrock that hosts the hematite has spectral features consistent with Al-rich opaline silica, Mg-, Ca-, and Fe-bearing sulfates, plagioclase feldspar, and nontronite. Rare rocks at both sites are recognizable as iron meteorites from their infrared reflective properties.

Visible to short-wave infrared (VSWIR, 0.4–5.0 µm) reflectance spectroscopy is a powerful tool to identify and map mineral groups on the martian surface. The Mars Express/OMEGA and Mars Reconnaissance Orbiter/CRISM instruments have characterized more than 30 mineral groups, revolutionizing previous understanding of martian crustal composition and the role of water in altering it. Analyses of these spectral images revealed the primary structure of the crust to be dominated by basalt, over a deep layer of segregated pyroxene- and olivine-rich plutons, with sparse feldspar-rich, differentiated intrusions. Martian volatile-bearing environments have evolved through four phases: the pre-Noachian to early Noachian period when alteration by liquid water occurred near the surface and deep in the subsurface, in chemically neutral to alkaline environments that formed hydrous silicates and carbonates; the middle to late Noachian period when liquid water was widely present at the surface forming valley networks, lacustrine deposits, and clay-rich pedogenic horizons; the early Hesperian to early Amazonian period during which water became increasingly acidic and saline, forming deposits rich in sulfate salts, chlorides, and hydrated silica; and the Amazonian period when surface water has existed predominantly as ice, with only localized reaction with regolith and briny flow on the surface.

Visible/near-infrared (VNIR) reflectance spectra are used in laboratory, field, and airborne studies to characterize geologic materials. This chapter covers the region 0.3–5 µm and describes the species responsible for the absorption of radiation at specific wavelengths that create spectral features used to identify minerals, rocks, and other geologic materials. Fe contributes greatly to VNIR spectral signatures, producing features near 1 and 2 µm for Fe2+ in spectra of pyroxene and glass, while a broad, strong band from ~0.9 to 1.3 µm is characteristic of Fe2+ in olivine, carbonate, and many sulfates; a weak band near 1.2 µm is due to Fe2+ in feldspar; and bands near 0.6 and 0.9 µm arise from Fe3+ in ferric oxides/hydroxides. Water bands occur near 0.96, 1.15, 1.4, 1.9, and 2.9 µm, depending on the mineral structure, while structural OH bands occur near 1.4, 2.1–2.5, and 2.7 µm. Additional features are observed for carbonates, nitrates, sulfates, phosphates, chlorides, and perchlorates. The spectral signatures of geologic samples are also affected by how photons interact with particles in the sample. Factors such as grain size, coatings and mixtures influence the reflectance, transmittance, and absorption of photons at grain boundaries and contribute to the VNIR spectral properties of geologic materials.

Radar has proven to be a powerful tool in planetary exploration. Most of the major solid bodies of the Solar System have been observed with radar, either from Earth or from spacecraft. Planetary radar studies are reviewed in this chapter, with information on the various techniques of radar remote sensing provided along with key results. Recent radar results are emphasized. Concluding remarks are provided on future directions in planetary radar remote sensing.