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We present the analysis of 205 spatially resolved measurements of the surface composition of Mercury from MESSENGER's X‐Ray Spectrometer. The surface footprints of these measurements are categorized according to geological terrain. Northern smooth plains deposits and the plains interior to the Caloris basin differ compositionally from older terrain on Mercury. The older terrain generally has higher Mg/Si, S/Si, and Ca/Si ratios, and a lower Al/Si ratio than the smooth plains. Mercury's surface mineralogy is likely dominated by high‐Mg mafic minerals (e.g., enstatite), plagioclase feldspar, and lesser amounts of Ca, Mg, and/or Fe sulfides (e.g., oldhamite). The compositional difference between the volcanic smooth plains and the older terrain reflects different abundances of these minerals and points to the crystallization of the smooth plains from a more chemically evolved magma source. High‐degree partial melts of enstatite chondrite material provide a generally good compositional and mineralogical match for much of the surface of Mercury. An exception is Fe, for which the low surface abundance on Mercury is still higher than that of melts from enstatite chondrites and may indicate an exogenous contribution from meteoroid impacts. Key Points Analysis of spatially resolved X‐ray spectrometry data from MESSENGER Volcanic smooth plains units differ compositionally from older terrains Mercury's surface consists of high‐Mg mafic minerals, plagioclase, and sulfides

MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury's high northern latitudes and occupies more than 6% of the planet's surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury's post—heavy bombardment era.

A technique for converting gamma‐ray count rates measured by the Gamma‐Ray Spectrometer on the MESSENGER spacecraft to spatially resolved maps of the gamma‐ray emission from the surface of Mercury is utilized to map the surface distributions of the elements Si, O, and K over the planet's northern hemisphere. Conversion of the K gamma‐ray count rates to elemental abundances on the surface reveals variations from 300 to 2400 ppm. A comparison of these abundances with models for the maximum surface temperature suggests the possibility that a temperature‐related process is controlling the K abundances on the surface as well as providing K to the exosphere. The abundances of K and Th have been determined for several geologically distinct regions, including Mercury's northern smooth plains and the plains interior to the Caloris basin. The lack of a significant variation in the measured Th abundances suggests that there may be considerable variability in the K/Th abundance ratio over the mapped regions. Key Points First spatially resolved measurements of Si, O, K, and Th on Mercury K abundances vary significantly across the surface Potassium abundances may be driven by the thermal conditions on the surface

We present an overview of the operations, calibration, geodetic control, photometric standardization, and processing of images from the Mercury Dual Imaging System (MDIS) acquired during the orbital phase of the MESSENGER spacecraft’s mission at Mercury (18 March 2011–30 April 2015). We also provide a summary of all of the MDIS products that are available in NASA’s Planetary Data System (PDS). Updates to the radiometric calibration included slight modification of the frame-transfer smear correction, updates to the flat fields of some wide-angle camera (WAC) filters, a new model for the temperature dependence of narrow-angle camera (NAC) and WAC sensitivity, and an empirical correction for temporal changes in WAC responsivity. Further, efforts to characterize scattered light in the WAC system are described, along with a mosaic-dependent correction for scattered light that was derived for two regional mosaics. Updates to the geometric calibration focused on the focal lengths and distortions of the NAC and all WAC filters, NAC–WAC alignment, and calibration of the MDIS pivot angle and base. Additionally, two control networks were derived so that the majority of MDIS images can be co-registered with sub-pixel accuracy; the larger of the two control networks was also used to create a global digital elevation model. Finally, we describe the image processing and photometric standardization parameters used in the creation of the MDIS advanced products in the PDS, which include seven large-scale mosaics, numerous targeted local mosaics, and a set of digital elevation models ranging in scale from local to global.

Magnetized rocks can record the history of the magnetic field of a planet, a key constraint for understanding its evolution. From orbital vector magnetic field measurements of Mercury taken by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft at altitudes below 150 kilometers, we have detected remanent magnetization in Mercury's crust. We infer a lower bound on the average age of magnetization of 3.7 to 3.9 billion years. Our findings indicate that a global magnetic field driven by dynamo processes in the fluid outer core operated early in Mercury's history. Ancient field strengths that range from those similar to Mercury's present dipole field to Earth-like values are consistent with the magnetic field observations and with the low iron content of Mercury's crust inferred from MESSENGER elemental composition data.

Mapping the distribution and extent of major terrain types on a planet's surface helps to constrain the origin and evolution of its crust. Together, MESSENGER and Mariner 10 observations of Mercury now provide a near-global look at the planet, revealing lateral and vertical heterogeneities in the color and thus composition of Mercury's crust. Smooth plains cover approximately 40% of the surface, and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically. A low-reflectance, relatively blue component affects at least 15% of the surface and is concentrated in crater and basin ejecta. Its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron- and titanium-bearing oxides.

On the Moon, extensional tectonic features have only been observed close to the influence of the mare basalt-filled basins and floor-fractured craters. Analysis of Lunar Reconnaissance Orbiter Camera images reveals several potentially very young extensional tectonic features in the farside highlands, implying that extensional stresses may locally exceed compressional ones. Large-scale expressions of lunar tectonics—contractional wrinkle ridges and extensional rilles or graben—are directly related to stresses induced by mare basalt-filled basins 1 , 2 . Basin-related extensional tectonic activity ceased about 3.6 Gyr ago, whereas contractional tectonics continued until about 1.2 Gyr ago 2 . In the lunar highlands, relatively young contractional lobate scarps, less than 1 Gyr in age, were first identified in Apollo-era photographs 3 . However, no evidence of extensional landforms was found beyond the influence of mare basalt-filled basins and floor-fractured craters. Here we identify previously undetected small-scale graben in the farside highlands and in the mare basalts in images from the Lunar Reconnaissance Orbiter Camera. Crosscut impact craters with diameters as small as about 10 m, a lack of superposed craters, and graben depths as shallow as ∼1 m suggest these pristine-appearing graben are less than 50 Myr old. Thus, the young graben indicate recent extensional tectonic activity on the Moon where extensional stresses locally exceeded compressional stresses. We propose that these findings may be inconsistent with a totally molten early Moon, given that thermal history models for this scenario predict a high level of late-stage compressional stress 4 , 5 , 6 that might be expected to completely suppress the formation of graben.

During its first two flybys of Mercury, the MESSENGER spacecraft acquired images confirming that pervasive volcanism occurred early in the planet's history. MESSENGER's third Mercury flyby revealed a 290-kilometer-diameter peak-ring impact basin, among the youngest basins yet seen, having an inner floor filled with spectrally distinct smooth plains. These plains are sparsely cratered, postdate the formation of the basin, apparently formed from material that once flowed across the surface, and are therefore interpreted to be volcanic in origin. An irregular depression surrounded by a halo of bright deposits northeast of the basin marks a candidate explosive volcanic vent larger than any previously identified on Mercury. Volcanism on the planet thus spanned a considerable duration, perhaps extending well into the second half of solar system history.

The Planetary Surface Texture Laboratory at the Johns Hopkins University Applied Physics Laboratory is a facility for the study of the photometric and polarization behavior of regolith analogs using an ∼1.5‐m‐radius arc goniometer system. This system characterizes the photometric and polarization response of granular materials (planetary regolith analogs) at various illumination and viewing angles in the principal plane, enabling both forward and backward scattering observations at phase angles as low as 20 ± 5°. A polarimetric camera with linear polarizers at 0°, 45°, 90°, and 135° captures visible‐wavelength images of samples illuminated by semi‐collimated, unpolarized light. Imaging polarimetry and photometric studies are important for their ability to reveal sub‐pixel information about the nature of planetary surfaces, in particular, texture‐related properties such as porosity and roughness. We develop a calibration pipeline for the system and illustrate how the system can be utilized to understand the polarization and photometric response of planetary surface analog materials relevant to a range of airless Solar System bodies. We present an initial case study using a lunar regolith simulant, JSC‐1A, illuminated with broadband white light and observed between phase angles of 20 ± 5° and 120 ± 5°. We find that the radiance reflected from JSC‐1A decreases with increasing phase angle, while the degree of linear polarization increases, consistent with previous studies. This work illustrates the system's potential to enhance the interpretation of data sets such as those from the PolCam instrument on the Danuri lunar orbiter, and astronomical observations of asteroids. Plain Language Summary We present a new laboratory facility called the Planetary Surface Texture Laboratory (PSTL). We describe the design and calibration of the system. The light source and camera may be moved to a range of angles relative to a sample that is being studied. Thus, the system simulates conditions encountered when a spacecraft observes a planetary surface, or a ground‐based telescope observes solar system bodies such as asteroids. We illustrate the system's capabilities with a study of the general‐use lunar soil simulant JSC‐1A. Key Points A new facility to measure the photometric and polarization response of regolith analogs System polarimetric capabilities are demonstrated on lunar simulant JSC‐1A

Mercury appears darker globally than expected. Remote sensing evidence from the MESSENGER spacecraft indicates that the planet’s darkening agent is carbon and suggests that it originates from an ancient graphite-rich crust. Mercury’s global surface is markedly darker than predicted from its measured elemental composition. The darkening agent, which has not been previously identified, is most concentrated within Mercury’s lowest-reflectance spectral unit, the low-reflectance material 1 . This low-reflectance material is generally found in large impact craters and their ejecta 2 , 3 , which suggests a mid-to-lower crustal origin. Here we present neutron spectroscopy measurements of Mercury’s surface from the MESSENGER spacecraft that reveal increases in thermal-neutron count rates that correlate spatially with deposits of low-reflectance material. The only element consistent with both the neutron measurements and visible to near-infrared spectra 4 of low-reflectance material is carbon, at an abundance that is 1–3 wt% greater than surrounding, higher-reflectance material. We infer that carbon is the primary darkening agent on Mercury and that the low-reflectance material samples carbon-bearing deposits within the planet’s crust. Our findings are consistent with the formation of a graphite flotation crust from an early magma ocean 5 , and we propose that the heavily disrupted remnants of this ancient layer persist beneath the present upper crust. Under this scenario, Mercury’s globally low reflectance results from mixing of the ancient graphite-rich crust with overlying volcanic materials via impact processes or assimilation of carbon into rising magmas during secondary crustal formation.