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X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets. Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust. The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles. Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury's low and variable surface reflectance.

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

Measurements by the Neutron Spectrometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft show decreases in the flux of epithermal and fast neutrons from Mercury's north polar region that are consistent with the presence of water ice in permanently shadowed regions. The neutron data indicate that Mercury's radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surficial layer 10 to 30 cm thick that is less rich in hydrogen. Combined neutron and radar data are best matched if the buried layer consists of nearly pure water ice. The upper layer contains less than 25 weight % water-equivalent hydrogen. The total mass of water at Mercury's poles is inferred to be 2 × 10 16 to 10 18 grams and is consistent with delivery by comets or volatile-rich asteroids.

We have conducted petrologic modeling of MESSENGER‐derived compositions and analog compositions to gain a better understanding of the petrogenesis of the crust of Mercury. Analog compositions included a 1425°C partial melt of the Indarch (EH4) meteorite and a range of Mg‐rich terrestrial rocks (magnesian basalt, basaltic komatiite, and peridotitic komatiite). All models were held at the iron‐wüstite buffer to simulate the reducing conditions that likely existed during Mercury's formation. We then compared modeled mineral compositions and abundances, liquidus temperatures, and viscosities to better constrain the characteristics of the lavas that erupted on Mercury's surface. Our results show that the surface composition of Mercury is most similar to that of a terrestrial magnesian basalt (with lowered FeO), composed mainly of Mg‐rich orthopyroxene and plagioclase. Because the model magmas are Mg‐rich, their counterparts on Mercury would have erupted at high temperatures and displayed low viscosities. Producing melts of these compositions would have required high temperatures at the mantle source regions on Mercury. The inferred low‐viscosity lavas would have erupted as thin, laterally extensive flows (depending upon their effusion rate) and would be expected to display surficial flow features that might be preserved to the present. Key Points We conducted petrologic modeling of MESSENGER‐derived and analog compositions Mercury's surface is composed mainly of Mg‐rich orthopyroxene and plagioclase

Mercury’s regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury’s exosphere (micrometeoroid flux and solar wind interactions) and are moderated by the local space environment and the presence of a global magnetic field. To comprehend how space weathering acts on Mercury’s regolith, an understanding is needed of how contributing processes act as an interactive system. As no direct information (e.g., from returned samples) is available about how the system of space weathering affects Mercury’s regolith, we use as a basis for comparison the current understanding of these same processes on lunar and asteroidal regoliths as well as laboratory simulations. These comparisons suggest that Mercury’s regolith is overturned more frequently (though the characteristic surface time for a grain is unknown even relative to the lunar case), more than an order of magnitude more melt and vapor per unit time and unit area is produced by impact processes than on the Moon (creating a higher glass content via grain coatings and agglutinates), the degree of surface irradiation is comparable to or greater than that on the Moon, and photon irradiation is up to an order of magnitude greater (creating amorphous grain rims, chemically reducing the upper layers of grains to produce nanometer-scale particles of metallic iron, and depleting surface grains in volatile elements and alkali metals). The processes that chemically reduce the surface and produce nanometer-scale particles on Mercury are suggested to be more effective than similar processes on the Moon. Estimated abundances of nanometer-scale particles can account for Mercury’s dark surface relative to that of the Moon without requiring macroscopic grains of opaque minerals. The presence of nanometer-scale particles may also account for Mercury’s relatively featureless visible–near-infrared reflectance spectra. Characteristics of material returned from asteroid 25143 Itokawa demonstrate that this nanometer-scale material need not be pure iron, raising the possibility that the nanometer-scale material on Mercury may have a composition different from iron metal [such as (Fe,Mg)S]. The expected depletion of volatiles and particularly alkali metals from solar-wind interaction processes are inconsistent with the detection of sodium, potassium, and sulfur within the regolith. One plausible explanation invokes a larger fine fraction (grain size <45 μm) and more radiation-damaged grains than in the lunar surface material to create a regolith that is a more efficient reservoir for these volatiles. By this view the volatile elements detected are present not only within the grain structures, but also as adsorbates within the regolith and deposits on the surfaces of the regolith grains. The comparisons with findings from the Moon and asteroids provide a basis for predicting how compositional modifications induced by space weathering have affected Mercury’s surface composition.

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

The X‐Ray Spectrometer (XRS) on the MESSENGER spacecraft measures elemental abundances on the surface of Mercury by detecting fluorescent X‐ray emissions induced on the planet's surface by the incident solar X‐ray flux. The XRS began orbital observations on 23 March 2011 and has observed X‐ray fluorescence (XRF) from the surface of the planet whenever a sunlit portion of Mercury has been within the XRS field of view. Solar flares are generally required to provide sufficient signal to detect elements that fluoresce at energies above ∼2 keV, but XRF up to the calcium line (3.69 keV) has been detected from Mercury's surface at times when the XRS field of view included only unlit portions of the planet. Many such events have been detected and are identified as electron‐induced X‐ray emission produced by the interaction of ∼1–10 keV electrons with Mercury's surface. Electrons in this energy range were detected by the XRS during the three Mercury flybys and have also been observed regularly in orbit about Mercury. Knowledge of the energy spectrum of the electrons precipitating at the planet's surface makes it possible to infer surface composition from the measured fluorescent spectra, providing additional measurement opportunities for the XRS. Abundance results for Mg, Al, and Si are in good agreement with those derived from solar‐induced XRF data, providing independent validation of the analysis methodologies. Derived S and Ca abundances are somewhat higher than derived from the solar‐induced fluorescence data, possibly reflecting incomplete knowledge of the energy spectra of electrons impacting the planet. Key Points Electron induced XRF detected from Mercury's surface Exciting electron spectrum known from model Surface elemental abundances inferred from detected XRF

Orbital gamma‐ray measurements obtained by the MESSENGER spacecraft have been analyzed to determine the abundances of the major elements Al, Ca, S, Fe, and Na on the surface of Mercury. The Si abundance was determined and used to normalize those of the other reported elements. The Na analysis provides the first abundance estimate of 2.9 ± 0.1 wt% for this element on Mercury's surface. The other elemental results (S/Si = 0.092 ± 0.015, Ca/Si = 0.24 ± 0.05, and Fe/Si = 0.077 ± 0.013) are consistent with those previously obtained by the MESSENGER X‐Ray Spectrometer, including the high sulfur and low iron abundances. Because of different sampling depths for the two techniques, this agreement indicates that Mercury's regolith is, on average, homogenous to a depth of tens of centimeters. The elemental results from gamma‐ray and X‐ray spectrometry are most consistent with petrologic models suggesting that Mercury's surface is dominated by Mg‐rich silicates. We also compare the results with those obtained during the MESSENGER flybys and with ground‐based observations of Mercury's surface and exosphere. Key Points Abundances of the major elements on the surface of Mercury MESSENGER mission orbital gamma ray measurements The first abundance measurements of sodium

Mercury holds valuable clues to the distribution of elements at the birth of the solar system and how planets form and evolve in close proximity to their host stars. This Mercury Lander mission concept returns in situ measurements that address fundamental science questions raised by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission’s pioneering exploration of Mercury. Such measurements are needed to understand Mercury's unique mineralogy and geochemistry, characterize the proportionally massive core's structure, measure the planet's active and ancient magnetic fields at the surface, investigate the processes that alter the surface and produce the exosphere, and provide ground truth for remote data sets. The mission concept achieves one full Mercury year (∼88 Earth days) of surface operations with an 11-instrument, high-heritage payload delivered to a landing site within Mercury's widely distributed low-reflectance material, and it addresses science goals encompassing geochemistry, geophysics, the Mercury space environment, and geology. The spacecraft launches in 2035, and the four-stage flight system uses a solar electric propulsion cruise stage to reach Mercury in 2045. Landing is at dusk to meet thermal requirements, permitting ∼30 hr of sunlight for initial observations. The radioisotope-powered lander continues operations through the Mercury night. Direct-to-Earth communication is possible for the initial 3 weeks of landed operations, drops out for 6 weeks, and resumes for the final month. Thermal conditions exceed lander operating temperatures shortly after sunrise, ending operations. Approximately 11 GB of data are returned to Earth. The cost estimate demonstrates that a Mercury Lander mission is feasible and compelling as a New Frontiers–class mission.