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Mercury’s polar regions host deposits of radar-bright material in regions of permanent shadow, commonly the interior of impact craters, and the deposits are hypothesised to be water ice (i.e., Chabot et al., 2018). Thermal modelling, prior to the arrival of the MESSENGER mission, found that water ice is not thermally stable in craters smaller than 10 km, assuming the craters had a depth-to-diameter ratio of 0.2 (Vasavada et al., 1999). Studies of the distribution of radar-bright deposits have identified deposits in craters under 10 km (Deutsch, et al., 2016). In this study, we used the high-resolution north polar topography from the MESSENGER mission to evaluate the morphometry and temperatures of craters with diameters of 5-10 km to explore if these craters could host stable water ice on geologic timescales. We measured the depth and diameter of 201 5-10 km in diameter craters between 75-85° N. MLA tracks that bisected the crater were used to measure the depth and diameter of 99 craters, spanning all longitudes of this north polar region. Thermal models for the north polar region of Mercury use the gridded MLA topography sampled at 1 km resolution (Paige et al., 2013; Chabot et al., 2018), so it was important to ensure the gridded topography accurately captured the craters’ shapes before using the results of these thermal models for these small craters, Comparisons between the MLA track profiles and the profiles taken through the gridded MLA product showed consistent depth to diameter profiles in both datasets, substantiating the use of the gridded MLA product to be used to determine depth and diameter values for these craters and the thermal models for these craters to be used to explore the stability of water ice in these craters. The average depth-to-diameter ratio of the 201 craters is 0.15, 25% lower than the estimate used in pre-MESSENGER thermal study (Vasavada et al., 1999). Thermal measurements of the 156 craters show that many of them have average temperatures below 110 K, meaning that they have thermal conditions that would allow water ice to be stable on geologic timescales under a thin layer of insulating material. Only three craters had small, single-pixel regions with maximum temperatures under 110 K, suggesting that water ice is not stable on the surface in the majority of small craters, except for isolated regions or below the 1-km scale of the thermal model. These results show that water ice would be stable in simple, sub-10 km diameter craters on Mercury and that the presence of radar-bright deposits in these craters is not a constraint on the age of radar-bright deposits. However, our mapping results do show a clear correlation with radar-bright signatures and longitude. In particular, around 60°E longitude, we observe a higher percentage of radar-bright craters. One of Mercury’s two cold poles is nearby at 90°E, but a large complex crater, Prokofiev-112 km in diameter, is also located at 64°E and many of the craters that are radar-bright appear to be secondaries of Prokofiev. Possible explanations for this longitude distribution are being actively investigated, including association with Prokofiev, cold-pole thermal conditions, effects of radar visibility, and the potential for uneven water ice distribution in the small craters near Mercury’s north pole.

Data from Mariner 10, MESSENGER, and ground-based telescopic observations have facilitated great advancements towards understanding the geochemistry, geology, internal structure, exosphere, and magnetosphere of Mercury. However, there are critical science questions that can be only addressed via examination of a sample in Earth-based laboratories, where numerous highly sensitive analytical measurements are possible. Collecting a sample from the surface of Mercury and bringing it to Earth for in-depth analysis would allow for transformative Solar System science to be conducted, examining aspects of our Solar System such as the evolution of the protoplanetary disk, space weathering on airless bodies, the geochemical behavior of elements at extreme conditions, and the origin and distribution of volatiles across the terrestrial planets. Furthermore, our knowledge of Mercury’s differentiation and geochemical processes, chronology and geologic evolution, tectonism and geomechanical properties, and past and ongoing magnetism would be greatly advanced via analysis of a sample from Mercury. Although there are ample challenges and knowledge gaps associated with sample return from Mercury in terms of both spacecraft requirements and material requirements for curatorial facilities, a sample from the planet would be an invaluable scientific resource for generations to come, enabling the most sophisticated measurements to be brought to bear for decades and helping to truly unlock the mysteries of our Solar System.

Multiple lines of evidence support the hypothesis that Mercury’s polar regions host deposits of water ice in permanently shadowed regions, often within the interiors of craters. Pre-MErcury, Surface, Space, ENvironment, GEochemisty, and Ranging (MESSENGER) thermal modeling of the temperature of idealized simple craters found that the interiors of these craters were too hot to host near-surface ice on geologic timescales unless within 2° of the poles. However, results from the Arecibo Observatory and the MESSENGER mission identified many small

The planet Mercury has contracted over its history. The identification of small thrust fault scarps suggests the occurrence of tectonic activity on Mercury within the past 50 million years and thus a slow-cooling planetary interior. Large tectonic landforms on the surface of Mercury, consistent with significant contraction of the planet, were revealed by the flybys of Mariner 10 in the mid-1970s1. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission confirmed that the planet’s past 4 billion years of tectonic history have been dominated by contraction expressed by lobate fault scarps that are hundreds of kilometres long2,3,4,5. Here we report the discovery of small thrust fault scarps in images from the low-altitude campaign at the end of the MESSENGER mission that are orders of magnitude smaller than the large-scale lobate scarps. These small scarps have tens of metres of relief, are only kilometres in length and are comparable in scale to small young scarps on the Moon6,7,8. Their small-scale, pristine appearance, crosscutting of impact craters and association with small graben all indicate an age of less than 50 Myr. We propose that these scarps are the smallest members of a continuum in scale of thrust fault scarps on Mercury. The young age of the small scarps, along with evidence for recent activity on large-scale scarps, suggests that Mercury is tectonically active today and implies a prolonged slow cooling of the planet’s interior.

High resolution images from MESSENGER provide morphological information on the nature and origin of Mercury's hollows, small depressions that likely formed when a volatile constituent was lost from the surface. Because graphite may be a component of the low-reflectance material that hosts hollows, we suggest that loss of carbon by ion sputtering or conversion to methane by proton irradiation could contribute to hollows formation. Measurements of widespread hollows in 565 images with pixel scales

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.

A principal data product from MESSENGER’s primary orbital mission at Mercury is a global multispectral map in eight visible to near-infrared colors, at an average pixel scale of 1 km, acquired by the Mercury Dual Imaging System. The constituent images have been calibrated, photometrically corrected to a standard geometry, and map projected. Global analysis reveals no spectral units not seen during MESSENGER’s Mercury flybys and supports previous conclusions that most spectral variation is related to changes in spectral slope and reflectance between spectral end-member high-reflectance red plains (HRP) and low-reflectance material (LRM). Comparison of color properties of plains units mapped on the basis of morphology shows that the two largest unambiguously volcanic smooth plains deposits (the interior plains of Caloris and the northern plains) are close to HRP end members and have average color properties distinct from those of most other smooth plains and intercrater plains. In contrast, smaller deposits of smooth plains are nearly indistinguishable from intercrater plains on the basis of their range of color properties, consistent with the interpretation that intercrater plains are older equivalents of smooth plains. LRM having nearly the same reflectance is exposed in crater and basin ejecta of all ages, suggesting impact excavation from depth of material that is intrinsically dark or darkens very rapidly, rather than gradual darkening of exposed material purely by space weathering. A global search reveals no definitive absorptions attributable to Fe 2+ -containing silicates or to sulfides over regions 20 km or more in horizontal extent, consistent with results from MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer. The only absorption-like feature identified is broad upward curvature of the spectrum centered near 600 nm wavelength. The feature is strongest in freshly exposed LRM and weak or absent in older exposures of LRM. We modeled spectra of LRM as intimate mixtures of HRP with candidate low-reflectance phases having a similar 600-nm spectral feature, under the assumption that the grain size is 1 μm or larger. Sulfides measured to date in the laboratory and coarse-grained iron are both too bright to produce LRM from HRP. Ilmenite is sufficiently dark but would require Ti abundances too high to be consistent with MESSENGER X-Ray Spectrometer measurements. Three phases or mixtures of phases that could be responsible for the low reflectance of LRM are consistent with our analyses. Graphite, in amounts consistent with upper limits from the Gamma-Ray Spectrometer, may be consistent with geochemical models of Mercury’s differentiation calling for a graphite-enriched primary flotation crust from an early magma ocean and impact mixing of that early crust before or during the late heavy bombardment (LHB) into material underlying the volcanic plains. The grain size of preexisting iron or iron sulfide could have been altered to a mix of nanophase and microphase grains by shock during those impacts, lowering reflectance. Alternatively, iron-bearing phases and carbon in a late-accreting carbonaceous veneer may have been stirred into the lower crust or upper mantle. Decoupling of variations in color from abundances of major elements probably results from the very low content and variation of Fe 2+ in crustal silicates, such that reflectance is controlled instead by one or more minor opaque phases and the extent of space weathering.

Caloris basin, Mercury's youngest large impact basin, is filled by volcanic plains that are spectrally distinct from surrounding material. Post-plains impact craters of a variety of sizes populate the basin interior, and the spectra of the material they have excavated enable the thickness of the volcanic fill to be estimated and reveal the nature of the subsurface. The thickness of the interior volcanic plains is consistently at least 2.5 km, reaching 3.5 km in places, with thinner fill toward the edge of the basin. No systematic variations in fill thickness are observed with long-wavelength topography or azimuth. The lack of correlation between plains thickness and variations in elevation at large horizontal scales within the basin indicates that plains emplacement must have predated most, if not all, of the changes in long-wavelength topography that affected the basin. There are no embayed or unambiguously buried (ghost) craters with diameters greater than 10 km in the Caloris interior plains. The absence of such ghost craters indicates that one or more of the following scenarios must hold: the plains are sufficiently thick to have buried all evidence of craters that formed between the Caloris impact event and the emplacement of the plains; the plains were emplaced soon after basin formation; or the complex tectonic deformation of the basin interior has disguised wrinkle-ridge rings localized by buried craters. That low-reflectance material (LRM) was exposed by every impact that penetrated through the surface volcanic plains provides a means to explore near-surface stratigraphy. If all occurrences of LRM are derived from a single layer, the subsurface LRM deposit is at least 7.5-8.5 km thick and its top likely once made up the Caloris basin floor. The Caloris-forming impact would have generated a layer of impact melt 3-15 km thick; such a layer could account for the entire thickness of LRM. This material would have been derived from a combination of lower crust and upper mantle.

Images acquired by NASA's MESSENGER spacecraft have revealed the morphology of frozen volatiles in Mercury's permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits. The images show extensive, spatially continuous regions with distinctive reflectance properties. A site within Prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance, indicating that the surficial ice was emplaced after formation of the underlying craters. In areas where water ice is inferred to be present but covered by a thin layer of dark, organic-rich volatile material, regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries. The sharp boundaries indicate that the volatile deposits at Mercury's poles are geologically young, relative to the time scale for lateral mixing by impacts, and either are restored at the surface through an ongoing process or were delivered to the planet recently.

Images obtained during the low‐altitude campaign in the final year of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission provide the highest‐spatial‐resolution views of Mercury's polar deposits. Images for distinct areas of permanent shadow within 35 north polar craters were successfully captured during the campaign. All of these regions of permanent shadow were found to have low‐reflectance surfaces with well‐defined boundaries. Additionally, brightness variations across the deposits correlate with variations in the biannual maximum surface temperature across the permanently shadowed regions, supporting the conclusion that multiple volatile organic compounds are contained in Mercury's polar deposits, in addition to water ice. A recent large impact event or ongoing bombardment by micrometeoroids could deliver water as well as many volatile organic compounds to Mercury. Either scenario is consistent with the distinctive reflectance properties and well‐defined boundaries of Mercury's polar deposits and the presence of volatiles in all available cold traps.

Analysis of images that were obtained at different phase angles can give clues to the texture of a planetary surface. Here we describe MESSENGER phase-ratio images for features on the surface of Mercury, including hollows on the floor of Eminescu basin, a pyroclastic deposit found within the Caloris basin, and a dark impact-melt flow produced by the impact that formed Waters crater. The results indicate that hollows and the pyroclastic material are characterized by finer particle size or smoother sub-resolution roughness than the ordinary impact-generated regolith. The impact melt flow appears to have a surface that is rougher or consists of coarser particle sizes than the nearby background.

[1] Observations of rayed craters at optical and radar wavelengths provide insight into the processes that lead to ray formation and degradation on terrestrial planets. We have compared optical and S-Band radar data for several large (> 20 km diameter), young craters on the Moon and Mercury and find evidence that secondary cratering plays a significant role in the formation of crater rays. Regions where rays appear bright to optical and radar sensors correspond to dense concentrations of secondary craters, and the observed radar enhancement appears to be a result of the deposition of blocky, immature ejecta from the secondary craters and/or the rocky, immature interior walls of the secondary craters. We define a new optical maturity index for Mercury and find that rays in radar and optical images correspond closely, indicating that the rays are rich in centimeter- to decimeter-sized clasts. Rays on the Moon are less prominent at radar wavelengths, suggesting that they are currently composed of smaller clasts, centimeter sized or less. This difference suggests that secondary craters are larger on Mercury and capable of excavating more decimeter-sized clasts. Furthermore, observations of rayed craters provide an opportunity to assign relative ages to the youngest craters on the Moon and Mercury. Although rayed craters on Mercury appear most similar to the youngest craters on the Moon, the apparent ages are more likely influenced by inherent differences in impact velocity, surface gravitational acceleration, and target properties that result in the formation of larger secondary craters on Mercury.

[1] Landforms unique to Mercury, hollows are shallow, flat-floored irregular depressions notable for their relatively high reflectance and characteristic color. Here we document the range of geological settings in which hollows occur. Most are associated with impact structures (simple bowl-shaped craters to multiring basins, and ranging from Kuiperian to Calorian in age). Hollows are found in the low-reflectance material global color unit and in low-reflectance blue plains, but they appear to be absent from high-reflectance red plains. Hollows may occur preferentially on equator- or hot-pole-facing slopes, implying that their formation is linked to solar heating. Evidence suggests that hollows form because of loss of volatile material. We describe hypotheses for the origin of the volatiles and for how such loss proceeds. Intense space weathering and solar heating are likely contributors to the loss of volatiles; contact heating by melts could promote the formation of hollows in some locations. Lunar Ina-type depressions differ from hollows on Mercury in a number of characteristics, so it is unclear if they represent a good analog. We also use MESSENGER multispectral images to characterize a variety of surfaces on Mercury, including hollows, within a framework defined by laboratory spectra for analog minerals and lunar samples. Data from MESSENGER's X-Ray Spectrometer indicate that the planet's surface contains up to 4% sulfur. We conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low-reflectance material relative to average surface material. Hollows may owe their relatively high reflectance to destruction of the darkening agent (sulfides), the presence of alteration minerals, and/or physical differences in particle size, texture, or scattering behavior.

[1] Orbital images from the MESSENGER spacecraft show that ~27% of Mercury's surface is covered by smooth plains, the majority (>65%) of which are interpreted to be volcanic in origin. Most smooth plains share the spectral characteristics of Mercury's northern smooth plains, suggesting they also share their magnesian alkali-basalt-like composition. A smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope, suggesting more ultramafic compositions, an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation. The knobby and hummocky plains surrounding the Caloris basin, known as Odin-type plains, occupy an additional 2% of Mercury's surface. The morphology of these plains and their color and stratigraphic relationships suggest that they formed as Caloris ejecta, although such an origin is in conflict with a straightforward interpretation of crater size–frequency distributions. If some fraction is volcanic, this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions. Smooth plains are widespread on Mercury, but they are more heavily concentrated in the north and in the hemisphere surrounding Caloris. No simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed. A likely volcanic origin for some older terrain on Mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large-scale volcanic deposits rather than differences in crustal formational process.

Wet Mercury Radar observations of Mercury's poles in the 1990s revealed regions of high backscatter that were interpreted as indicative of thick deposits of water ice; however, other explanations have also been proposed (see the Perspective by Lucey ). MESSENGER neutron data reported by Lawrence et al. (p. 292 , published online 29 November) in conjunction with thermal modeling by Paige et al. (p. 300 , published online 29 November) now confirm that the primary component of radar-reflective material at Mercury's north pole is water ice. Neumann et al. (p. 296 , published online 29 November) analyzed surface reflectance measurements from the Mercury Laser Altimeter onboard MESSENGER and found that while some areas of high radar backscatter coincide with optically bright regions, consistent with water ice exposed at the surface, some radar-reflective areas correlate with optically dark regions, indicative of organic sublimation lag deposits overlying the ice. Dark areas that fall outside regions of high radio backscatter suggest that water ice was once more widespread.

the basin floor, and (3) subsidence following volcanic loading. Our results suggest that only thermal contraction can account for the observed pattern of graben, whereas some combination of subsidence and global contraction is the most likely explanation for the central ridges in Rachmaninoff and Mozart. Thermal contraction models, however, predict the formation of graben in the centermost region of each basin, where no graben are observed. We hypothesize that graben in this region were buried by a thin, late-stage flow of plains material, and images of partially filled graben provide evidence of such late-stage plains emplacement. These results suggest that the smooth plains units in these three basins are volcanic in origin. The thermal contraction models also imply a cooling unit ~1km thick near the basin center, further supporting the view that plains-forming lavas on Mercury were often of sufficiently high volume and low viscosity to pool to substantial thicknesses within basins and craters.

[1] Radar-bright features near Mercury's poles were discovered in Earth-based radar images and proposed to be water ice present in permanently shadowed areas. Images from MESSENGER's one-year primary orbital mission provide the first nearly complete view of Mercury's north polar region, as well as multiple images of the surface under a range of illumination conditions. We find that radar-bright features near Mercury's north pole are associated with locations persistently shadowed in MESSENGER images. Within 10° of the pole, almost all craters larger than 10 km in diameter host radar-bright deposits. There are several craters located near Mercury's north pole with sufficiently large diameters to enable long-lived water ice to be thermally stable at the surface within regions of permanent shadow. Craters located farther south also host radar-bright deposits and show a preference for cold-pole longitudes; thermal models suggest that a thin insulating layer is required to cover these deposits if the radar-bright material consists predominantly of long-lived water ice. Many small (

Data acquired by the Mercury Laser Altimeter and the Mercury Dual Imaging System on the MESSENGER spacecraft in orbit about Mercury provide a means to measure the geometry of many of the impact craters in Mercury's northern hemisphere in detail for the first time. The combination of topographic and imaging data permit a systematic evaluation of impact crater morphometry on Mercury, a new calculation of the diameter Dt at which craters transition with increasing diameter from simple to complex forms, and an exploration of the role of target properties and impact velocity on final crater size and shape. Measurements of impact crater depth on Mercury confirm results from previous studies, with the exception that the depths of large complex craters are typically shallower at a given diameter than reported from Mariner 10 data. Secondary craters on Mercury are generally shallower than primary craters of the same diameter. No significant differences are observed between the depths of craters within heavily cratered terrain and those of craters within smooth plains. The morphological attributes of craters that reflect the transition from simple to complex craters do not appear at the same diameter; instead flat floors first appear with increasing diameter in craters at the smallest diameters, followed with increasing diameter by reduced crater depth and rim height, and then collapse and terracing of crater walls. Differences reported by others in Dt between Mercury and Mars (despite the similar surface gravitational acceleration on the two bodies) are confirmed in this study. The variations in Dt between Mercury and Mars cannot be adequately attributed to differences in either surface properties or mean projectile velocity.

Orbital images of Mercury obtained by the MESSENGER spacecraft have revealed families of troughs, interpreted to be graben, on volcanic plains material that largely or completely buried preexisting craters and basins. The graben are partially to fully encircled by rings of contractional wrinkle ridges localized over the rims of the buried impact features to form systems of associated contractional and extensional landforms. Most of the buried craters and basins with graben identified to date are located in the extensive volcanic plains that cover much of Mercury’s northern high latitudes. The distinctive relationship between wrinkle ridges and graben in buried craters and basins on Mercury is interpreted to be the result of a combination of extensional stresses from cooling and thermal contraction of thick lava flow units and compressional stresses from cooling and contraction of the planet’s interior.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of Mercury in 2008–2009. During the first and third of those flybys, MESSENGER passed behind the planet from the perspective of Earth, occulting the radio-frequency (RF) transmissions. The occultation start and end times, recovered with 0.1 s accuracy or better by fitting edge-diffraction patterns to the RF power history, are used to estimate Mercury's radius at the tangent point of the RF path. To relate the measured radius to the planet shape, we evaluate local topography using images to identify the high-elevation feature that defines the RF path or using altimeter data to quantify surface roughness. Radius measurements are accurate to 150 m, and uncertainty in the average radius of the surrounding terrain, after adjustments are made from the local high at the tangent point of the RF path, is 350 m. The results are consistent with Mercury's equatorial shape as inferred from observations by the Mercury Laser Altimeter and ground-based radar. The three independent estimates of radius from occultation events collectively yield a mean radius for Mercury of 2439.2±0.5 km.

Peak-ring basins represent an impact-crater morphology that is transitional between complex craters with central peaks and large multi-ring basins. Therefore, they can provide insight into the scale dependence of the impact process. Here the transition with increasing crater diameter from complex craters to peak-ring basins on Mercury is assessed through a detailed analysis of Eminescu, a geologically recent and well-preserved peak-ring basin. Eminescu has a diameter (∼125 km) close to the minimum for such crater forms and is thus representative of the transition. Impact crater size-frequency distributions and faint rays indicate that Eminescu is Kuiperian in age, geologically younger than most other basins on Mercury. Geologic mapping of basin interior units indicates a distinction between smooth plains and peak-ring units. Our mapping and crater retention ages favor plains formation by impact melt rather than post-impact volcanism, but a volcanic origin for the plains cannot be excluded if the time interval between basin formation and volcanic emplacement was less than the uncertainty in relative ages. The high-albedo peak ring of Eminescu is composed of bright crater-floor deposits (BCFDs, a distinct crustal unit seen elsewhere on Mercury) exposed by the impact. We use our observations to assess predictions of peak-ring formation models. We interpret the characteristics of Eminescu as consistent with basin formation models in which a melt cavity forms during the impact formation of craters at the transition to peak ring morphologies. We suggest that the smooth plains were emplaced via impact melt expulsion from the central melt cavity during uplift of a peak ring composed of BCFD-type material. In this scenario the ringed cluster of peaks resulted from the early development of the melt cavity, which modified the central uplift zone.

The study of peak-ring basins and other impact crater morphologies transitional between complex craters and multi-ring basins is important to our understanding of the mechanisms for basin formation on the terrestrial planets. Mercury has the largest population, and the largest population per area, of peak-ring basins and protobasins in the inner solar system and thus provides important data for examining questions surrounding peak-ring basin formation. New flyby images from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have more than doubled the area of Mercury viewed at close range, providing nearly complete global coverage of the planet's surface when combined with flyby data from Mariner 10. We use this new near-global dataset to compile a catalog of peak-ring basins and protobasins on Mercury, including measurements of the diameters of the basin rim crest, interior ring, and central peak (if present). Our catalog increases the population of peak-ring basins by ∼150% and protobasins by ∼100% over previous catalogs, including 44 newly identified peak-ring basins (total=74) and 17 newly identified protobasins (total=32). A newly defined transitional basin type, the ringed peak-cluster basin (total=9), is also described. The new basin catalog confirms that Mercury has the largest population of peak-ring basins of the terrestrial planets and also places the onset rim-crest diameter for peak-ring basins at 126 − 26 + 33 km , which is intermediate between the onset diameter for peak-ring basins on the Moon and those for the other terrestrial planets. The ratios of ring diameter to rim-crest diameter further emphasize that protobasins and peak-ring basins are parts of a continuum of basin morphologies relating to their processes of formation, in contrast to previous views that these forms are distinct. Comparisons of the predictions of peak-ring basin-formation models with the characteristics of the basin catalog for Mercury suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. The relationship between impact-melt production and peak-ring formation is strengthened further by agreement between power laws fit to ratios of ring diameter to rim-crest diameter for peak-ring basins and protobasins and the power-law relation between the dimension of a melt cavity and the crater diameter. More detailed examination of Mercury's peak-ring basins awaits the planned insertion of the MESSENGER spacecraft into orbit about Mercury in 2011.

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.

High-resolution images of Mercury's surface from orbit reveal that many bright deposits within impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or hollows, range from tens of meters to a few kilometers across, and many have high-reflectance interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, and walls, are interpreted to have been excavated from depth by the crater-forming process. The most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation, space weathering, outgassing, or pyroclastic volcanism. These features support the inference that Mercury's interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the solar system's innermost planet.

MESSENGER’s Mercury Dual Imaging System (MDIS) obtained multispectral images for more than 80% of the surface of Mercury during its first two flybys. Those images have confirmed that the surface of Mercury exhibits subtle color variations, some of which can be attributed to compositional differences. In many areas, impact craters are associated with material that is spectrally distinct from the surrounding surface. These deposits can be located on the crater floor, rim, wall, or central peak or in the ejecta deposit, and represent material that originally resided at depth and was subsequently excavated during the cratering process. The resulting craters make it possible to investigate the stratigraphy of Mercury’s upper crust. Studies of laboratory, terrestrial, and lunar craters provide a means to bound the depth of origin of spectrally distinct ejecta and central peak structures. Excavated red material (RM), with comparatively steep (red) spectral slope, and low-reflectance material (LRM) stand out prominently from the surrounding terrain in enhanced-color images because they are spectral end-members in Mercury’s compositional continuum. Newly imaged examples of RM were found to be spectrally similar to the relatively red, high-reflectance plains (HRP), suggesting that they may represent deposits of HRP-like material that were subsequently covered by a thin layer (∼1 km thick) of intermediate plains. In one area, craters with diameters ranging from 30 km to 130 km have excavated and incorporated RM into their rims, suggesting that the underlying RM layer may be several kilometers thick. LRM deposits are useful as stratigraphic markers, due to their unique spectral properties. Some RM and LRM were excavated by pre-Tolstojan basins, indicating a relatively old age (>4.0 Ga) for the original emplacement of these deposits. Detailed examination of several small areas on Mercury reveals the complex nature of the local stratigraphy, including the possible presence of buried volcanic plains, and supports sequential buildup of most of the upper ∼5 km of crust by volcanic flows with compositions spanning the range of material now visible on the surface, distributed heterogeneously across the planet. This emerging picture strongly suggests that the crust of Mercury is characterized by a much more substantial component of early volcanism than represented by the phase of mare emplacement on Earth’s Moon.

Mercury is the innermost planet, with a high density and large metallic core. It is the only terrestrial planet besides Earth with an internally generated magnetic field, which gives rise to a complex magnetosphere. Mercury's surface is heavily cratered and has also been modified by volcanism, including pyroclastic volcanism. Surface compositions are similar to komatiites and basalts on Earth, but highly depleted in iron, which is located mostly in the planet's core. Mercury's tectonics is dominated by compressional deformation driven by global cooling and contraction. The planet's surface is surprisingly rich in sulfur and other volatiles, whose mobility at high surface temperatures creates erosional topographic hollows. Regions of permanent shadow near the poles trap water ice and low-albedo volatiles, possibly supplied by infalling comets and asteroids. The thin exosphere is populated with atoms removed from the surface by solar radiation, charged particle sputtering, and micrometeoroid bombardment.

[1] Radar-bright features near Mercury's poles have been postulated to be deposits of water ice trapped in cold, permanently shadowed interiors of impact craters. From its orbit about Mercury, MESSENGER repeatedly imaged the planet's south polar region over one Mercury solar day, providing a complete view of the terrain near the south pole and enabling the identification of areas of permanent shadow larger in horizontal extent than approximately 4 km. In Mercury's south polar region, all radar-bright features correspond to areas of permanent shadow. Application of previous thermal models suggests that the radar-bright deposits in Mercury's south polar cold traps are in locations consistent with a composition dominated by water ice provided that some manner of insulation, such as a thin layer of regolith, covers many of the deposits.

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.

MESSENGER's Third Set of Messages MESSENGER, the spacecraft en route to insertion into orbit about Mercury in March 2011, completed its third flyby of the planet on 29 September 2009. Prockter et al. (p. 668 , published online 15 July) present imaging data acquired during this flyby, showing that volcanism on Mercury has extended to much more recent times than previously assumed. The temporal extent of volcanic activity and, in particular, the timing of most recent activity had been missing ingredients in the understanding of Mercury's global thermal evolution. Slavin et al. (p. 665 , published online 15 July) report on magnetic field measurements made during the 29 September flyby, when Mercury's magnetosphere underwent extremely strong coupling with the solar wind. The planet's tail magnetic field increased and then decreased by factors of 2 to 3.5 during periods lasting 2 to 3 minutes. These observations suggest that magnetic open flux loads the magnetosphere, which is subsequently unloaded by substorms—magnetic disturbances during which energy is rapidly released in the magnetotail. At Earth, changes in tail magnetic field intensity during the loading/unloading cycle are much smaller and occur on much longer time scales. Vervack et al. (p. 672 , published online 15 July) used the Mercury Atmospheric and Surface Composition Spectrometer onboard MESSENGER to make measurements of Mercury's neutral and ion exospheres. Differences in the altitude profiles of magnesium, calcium, and sodium over the north and south poles of Mercury indicate that multiple processes are at play to create and maintain the exosphere.

MESSENGER from Mercury The spacecraft MESSENGER passed by Mercury in October 2008, in what was the second of three fly-bys before it settles into the planet's orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planet's surface. Now, after MESSENGER, only 10% of Mercury's surface remains to be imaged up close. Denevi et al. (p. 613 ) use this near-global data to look at the mechanisms that shaped Mercury's crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercury's surface is dotted with impact craters. Watters et al. (p. 618 ) describe a well-preserved impact basin, Rembrandt, which is second in size to the largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by material of volcanic origin, preserving clues to its formation and evolution. It displays unique patterns of tectonic deformation, some of which result from Mercury's contraction as its interior cooled over time. Mercury's exosphere and magnetosphere were also observed (see the Perspective by Glassmeier ). Magnetic reconnection is a process whereby the interplanetary magnetic field lines join the magnetospheric field lines and transfer energy from the solar wind into the magnetosphere. Slavin et al. (p. 606 ) report observations of intense magnetic reconnection 10 times as intense as that of Earth. McClintock et al. (p. 610 ) describe simultaneous, high-resolution measurements of Mg, Ca, and Na in Mercury's exosphere, which may shed light on the processes that create and maintain the exosphere.

Earth-based radar observations and results from the MESSENGER mission have provided strong evidence that permanently shadowed regions near Mercury's poles host deposits of water ice. MESSENGER's complete orbital image and topographic datasets enable Mercury's surface to be observed and modeled under an extensive range of illumination conditions. The shadowed regions of Mercury's north polar region from 65°N to 90°N were mapped by analyzing Mercury Dual Imaging System (MDIS) images and by modeling illumination with Mercury Laser Altimeter (MLA) topographic data. The two independent methods produced strong agreement in identifying shadowed areas. All large radar-bright deposits, those hosted within impact craters ≥6 km in diameter, collocate with regions of shadow identified by both methods. However, only ∼46% of the persistently shadowed areas determined from images and ∼43% of the permanently shadowed areas derived from altimetry host radar-bright materials. Some sizable regions of shadow that do not host radar-bright deposits experience thermal conditions similar to those that do. The shadowed craters that lack radar-bright materials show a relation with longitude that is not related to the thermal environment, suggesting that the Earth-based radar observations of these locations may have been limited by viewing geometry, but it is also possible that water ice in these locations is insulated by anomalously thick lag deposits or that these shadowed regions do not host water ice.