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We perform a detailed geomorphological and compositional analysis on three craters hosting hollows located in the Victoria and Kuiper Quadrangles of Mercury. Based on Mercury Dual Imaging System data, high‐resolution detailed geomorphological mapping is provided in order to fully characterize the geological framework where hollows formed. In addition, we apply an unsupervised spectral clustering, based on a K‐mean algorithm, to separate in clusters our data sets. The comparison between the spectral and the well‐defined geomorphological units reveals a spatially coherent distribution. In particular, all hollows are uniquely identified by a well‐defined spectrum showing a wide absorption band between 0.558 and 0.828, with a possible hint of absorption toward 1 μm. Our analysis suggests that the composition of hollows may be characterized by a mixture of different minerals contributing to the absorptions found in our spectra. Indeed, sulfides alone (CaS, MnS, and MgS) cannot explain the spectrum behavior of hollows, even if the mechanism forming hollows likely involves the loss of volatiles from the surface. Hence, we have to consider bedrock‐forming material as partial responsible of the absorptions. For the studied hollows we suggest that the bedrock‐forming minerals are pyroxenes presenting transitional elements, like Cr, Ti, and Ni in substitution of Mg and/or Fe. Therefore, we suggest that the spectral characteristics of hollows are related to both remnant material produced by devolatilization process and to bedrock in which the hollows formed. Plain Language Summary: The Mercury Surface, Space ENvironment, GEochemistry, and Ranging mission provided the first detailed view of Mercury's surface and its space environment. Among the many discoveries, it revealed for the first time the presence of unusual bright irregular and rimless flat‐floored depression, called hollows, which are usually found on crater walls, rims, floors, and central peaks. Understanding the nature of hollows is still a major challenge since it is difficult to define which is their source mechanism. By using multicolor images acquired through the Mercury Dual Imaging System, we analyzed the nature of hollows hosted by three different impact craters, both from a geomorphological and a compositional perspective. Such analysis revealed that different units identified in high‐resolution geological maps are characterized by different spectral behaviors. Hollows in all craters have a well‐defined visible spectrum that when compared with laboratory spectra is indicative of the presence of both sulfides and pyroxene presenting transitional elements. This provides new insights into the hollows' nature and composition, suggesting that hollow terrains are the expression of both the remnant material coming from a process that involve devolatilization and the bedrock‐forming material (planet crust) where they formed. Key Points: Spectral behavior of hollows is identified by a wide absorption band between 0.558 and 0.828 μm, with a hint of absorption toward 1 μmThe hollow absorption can be explained by pyroxenes presenting transitional elements, in addition to sulfidesHollows consist of both the remnant material coming from a degassing process and the bedrock‐forming material in which they formed [ABSTRACT FROM AUTHOR]

Highlights: [•] MESSENGER Gamma-Ray Spectrometer data reveal a north polar Na enhancement on Mercury. [•] Na varies from 2.6wt% in equatorial regions to 4.9wt% poleward of 80°N latitude. [•] Enhancement may be due to higher alkali feldspar abundance in volcanic smooth plains. [•] Also consistent with temperature-induced mobilization of Na. [ABSTRACT FROM AUTHOR]

Thermal models for the north polar region of Mercury, calculated from topographic measurements made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, show that the spatial distribution of regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice. MESSENGER measurements of near-infrared surface reflectance indicate bright surfaces in the coldest areas where water ice is predicted to be stable at the surface, and dark surfaces within and surrounding warmer areas where water ice is predicted to be stable only in the near subsurface. We propose that the dark surface layer is a sublimation lag deposit that may be rich in impact-derived organic material. [ABSTRACT FROM AUTHOR]

Measurements of surface reflectance of permanently shadowed areas near Mercury's north pole reveal regions of anomalously dark and bright deposits at 1064-nanometer wavelength. These reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near-surface water ice. Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice, whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation. Impacts of comets or volatile-rich asteroids could have provided both dark and bright deposits. [ABSTRACT FROM AUTHOR]

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 x 1016 to 1018 grams and is consistent with delivery by comets or volatile-rich asteroids. [ABSTRACT FROM AUTHOR]

Abstract: Lobate scarps, thought to be the surface expression of large thrust faults, are the most spectacular contractional tectonic features visible on Mercury. Most lobate scarps follow a general and relatively simple pattern, with a roughly arcuate or linear form in plan view, and an asymmetric cross section characterized by a steeply rising scarp face and a gently declining back scarp. In this work, we study two peculiar and complex scarps in the Rembrandt region of Mercury through MESSENGER imagery. On the one hand, the formation of these scarps resulted in the deformation of features such as impact craters, fractures, extensional faults, and volcanic plains, while on the other hand, the deformed features partly influenced the formation of the scarps. Evidence for structural control on the formation of the scarps includes their orientation, segmentation, bifurcation, change in structural trend and dip orientation, and transition into high-relief ridges or wrinkle ridge morphologies in some cases. Thus, these two lobate scarps provide examples of complex geological relations among other features, expanding the recognized richness of mercurian geology. Also, the southern scarp records a complex history of contraction, suggesting that the development of some mercurian lobate scarps may be more complex than usually thought. [Copyright &y& Elsevier]

Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately haft the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury's topography occurred after the earliest phases of the planet's geological history. [ABSTRACT FROM AUTHOR]

Abstract: Mercurian lobate scarps are interpreted to be the surface expressions of thrust faults formed by planetary cooling and contraction, which deformed the crust down to the brittle–ductile transition (BDT) depth at the time of faulting. In this work we have used a forward modeling procedure in order to analyze the relation between scarp topography and fault geometries and depths associated with a group of prominent lobate scarps (Santa Maria Rupes and two unnamed scarps) located in the Kuiper region of Mercury for which Earth-based radar altimetry is available. Also a backthrust associated with one of the lobate scarps has been included in this study. We have obtained best fits for depths of faulting between 30 and 39km; the results are consistent with the previous results for other lobate scarps on Mercury. The so-derived fault depths have been used to calculate surface heat flows for the time of faulting, taking into account crustal heat sources and a heterogeneous surface temperature due to the variable insolation pattern. Deduced surface heat flows are between 19 and 39mWm−2 for the Kuiper region, and between 22 and 43mWm−2 for Discovery Rupes. Both BDT depths and heat flows are consistent with the predictions of thermal history models for the range of time relevant for scarp formation. [Copyright &y& Elsevier]

The article presents research on depressions in the deposits in impact craters on the planet Mercury. The authors used high-resolution images taken by the MESSENGER spacecraft to examine both the depressions and host rocks associated with crater central peaks, walls, and floors. They conclude that the hollows were most like formed by a loss of volatile materials due to processes such as pyroclastic volcanism, sublimation, and space weathering. Photographs of hollows on Mercury in craters including the Sander crater, the Tyagaraja crater, and the Praxiteles crater are provided.

The article discusses the formation of smooth plains found in the high northern latitudes of the planet Mercury. Observations from the MESSENGER spacecraft found that the plains show flow features, have a distinct color, and in some instances bury impact craters. The authors interpret this information to be evidence of flood-basalt style emplacement and argue that the surface composition is intermediate between that of komatiites and basalts. This indicates that volcanism occurred on Mercury following the heavy bombardment. Photographs, diagrams, and graphs are provided which show geological features on the planet, the distribution of impact craters, and possible lava flows.

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. [ABSTRACT FROM AUTHOR]

Abstract: Current data reveal that Mercury is a dynamic system with a core which has not yet solidified completely and is at least partially decoupled from the mantle. Radar speckle displacement experiments have demonstrated that the accuracy in spin-dynamics determination for Earth-like planets can approach 10−5. The extended analysis of space–time correlation properties of radar echoes shows that the behavior of speckles does not prevent estimation of Mercury’s instantaneous spin-vector components to accuracy of a few parts in 107. This limit can be reached with more powerful radar facilities and leads to constraining the interior in more detail from effects of spin dynamics, e.g., from observation of the core–mantle interplay through high precision monitoring of the 88-day spin-variation of Mercury’s crust. [Copyright &y& Elsevier]

Abstract: A recent dynamo model for Mercury assumes that the upper part of the planet''s fluid core is thermally stably stratified because the temperature gradient at the core–mantle boundary is subadiabatic. Vigorous convection driven by a superadiabatic temperature gradient at the boundary of a growing solid inner core and by the associated release of light constituents takes place in a deep sub-layer and powers a dynamo. These models have been successful at explaining the observed weak global magnetic field at Mercury''s surface. They have been based on the concept of codensity, which combines thermal and compositional sources of buoyancy into a single variable by assuming the same diffusivity for both components. Actual diffusivities in planetary cores differ by a large factor. To overcome the limitation of the codensity model, we solve two separate transport equations with different diffusivities in a double diffusive dynamo model for Mercury. When temperature and composition contribute comparable amounts to the buoyancy force, we find significant differences to the codensity model. In the double diffusive case convection penetrates the upper layer with a net stable density stratification in the form of finger convection. Compared to the codensity model, this enhances the poloidal magnetic field in the nominally stable layer and outside the core, where it becomes too strong compared to observation. Intense azimuthal flow in the stable layer generates a strong axisymmetric toroidal field. We find in double diffusive models a surface magnetic field of the observed strength when compositional buoyancy plays an inferior role for driving the dynamo, which is the case when the sulphur concentration in Mercury''s core is only a fraction of a percent. [Copyright &y& Elsevier]

Abstract: Scheduled for launch on board the BepiColombo Mercury Planetary Orbiter (MPO) in 2014, the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an innovative instrument for studying the surface composition and mineralogy of planet Mercury. MERTIS combines an uncooled grating push broom IR-spectrometer (TIS) with a radiometer (TIR), which will operate in the wavelength region of 7–14 and 7–40μm, respectively. The spatial resolution of the MERTIS observations will be about 500m globally and better than 500m for approximately 5–10% of the surface. The thermal infrared range offers unique diagnostic capabilities to study the surface composition of Mercury. In particular, feldspars can easily be detected and characterized, because they show several diagnostic spectral signatures in the 7–14μm range: the Christiansen feature, reststrahlen bands, and the transparency feature. In addition, MERTIS will allow the identification and mapping of elemental sulfur, pyroxenes, olivines, and other complex minerals. The scientific objectives of MERTIS include: (1) characterization of Mercury''s surface composition, (2) identification of rock-forming minerals, (3) mapping of the surface mineralogy, and (4) study of surface temperature variations and the thermal inertia. In preparation for the MERTIS data interpretation, we are performing spectral measurements of appropriate analogue materials in the Planetary Emissivity Laboratory (PEL) and are building a spectral library (Berlin Emissivity Database (BED)) of these materials for a variety of grain sizes. [Copyright &y& Elsevier]

Abstract: The limited knowledge about the majority of the Mercury surface leaves many open questions regarding its geological evolution, the anomalously high metal/silicate ratio, the magnetic field generation and exosphere evolution. An integrated suite of instruments, Spectrometer and Imagers for MPO BepiColombo-Integrated Observatory SYStem (SIMBIO-SYS), which includes a stereo imaging system (STC), a high-resolution imager (HRIC) and a visible–near-infrared imaging spectrometer (VIHI), has been selected for the ESA BepiColombo mission to Mercury. SIMBIO-SYS will scan the hermean surface with the three channels and map the physical, morphological, tectonic and compositional properties of the entire planet. The availability of high-resolution images will unveil details of specific target at an unprecedented resolution. The main scientific objectives and performances along with technical characteristics of SIMBIO-SYS are described in this paper. [Copyright &y& Elsevier]

Abstract: We describe the contributions that we expect the BepiColombo mission to make towards increased knowledge and understanding of Mercury''s surface and composition. BepiColombo will have a larger and more capable suite of instruments relevant for determination of the topographic, physical, chemical and mineralogical properties of the surface than carried by NASA''s MESSENGER mission. We anticipate that the insights gained into the planet''s geological history and its current space weathering environment will enable us to understand the relationships between surface composition and the composition of different types of crust. This will enable estimation of the composition of the mantle from which the crust was derived, and lead to better constraints on models for Mercury''s origin and the nature of the material from which it formed. [Copyright &y& Elsevier]

Abstract: The size of the Mercurian core and the low ferrous iron bearing silicate content of its crust offer constraints on formation models for the planet. Here we consider a bulk composition that allows endogenous formation of the planet''s large core, and by processing the mantle through a magma ocean, would produce a low-iron oxide crust consistent with observations. More Earth-like bulk compositions require silicate removal, perhaps by a giant impact, to create the planet''s large core fraction. We find that the endogenous model can produce a large core with either a plagioclase flotation crust or a low-iron oxide magmatic crust. Because a magma ocean creates a gradient in iron oxide content in the resulting planetary mantle, the parts of the mantle removed by a putative giant impact could result in either a high-iron oxide mantle in contradiction to current crustal measurements, or a low-iron oxide mantle consistent with the current understanding of Mercury. If a giant impact cannot preferentially remove shallow mantle material then the proto-Mercury must have had a bulk low iron-oxide composition. Thus a specific bulk composition is required to make Mercury endogenously, and either a specific process or a specific composition is required to make it exogenously through giant impact. Measurements taken by the MESSENGER mission, when compared to predictions given here, may help resolve Mercury''s formation process. [Copyright &y& Elsevier]

Abstract: Stereo-derived topographic models have shown that the impact basins Beethoven and Tolstoj on Mercury are shallow for their size, with depths of 2.5 and 2 (±0.7) km, respectively, while Caloris basin has been estimated to be 9 (±3) km deep on the basis of photoclinometric measurements. We evaluate the depths of Beethoven and Tolstoj in the context of comparable basins on other planets and smaller craters on Mercury, using data from Mariner 10 and the first flyby of the MESSENGER spacecraft. We consider three scenarios that might explain the anomalous depths of these basins: (1) volcanic infilling, (2) complete crustal excavation, and (3) viscoelastic relaxation. None of these can be ruled out, but the fill scenario would imply a thick lithosphere early in Mercury''s history and the crustal-excavation scenario a pre-impact crustal thickness of 15–55 km, depending on the density of the crust, in the area of Beethoven and Tolstoj. The potential for viscous relaxation of Beethoven, Tolstoj, and Caloris is explored with a viscoelastic model. Results show that relaxation of these basins could occur at plausible heat flux values for a range of crustal thicknesses. However, the amplitude of current topographic relief points to a crustal thickness of at least 60 km under this hypothesis. Relaxation of Caloris may have occurred if the floor is underlain by crust at least 20 km thick. We discuss future observations by MESSENGER that should distinguish among these scenarios. [Copyright &y& Elsevier]

Abstract: Images from MESSENGER''s first flyby of Mercury have shown convincing evidence for surface volcanism. Here we report on evidence in the new data for several features that are characterized by fractures and graben — rare features on a planet dominated by contractional deformation — that may be linked to intrusive activity. These features include: (1) A floor-fractured crater, interpreted to have been the site of laccolith-like sill intrusions; the feature is similar to some floor-fractured craters on the Moon and shows evidence for individual fractured dome-like uplifts on the floor. (2) A concentric complex of graben, observed inside the peak ring on the floor of the ~250-km-diameter Raditladi basin and associated with dark plains and possibly embayed by them; the feature may represent an unusual type of floor-fracturing associated with deeper intrusions and related ring dikes or cone sheets, or the graben may instead be the product of non-magmatic uplift of the basin floor. (3) A large radial graben swarm, Pantheon Fossae, located near the center of the Caloris basin, thus far unique on Mercury, and characterized by hundreds of individual graben segments ranging from ~5 km to ~110 km in length. In the nexus, graben crosscut one another and produce a local polygonal pattern; others curve away from the center as the nexus is approached. Two scales of graben length are observed; the radius of the dense radially symmetric plexus of graben is ~175 km, and a few graben extend to greater radial distances to the north and southwest out to distances that intersect with a ring of generally concentric graben around the outer basin floor. Two width scales of graben are observed; a large graben about 8 km wide emerges from the nexus and extends for ~100 km; most graben are less than half this width. Some graben walls appear cuspate, with convex-outward wall segments that resemble crater chain segments. One crater chain with distinctive raised rims parallels nearby graben. Locally, some graben appear in en echelon patterns, and smaller graben sometimes show cross-cutting (superposition) relationships. Abundant impact craters, the most prominent being Apollodorus, and secondary crater clusters and chains are superposed on the graben system; there is little evidence that craters greater than 5 km in diameter have been cut by a graben. This relation implies that the graben swarm formed soon after the emplacement of the Caloris floor plains. These graben are interpreted to be the surface expression of a radial dike swarm emanating from a subsurface magma reservoir. Similar features, in which the dikes contribute to a near-surface stress field that favors radial graben, are known on the Earth, Venus, and Mars. The location of Pantheon Fossae in the center of the Caloris basin suggests that formation of the radial graben structure is linked to basin evolution. [Copyright &y& Elsevier]

Abstract: High-resolution images obtained during the first MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) flyby of Mercury show evidence for igneous modification of the surface through the formation of pit craters on the floors of several impact craters. Pit craters are rimless steep-sided depressions that are inferred to have formed by non-impact processes. Among the arguments in favor of this interpretation are that the features lack evident rims; exhibit no observable ejecta; are irregularly shaped (e.g., subcircular, elliptical, or arcuate), with the long axis of the pit crater often concentric to the rim of the host impact crater; and are superposed on smooth plains deposits within the craters. Pit craters documented to date do not exhibit signs of extrusive flows. Impact craters hosting pit craters, termed pit-floor craters, vary in size from 55 to 120 km in diameter, while the associated pit craters range in maximum horizontal dimension from 20 to almost 40 km. Pit crater radius and host crater radius are not well correlated for pit-floor craters on Mercury, in contrast to the situation for otherwise similar features on Mars. On the basis of morphology, structural association, relative age, and proximity to smooth plains for pit craters on Mercury, we test mechanisms of pit crater formation previously proposed for such features on other planetary bodies (e.g., Earth, Moon, Mars, and Venus). We conclude that pit craters on Mercury formed through collapse into an underlying drained magma chamber. Pit-floor craters thus provide evidence for near-surface magmatic activity on Mercury and extend the range of evidence for magmatism beyond such surface expressions as smooth plains and pyroclastic deposits. [Copyright &y& Elsevier]

Abstract: The first MESSENGER flyby of Mercury obtained images of 21% of the surface not seen by Mariner 10, including the center and western half of the Caloris basin and regions near the terminator that show details of the nature of smooth and intercrater plains. These new data have helped to address and resolve a series of longstanding questions on the existence and nature of volcanism on Mercury and the distribution of volcanic materials. Data from the Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft have shown the following: (1) Numerous volcanic vents, in the form of irregularly shaped rimless depressions, are concentrated around the interior edge of the Caloris basin. (2) These vents appear to be sources for effusive volcanism that in one case built a shield in excess of 100 km in diameter and in some cases formed bright haloes around the vents that are interpreted to represent pyroclastic eruptions. (3) Lobate margins of plains units, seen previously in Mariner 10 data, are documented in MESSENGER images with more clarity and are often distinctive in morphology and color properties, supporting the interpretation that these features are the edges of lava flow units. (4) The interior of the Caloris basin is filled with plains units spectrally distinctive from the rim deposits, and comparison with the lunar Imbrium basin and superposed impact crater stratigraphy provide evidence that these units are volcanic in origin; detailed differences in the mineralogy of lava flow units, so prominent in Imbrium, are not seen in the Caloris interior. (5) Some of the smooth plains surrounding the exterior of the Caloris basin show distinct differences in color and morphological properties, supporting a volcanic origin. (6) Some smooth and intercrater plains units distant from the Caloris basin show evidence of flooding and embayment relations unrelated to Caloris ejecta emplacement; local and regional geological and color relationships support a volcanic origin for these plains. (7) Large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes. (8) Crater embayment and flooding relationships in selected areas suggest volcanic plains thicknesses of many hundreds of meters and local thicknesses inside impact craters of up to several kilometers. (9) Impact crater size–frequency distributions for Caloris exterior deposits, including the facies of the Caloris Group and relatively high- and low-albedo smooth plains, show that they are younger than plains interior to Caloris and thus must be dominantly the product of post-Caloris volcanism. These new data provide evidence that supports and confirms earlier hypotheses from Mariner 10 data that volcanism was important in shaping the surface of Mercury. The emerging picture of the volcanic style of Mercury is similar to that of the Moon, the other small, one-plate planetary body: there are no major shield volcanoes (e.g., comparable to Tharsis Montes on Mars), shallow magma reservoirs are rare, and there is little evidence for surface deformation or long-lived volcanic sources related to sites of upwelling mantle. The close association of volcanic plains and surface deformation features suggests that future observations and analyses can help document the relation between the volcanic flux and the evolving state and magnitude of stress in the lithosphere of Mercury. [Copyright &y& Elsevier]

Abstract: The ~40-km-diameter Apollodorus impact crater lies near the center of Pantheon Fossae, a complex of radiating linear troughs itself at the approximate center of the 1500-km-diameter Caloris basin on Mercury. Here we use a series of finite element models to explore the idea that the Apollodorus crater-forming impact induced the formation of radially oriented graben by altering a pre-existing extensional stress state. Graben in the outer portions of the Caloris basin, which display predominantly circumferential orientations, have been taken as evidence that the basin interior was in a state of horizontal extensional stress as a result of uplift. If the Apollodorus crater formed at the time of such a stress state, impact-induced damage to basin fill material would have caused basin material to move radially outward, leading to a decrease in the radial extensional stress and an increase in the circumferential stress. If this change in differential stress was sufficient to induce failure, the predicted style of faulting would be radial graben extending outward from the exterior crater rim. The ~230-km radial extent of Pantheon Fossae implies, by this scenario, that the Apollodorus impact generated a large damage zone, extending to perhaps three crater radii (~60 km) or more. The calculations also suggest, under this scenario, that the Caloris basin fill had greater strength than the surrounding crust and that the basin uplift and extensional stress field prior to the Apollodorus impact were close to azimuthally symmetric. The location of Pantheon Fossae very near the center of the Caloris basin appears to be coincidental; any crater similar in size to Apollodorus and located within ~300 km of the basin center could have produced a radiating set of graben by the mechanism explored here. [Copyright &y& Elsevier]

Abstract: MESSENGER''s first flyby of Mercury provided the first complete view of the Caloris basin and its interior smooth plains. Multispectral imaging shows that the interior plains are spectrally distinct from surrounding terrain, with a reflectance about 15–20% higher and a steeper spectral slope than the global average. The Caloris basin rim massifs and hummocky material, in contrast, have lower reflectance and a shallower spectral slope than the global average and extend around the entire basin. Isolated, relatively red patches along the margins of Caloris each surround irregularly shaped, scalloped-edged, rimless depressions that are interpreted to be volcanic vents, some with associated pyroclastic deposits. A nearly continuous annulus of smooth plains exterior to the basin displays spectral characteristics that contrast sharply with those of the basin-interior smooth plains. The exterior smooth plains are lower in reflectance and spectrally similar to the Caloris rim massifs and the hummocky materials. The rims and floors of larger impact craters in the interior plains expose low-reflectance material. There is widespread evidence of contractional and extensional tectonic deformation of the Caloris interior smooth plains. A set of linear radial graben, Pantheon Fossae, radiates outward from a zone near the basin center, and some of these graben extend to and intersect a set of generally basin-concentric graben distributed along the outer margin of the Caloris basin floor. The average areal extensional strain expressed by the mapped system of graben is ~0.08%. Wrinkle ridges also deform the interior smooth plains and predate both basin-radial and basin-concentric graben. The average areal contractional strain expressed by the mapped wrinkle ridges is ~0.07%. The density of wrinkle ridges on the western interior smooth plains is less than on the eastern interior plains, in contrast to the more even distribution of graben. A similar difference is seen between the density of wrinkle ridges on the western and eastern exterior smooth plains, with far fewer wrinkle ridges per unit area on the western annular plains. Superposition relations, spectral contrast, and the occurrence of volcanic vents and possible pyroclastic deposits all support a volcanic origin for the Caloris interior plains. Crater counts indicate that the Caloris exterior plains and hummocky material are younger than the Caloris rim and interior plains and are therefore also likely volcanic in origin. The contrast in spectral properties between the interior and exterior plains indicates a difference in the composition of the two expanses of volcanic material. Models for extensional tectonic deformation of the Caloris basin interior inferred from the distribution of graben viewed by Mariner 10 must be reevaluated to account for the discovery of Pantheon Fossae. [Copyright &y& Elsevier]

Abstract: The Mariner 10 flybys revealed the eastern third of the Caloris basin and demonstrated its importance as a chronostratigraphic marker for the geological history of Mercury. Uncertain after that mission were the size of the basin, the full distribution of materials deposited or modified during the impact process, and the stratigraphic and age relationships between Caloris and surrounding terrain. Images obtained during MESSENGER''s January 2008 flyby of Mercury revealed that the Caloris basin is ∼15% larger than previously estimated and moderately elliptical (∼1525 by ∼1315 km). Basin-related sculpture and secondary craters are dispersed widely in areas surrounding the basin, confirming the widespread significance of this event as a stratigraphic marker. Units mapped around Caloris on the basis of Mariner 10 data are generally recognized in regions observed for the first time by MESSENGER, including most facies of the Caloris Group (the Caloris Montes, Odin Formation, and Van Eyck Formation). The only unit without obvious exposures in MESSENGER images is the Nervo Formation, which was originally interpreted as impact melt or fallback ejecta. New measurements of the size-frequency distribution of impact craters both within the Caloris basin and on surrounding units, particularly the Odin Formation, support the hypothesis that the vast majority of circum-Caloris plains postdate the basin and are likely volcanic. The MESSENGER observations re-emphasize the importance of understanding the exterior stratigraphy of the basin in order to utilize the Caloris event as a discrete time horizon in the geological history of Mercury. [Copyright &y& Elsevier]

During its first flyby of Mercury, MESSENGER imaged many tectonic landforms, most of which are contractional in nature and include lobate scarps, high-relief ridges, and wrinkle ridges. Wrinkle ridges are found on the smooth plains that partially fill the interior and surround the exterior of the Caloris basin and also on smooth plains that fill the interiors of smaller impact basins and larger craters. MESSENGER revealed a radial graben complex, Pantheon Fossae, nearly co-centered with the Caloris basin. Pantheon Fossae and a network of mostly basin-concentric graben in the outer portions of the Caloris basin interior form a pattern of extension not seen elsewhere on Mercury. The first clear example of extensional faults outside of the Caloris basin has been documented on smooth plains inside the peak ring of the relatively young Raditladi basin. A map of the distribution of tectonic landforms imaged by MESSENGER and Mariner 10 shows that lobate scarps are the most widespread type of feature. Estimates of the horizontal shortening associated with lobate scarps that crosscut and overthrust small impact craters imaged by MESSENGER range from ~1 to 3 km. Previously unrecognized lobate scarps detected in areas imaged by Mariner 10 indicate that earlier estimates of contractional strain are low. A new estimate of the average areal contractional strain (~0.06%) accommodated by scarps is at least one third greater than comparable previous estimates and corresponds to a decrease in Mercury''s radius of at least 0.8 km since the end of heavy impact bombardment of the inner solar system. These figures are lower bounds because of the likelihood that not all lobate scarps have been identified even in areas imaged to date. Crosscutting and embayment relations revealed by MESSENGER suggest that lobate scarps formed before the end of smooth plains emplacement and continued to be active after the emplacement of the youngest smooth plains deposits. Relatively recent activity on lobate scarps may be the expression of slow but continuous radial contraction that accompanied cooling of Mercury''s interior and the growth of the planet''s solid inner core. [Copyright &y& Elsevier]

Abstract: Images obtained by the MESSENGER spacecraft have revealed evidence for pyroclastic volcanism on Mercury. Because of the importance of this inference for understanding the interior volatile inventory of Mercury, we focus on one of the best examples determined to date: a shield-volcano-like feature just inside the southwestern rim of the Caloris impact basin characterized by a near-central, irregularly shaped depression surrounded by a bright deposit interpreted to have a pyroclastic origin. This candidate pyroclastic deposit has a mean radius of ~24 km, greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions. From the extent of the candidate pyroclastic deposit, we characterize the eruption parameters of the event that emplaced it, including vent speed and candidate volatile content. The minimum vent speed is ~300 m/s, and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million (ppm) of the volatiles typically associated with pyroclastic eruptions on other bodies (e.g., CO, CO2, H2O, SO2, H2S). For comparison, measurements of the exsolution of volatiles (H2O, CO2, S) from basaltic eruptive episodes at Kilauea volcano, Hawaii, indicate values of ~1300–6500 ppm for the terrestrial mantle source. Evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of Mercury is an unexpected result and provides a new constraint on models for the planet''s formation and early evolution. [Copyright &y& Elsevier]

Abstract: Among the terrestrial planets, Mercury''s composition is characterized by two specific features: a high density and a low surface FeO content. Based on these two constraints, different geochemical models have been proposed, according to different formation scenarios. Here thermodynamical modeling is used to derive the mantle and crust mineralogy associated with these geochemical models. For each mineralogy, the electrical conductivity profile and associated electromagnetic data are computed. Due to the very different oxide/silicate ratios, most geochemical models proposed for Mercury''s formation show very different electromagnetic signatures. As a result, future measurements with MESSENGER and BepiColombo missions will help differentiating between different interior models and different formation scenarios. [Copyright &y& Elsevier]

Abstract: Mariner 10 clear filter (490 nm) images of Mercury were recalibrated and photometrically normalized to produce a mosaic of nearly an entire hemisphere of the planet. Albedo contrasts are slightly larger than seen in the lunar highlands (excluding maria). Variegations indicative of compositional differences include diffuse low albedo units often overlain by smooth plains, the high albedo smooth plains of Borealis Planitia, and high-albedo enigmatic crater floor deposits. A higher level of contrast between immature crater ejecta and average mature material on Mercury compared to the Moon is consistent with a more intense space weathering environment on Mercury that results in a more mature regolith. Immature lunar highlands materials are ∼1.5 times higher in reflectance than analogous immature mercurian materials. Immature materials of the same composition would have the same reflectance on both bodies, thus this observation requires that Mercury''s crust contains a significant darkening agent, either opaque minerals or ferrous iron bearing silicates, in abundances significantly higher than those of the lunar highlands. If the darkening agent is opaque minerals (e.g. ilmenite or ulvospinel) Mercury''s crust may contain significant ferrous iron and yet not exhibit a 1-μm absorption band. [Copyright &y& Elsevier]

By applying our understanding of lunar space weathering processes, we can predict how space weathering will effect the soil properties on Mercury. In particular, the extreme temperature range on Mercury may result in latitudinal variations in the size distribution of npFe0, and therefore the spectral properties of the soil. [ABSTRACT FROM AUTHOR]

The article discusses the U.S. National Aeronautics and Space Administration's (NASA's) MESSENGER mission to the planet Mercury. An overview of two reports within the issue regarding the results of analyses of MESSENGER data by researcher D. E. Smith and colleagues investigating Mercury's moment-of-inertia factor and M. T. Zuber and colleagues investigating the dynamic range of mercurian topography is presented. Also discussed is the internal structure of Mercury, which MESSENGER measurements revealed may contain an anti-crust, and the geological and geophysical effects of the anti-crust.

The article focuses on the geology of the planet Mercury detected by the MESSENGER spacecraft which reveals an abundance of elements on Mercury's surface that could easily be boiled out of hot rock, which rules out the possibility that Mercury had its outer parts blasted away by a young Sun.