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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]

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 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]