Mercury surface composition: Integrating petrologic modeling and remote sensing data to place constraints on FeO abundance

A significant opaque component in Mercury’s crust is inferred based on albedo and spectral observations. Previous workers have favored iron-titanium bearing oxide minerals as the spectrally neutral opaque. A consequence of this hypothesis is that Mercury’s surface would have a high FeO content. An array of remote sensing techniques have not provided definitive constraints on the FeO content of Mercury’s surface. However, spectral observations have not detected a diagnostic 1 μm absorption band and have thus limited the FeO in coexisting silicates to <2 wt.% FeO. In this paper, we assess equilibrium among oxide and silicate minerals to constrain the distribution of iron between opaque oxides and silicates under a variety of environmental conditions. Equilibrium modeling is favored here because the geologic process that produced Mercury’s low-albedo intermediate terrain must have occurred globally, which favors a common widespread igneous process. Based on our modeling, we find that iron-rich ilmenite cannot occur with silicates that do not display a 1 μm absorption feature unless plagioclase abundances are high. However, such high plagioclase abundances are precluded by Mercury’s low albedo. Incorporating equilibrium crystallization modeling with spectral and albedo constraints we find the iron abundance of Mercury’s intermediate terrain is ?10 wt.% FeO. This intermediate iron composition matches constraints provided by visible albedo and total neutron absorption observed by MESSENGER. In fact, the total neutron absorption of mixtures of oxide, plagioclase, olivine and pyroxene for the oxide abundances estimated for Mercury, favor Mg-rich members of the ilmenite-geikielite solid-solution series. This work offers compositional constraints for Fe, Ti, and Mg that will be testable by various MESSENGER instrument data sets after it begins its orbital mission.     

Orbital effects on Mercury’s escaping sodium exosphere

We present results from coronagraphic imaging of Mercury’s sodium tail over a 7° field of view. Several sets of observations made at the McDonald Observatory since May 2007 show a tail of neutral sodium atoms stretching more than 1000 Mercury radii (R_m) in length, or a full degree of sky. However, no tail was observed extending beyond 120 R_m during the January 2008 MESSENGER fly-by period, or during a similar orbital phase of Mercury in July 2008. Large changes in Mercury’s heliocentric radial velocity cause Doppler shifts about the Fraunhofer absorption features; the resultant change in solar flux and radiation pressure is the primary cause of the observed variation in tail brightness. Smaller fluctuations in brightness may exist due to changing source rates at the surface, but we have no explicit evidence for such changes in this data set. The effects of radiation pressure on Mercury’s escaping atmosphere are investigated using seven observations spanning different orbital phases. Total escape rates of atmospheric sodium are estimated to be between 5 and 13 × 10²³ atoms/s and show a correlation to radiation pressure. Candidate sources of Mercury’s sodium exosphere include desorption by UV sunlight, thermal desorption, solar wind channeled along Mercury’s magnetic field lines, and micro-meteor impacts. Wide-angle observations of the full extent of Mercury’s sodium tail offer opportunities to enhance our understanding of the time histories of these source rates.     

Self-consistent modelling of Mercury’s exosphere by sputtering, micro-meteorite impact and photon-stimulated desorption

A Monte-Carlo model of exospheres (Wurz and Lammer, 2003) was extended by treating the ion-induced sputtering process, photon-stimulated desorption, and micro-meteorite impact vaporisation quantitatively in a self-consistent way starting with the actual release of particles from the mineral surface of Mercury. Based on available literature data we established a global model for the surface mineralogy of Mercury and from that derived the average elemental composition of the surface. This model serves as a tool to estimate densities of species in the exosphere depending on the release mechanism and the associated physical parameters quantitatively describing the particle release from the surface.Our calculation shows that the total contribution to the exospheric density at the Hermean surface by solar wind sputtering is about 4×10⁷ m^-3, which is much less than the experimental upper limit of the exospheric density of 10¹² m^-3. The total calculated exospheric density from micro-meteorite impact vaporisation is about 1.6×10⁸ m^-3, also much less than the observed value. We conclude that solar wind sputtering and micro-meteorite impact vaporisation contribute only a small fraction of Mercury’s exosphere, at least close to the surface. Because of the considerably larger scale height of atoms released via sputtering into the exosphere, sputtered atoms start to dominate the exosphere at altitudes exceeding around 1000 km, with the exception of some light and abundant species released thermally, e.g. H₂ and He. Because of Mercury’s strong gravitational field not all particles released by sputtering and micro-meteorite impact escape. Over extended time scales this will lead to an alteration of the surface composition.     

Constraints on Mercury’s Na exosphere: Combined MESSENGER and ground-based data

We have used observations of sodium emission obtained with the McMath-Pierce solar telescope and MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) to constrain models of Mercury’s sodium exosphere. The distribution of sodium in Mercury’s exosphere during the period January 12-15, 2008, was mapped using the McMath-Pierce solar telescope with the 5″ × 5″ image slicer to observe the D-line emission. On January 14, 2008, the Ultraviolet and Visible Spectrometer (UVVS) channel on MASCS sampled the sodium in Mercury’s anti-sunward tail region. We find that the bound exosphere has an equivalent temperature of 900-1200 K, and that this temperature can be achieved if the sodium is ejected either by photon-stimulated desorption (PSD) with a 1200 K Maxwellian velocity distribution, or by thermal accommodation of a hotter source. We were not able to discriminate between the two assumed velocity distributions of the ejected particles for the PSD, but the velocity distributions require different values of the thermal accommodation coefficient and result in different upper limits on impact vaporization. We were able to place a strong constraint on the impact vaporization rate that results in the release of neutral Na atoms with an upper limit of 2.1 × 10⁶ cm⁻² s⁻¹. The variability of the week-long ground-based observations can be explained by variations in the sources, including both PSD and ion-enhanced PSD, as well as possible temporal enhancements in meteoroid vaporization. Knowledge of both dayside and anti-sunward tail morphologies and radiances are necessary to correctly deduce the exospheric source rates, processes, velocity distribution, and surface interaction.     

Constraints on Mercury’s surface composition from MESSENGER and ground-based spectroscopy

The composition and chemistry of Mercury’s regolith has been calculated from MESSENGER MASCS 0.3-1.3 μm spectra from the first flyby, using an implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 μm. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at ∼0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury’s low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35-70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, ∼20-45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals. Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40-50 wt.% SiO₂, 10-35 wt.% Al₂O₃, 1-8 wt.% FeO, and <25 wt.% TiO₂; the inferred rock type is basalt. Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury’s low brightness. The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt.% MnO and the presence of a Mn-rich hedenbergite. Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 μm crystal field absorption bands and may thus act as a strong darkening agent on Mercury. Also, one or more of thermally darkened silicates, Fe-poor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo. A major part of the total microphase iron present in Mercury’s regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources.     

Resolved spectroscopy of Mercury in the near-IR with SpeX/IRTF

We present resolved near-infrared spectra of Mercury scanning 70% of the surface in latitude and longitude from three separate observations, allowing us to perform a compositional investigation of its surface. By scanning the surface we find that all spectra in our sample are remarkably similar suggesting overall compositional homogeneity. We do, however, observe a slope difference between the spectra. These slope changes are most likely due to differences in the emission angle over different parts of the surface. We confirm the presence of a 1.1 μm feature that had been previously detected (Warell, J. et al. [2006]. Icarus 180, 281-291) and attributed to Ca-rich clinopyroxene. Finally, we investigated Mercury’s surface composition by comparing its spectrum with ground-based lunar spectra, lunar soil spectra collected in the laboratory, and analysis with a simple linear mixing model using various minerals as end-members. The result of this compositional investigation reveals that Mercury’s surface composition is likely to be quite different from the Moon’s. While low-Ca iron-rich pyroxenes are main surface components on the Moon (abundance varying from ∼5% to ∼35%), their abundance on Mercury may not exceed 5%. We also find that a Ca-rich clinopyroxene (in the hedenbergite-diopside series) is likely to be a main component of Mercury’s surface whereas this mineral is almost absent on the Moon. Our analysis also suggests the possible presence of olivine. We find that Mercury’s slope is less red than that of the Moon, in agreement with results from MESSENGER (McClintock, W.E., and 12 colleagues [2008]. Science 321, 62-65), and composition rather than variation of space weathering is likely the cause of this difference.     

Neutron capture effects on samarium,europium, and gadolinium in Apollo 15 deep drill‐core samples

Abstract— The isotopic compositions of Sm and Gd in seven lunar samples from the Apollo 15 deep drill core were determined to discuss the effects of neutron capture near the lunar surface. Large isotopic deviations of ¹⁵⁰Sm/¹⁴⁹Sm, ¹⁵⁶Gd/¹⁵⁵Gd, and ¹⁵⁸Gd/¹⁵⁷Gd derived from neutron capture effects were observed in all samples. Although neutron capture products in lunar samples were investigated extensively in the 1970s, our precise isotopic measurements resulted in several new findings. The neutron fluence in the Apollo 15 drill core is a function of depth with a symmetric peak at 190 g/cm² depth from the surface, confirming the results of earlier investigations. Neutron fluence values calculated from the isotopic shifts by comparison to artificially irradiated standard reagents were (5.16–7.49) × 10¹⁶ n/cm². These values are 1.3 to 1.4x larger than those previously reported. Variations of ε_Sm/ε_Gd with depth are interpreted as being due to variations in the neutron energy spectrum. Here ε_Sm and ε_Gd are defined as in previous studies of lunar neutron stratigraphy. Our data suggest that the neutron is more thermalized at the lower layers than it is at the upper layers. In addition to large isotopic shifts for ¹⁴⁹Sm, ¹⁵⁰Sm, ¹⁵⁵Gd, ¹⁵⁶Gd, ¹⁵⁷Gd, and ¹⁵⁸Gd, isotopic enrichments of ¹⁵²Gd and ¹⁵⁴Gd derived from neutron capture for ¹⁵¹Eu and ¹⁵³Eu, respectively, were also observed in all samples.     

Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field

The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3-6 log units below the iron-wüstite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO₂ being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core-mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe-S-Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe-S liquid, poor in Si; (ii) a middle layer of Fe-Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.     

Global contraction of planetary bodies due to despinning: Application to Mercury and Iapetus

We extend previous work on the global tectonic patterns generated by despinning with a self-consistent treatment of the isotropic despinning contraction that has been ignored. We provide simple analytic approximations that quantify the effect of the isotropic despinning contraction on the global shape and tectonic pattern. The isotropic despinning contraction of Mercury is ∼93 m (T/1 day)⁻², where T is the initial rotation period. If we take into account both the isotropic contraction and the degree-2 deformations associated with despinning, the preponderance of compressional tectonic features on Mercury’s surface requires an additional isotropic contraction ?1 km (T/1 day)⁻², presumably due to cooling of the interior and growth of the solid inner core. The isotropic despinning contraction of Iapetus is ∼9 m (T/16 h)⁻², and it is not sensitive to the presence of a core or the thickness of the elastic lithosphere. The tectonic pattern expected for despinning, including the isotropic contraction, does not explain Iapetus’ ridge. Furthermore, the ridge remains unexplained with the addition of any isotropic compressional stresses, including those generating by cooling.     

A neutron capture study of the Jilin chondrite

Abstract– The isotopic compositions of Sm and/or Gd of nine documented drill core samples of the Jilin H5 chondrite were determined to study the neutron capture records of individual meteorite samples. All the samples showed significant isotopic shifts of ¹⁵⁰Sm/¹⁴⁹Sm and/or ¹⁵⁸Gd/¹⁵⁷Gd corresponding to neutron fluences of (1.3–1.7) × 10¹⁵ n cm^?2. Considering the short 4π irradiation age of 0.32 Ma during the second stage, the 2π irradiation of 7 Ma during the first stage is the main influence on the Sm and Gd isotopic shifts of the Jilin chondrite. Although a depth dependence of the neutron capture effects was expected from the isotopic variations of ¹⁵⁰Sm/¹⁴⁹Sm and ¹⁵⁸Gd/¹⁵⁷Gd in the Jilin chondrite core samples that were possibly drilled perpendicular to the surface of the large object with a radius of >10 m in the 2π‐geometry, no clear evidence was observed in this study. The data from the combination of the isotopic shifts between Sm and Gd defined as ε_Sm/ε_Gd suggest that the neutrons produced in the 2π‐geometry of the Jilin chondrite follow the similar energy spectra as the neutrons in lunar samples, although the present analytical quality is not enough to discuss a critical discussion for the thermalization of the neutron energy levels.     

Modeling of the magnetosphere of Mercury at the time of the first MESSENGER flyby

The MESSENGER spacecraft flyby of Mercury on 14 January 2008 provided a new opportunity to study the intrinsic magnetic field of the innermost planet and its interaction with the solar wind. The model presented in this paper is based on the solution of the three-dimensional, bi-fluid equations for solar wind protons and electrons in the absence of mass loading. In this study we provide new estimates of Mercury’s intrinsic magnetic field and the solar wind conditions that prevailed at the time of the flyby. We show that the location of the boundary layers and the strength of the magnetic field along the spacecraft trajectory can be reproduced with a solar wind ram pressure P_sw = 6.8 nPa and a planetary magnetic dipole having a magnitude of 210 R_M³ − nT and an offset of 0.18 R_M to the north of the equator, where R_M is Mercury’s radius. Analysis of the plasma flow reveals the existence of a stable drift belt around the planet; such a belt can account for the locations of diamagnetic decreases observed by the MESSENGER Magnetometer. Moreover, we determine that the ion impact rate at the northern cusp was four times higher than at the southern cusp, a result that provides a possible explanation for the observed north-south asymmetry in exospheric sodium in the neutral tail.     

Mercury’s magnetospheric magnetic field after the first two MESSENGER flybys

The “paraboloid” model of Mercury’s magnetospheric magnetic field is used to determine the best-fit magnetospheric current system and internal dipole parameters from magnetic field measurements taken during the first and second MESSENGER flybys of Mercury on 14 January and 6 October 2008. Together with magnetic field measurements taken during the Mariner 10 flybys on 29 March 1974 and 16 March 1975, there exist three low-latitude traversals separated in longitude and one high-latitude encounter. From our model formulation and fitting procedure a Mercury dipole moment of 196 nT ·  (where R_M is Mercury’s radius) was determined. The dipole is offset from Mercury’s center by 405 km in the northward direction. The dipole inclination to Mercury’s rotation axis is relatively small, ∼4°, with an eastern longitude of 193° for the dipole northern pole. Our model is based on the a priori assumption that the dipole position and the moment orientation and strength do not change in time. The root mean square (rms) deviation between the Mariner 10 and MESSENGER magnetic field measurements and the predictions of our model for all four flybys is 10.7 nT. For each magnetic field component the rms residual is ∼6 nT or about 1.5% of the maximum measured magnetic field, ∼400 nT. This level of agreement is possible only because the magnetospheric current system parameters have been determined separately for each flyby. The magnetospheric stand-off distance, the distance from the planet’s center to the inner edge of the tail current sheet, the tail lobe magnetic flux, and the displacement of the tail current sheet relative to the Mercury solar-magnetospheric equatorial plane have been determined independently for each flyby. The magnetic flux in the tail lobes varied from 3.8 to 5.9 MWb; the subsolar magnetopause stand-off distance from 1.28 to 1.43 R_M; and the distance to the inner edge of the current sheet from 1.23 to 1.32 R_M. The differences in the current systems between the first and second MESSENGER flybys are attributed to the effects of strong magnetic reconnection driven by southward interplanetary magnetic field during the latter flyby.     

Exposure of spectrally distinct material by impact craters on Mercury: Implications for global stratigraphy

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.     

Radar measurements of Mercury’s north pole at 70 cm wavelength

We present radar imaging of Mercury using the Arecibo Observatory’s 70-cm wavelength radar system during the inferior conjunction of July 1999. At that time the sub-Earth latitude was ∼11°N and the highly reflective region at Mercury’s north pole that was first identified in radar images at the shorter wavelengths of 3.6 cm [Slade, M.A., Butler, B.J., Muhleman, D.O., 1992. Science 258, 635-640] and 13 cm [Harmon, J.K., Slade, M.A., 1992. Science 258, 640-643] was again clearly detected. The reflectivity averaged over a 75,000 km² region including the pole is similar to that measured at the other wavelengths over a comparable area, and the 70 cm circular polarization ratio of μ_C∼0.87 is possibly slightly lower. If this strong backscattering results from volume scattering in low absorption layers, the persistence of this effect over more than an order of magnitude change in wavelength scale has implications for the depth and thickness of the deposits responsible. The resolution of the radar maps at this wavelength is not sufficient to resolve individual craters, nor to discern features at other latitudes, but the planet’s total reflectivity is consistent with previous work and the scattering function suggests a surface roughness at this wavelength similar to the lunar highlands.     

The influence of Mercury’s inner core on its physical libration

As Mercury orbits the Sun, gravitational torques on its equatorial elliptical shape give rise to a planetary libration. The amplitude of Mercury’s libration, as determined from Earth-based radar speckle pattern observations, suggests that only the mantle participates in the motion. This indicates a decoupling between the core and the mantle, and therefore that the outermost part of the core must be fluid. If a solid inner core is present at the center of Mercury, the equatorial elliptical shape of the latter may become misaligned with that of Mercury’s mantle, leading to an internal gravitational torque between the two. If this torque is large, it may participate in the dynamics of Mercury’s libration. The goal of this work is to determine whether Mercury’s observed librations can be used to place constraints on the properties of its inner core. We present a comparison between predicted and observed librations for a range of interior models of Mercury, with various inner core sizes and fluid core densities. We show that a marginally better fit to observations can be achieved for interior models that have an inner core radius larger than 400 km. However, the improvement in fit is small, and it is not possible to draw robust conclusions on the size of Mercury’s inner core on the basis of existing libration data. Nevertheless, our study demonstrates that the influence of the inner core on the libration of Mercury could be detected with a decade worth of accurate observations.     

The origin of young mare basalts inferred from lunar meteorites Northwest Africa 4734, 032, and LaPaz Icefield 02205

Northwest Africa (NWA) 4734 is an unbrecciated basaltic lunar meteorite that is nearly identical in chemical composition to basaltic lunar meteorites NWA 032 and LaPaz Icefield (LAP) 02205. We have conducted a geochemical, petrologic, mineralogic, and Sm‐Nd, Rb‐Sr, and Ar‐Ar isotopic study of these meteorites to constrain their petrologic relationships and the origin of young mare basalts. NWA 4734 is a low‐Ti mare basalt with a low Mg* (36.5) and elevated abundances of incompatible trace elements (e.g., 2.00 ppm Th). The Sm‐Nd isotope system dates NWA 4734 with an isochron age of 3024 ± 27 Ma, an initial ε_Nd of +0.88 ± 0.20, and a source region ¹⁴⁷Sm/¹⁴⁴Nd of 0.201 ± 0.001. The crystallization age of NWA 4734 is concordant with those of LAP 02205 and NWA 032. NWA 4734 and LAP 02205 have very similar bulk compositions, mineral compositions, textures, and ages. Their source region ¹⁴⁷Sm/¹⁴⁴Nd values indicate that they are derived from similar, but distinct, source materials. They probably do not sample the same lava flow, but rather are similarly sourced, but isotopically distinct, lavas that probably originate from the same volcanic complex. They may have experienced slightly different assimilation histories in route to eruption, but can be source‐crater paired. NWA 032 remains enigmatic, as its source region ¹⁴⁷Sm/¹⁴⁴Nd definitively precludes a simple relationship with NWA 4734 and LAP 02205, despite a similar bulk composition. Their high Ti/Sm, low (La/Yb)_N, and Cl‐poor apatite compositions rule out the direct involvement of KREEP. Rather, they are consistent with low‐degree partial melting of late‐formed LMO cumulates, and indicate that the geochemical characteristics attributed to urKREEP are not unique to that reservoir. These and other basaltic meteorites indicate that the youngest mare basalts originate from multiple sources, and suggest that KREEP is not a prerequisite for the most recent known melting in the Moon.     

Searching for nonlocal lithologies in the Apollo 12 regolith: A geochemical and petrological study of basaltic coarse fines from the Apollo lunar soil sample 12023,155

New data from a petrological and geochemical examination of 12 coarse basaltic fines from the Apollo 12 soil sample 12023,155 provide evidence of additional geochemical diversity at the landing site. In addition to the bulk chemical composition, major, minor, and trace element analyses of mineral phases are employed to ascertain how these samples relate to the Apollo 12 lithological basalt groups, thereby overcoming the problems of representativeness of small samples. All of the samples studied are low‐Ti basalts (0.9–5.7 wt% TiO₂), and many fall into the established olivine, pigeonite, and ilmenite classification of Apollo 12 basaltic suites. There are five exceptions: sample 12023,155_1A is mineralogically and compositionally distinct from other Apollo 12 basalt types, with low pigeonite REE concentrations and low Ni (41–55 ppm) and Mn (2400–2556 ppm) concentrations in olivine. Sample 12023,155_11A is also unique, with Fe‐rich mineral compositions and low bulk Mg# (=100 × atomic Mg/[Mg+Fe]) of 21.6. Sample 12023,155_7A has different plagioclase chemistry and crystallization trends as well as a wider range of olivine Mg# (34–55) compared with other Apollo 12 basalts, and shows greater similarities to Apollo 14 high‐Al basalts. Two other samples (12023,155_4A, and _5A) are similar to the Apollo 12 feldspathic basalt 12038, providing additional evidence that feldspathic basalts represent a lava flow proximal to the Apollo 12 site rather than material introduced by impacts. We suggest that at least one parent magma, and possibly as many as four separate parent magmas, are required in addition to the previously identified olivine, pigeonite, and ilmenite basaltic suites to account for the observed chemical diversity of basalts found in this study.     

Derivation of elemental abundance maps at intermediate resolution from optical interpolation of lunar prospector gamma-ray spectrometer data

We propose a technique that interpolates available lunar prospector gamma-ray spectrometer (GRS) data using Clementine UVVIS spectral reflectance images. The main idea is to use low resolution GRS data as a “ground truth” to establish relationships linking optical data and geochemical information maximizing the respective correlation coefficients. Then the relationships and Clementine UVVIS data are used to derive elemental abundance maps with significantly improved spatial resolution. The main limitation of the technique is its dependence on how well the abundance of the elements correlates with the Clementine UVVIS data. The technique can also be applied to analysis of coming D-CIXS/Smart-1 and AMIE/Smart-1 data to increase resolution of lunar compositional maps. As an illustration of the suggested technique, maps for the elements Fe, Ti, O, Al, Ca, and Mg with pixel size 15 km×15 km are presented. The Fe and Ti distributions resemble qualitatively to the maps obtained with the well-known technique by lucey et al. (2000a. Lunar iron and titanium abundance algorithms based on final processing of Clementine ultraviolet-visible images. J. Geophys. Res. 105, 20,297-20,306), though in our case the ranges of Fe and Ti variations are, respectively, wider and narrower than for lucey's maps. New maps for the elements Fe, Ti, O, Al, Ca, and Mg appear to be informative. For instance, the map of oxygen abundance demonstrates an anomaly in the crater Tycho. The maps of Fe and Al contents show for highland regions slight variations related to maturity degree. Reliability of this relation is confirmed with lunar sample data. The reason of the correlation between chemical composition and exposition age of the lunar surface can be the global transport of the lunar surface material due to meteorite impacts.     

First disk-resolved spectroscopy of (4) Vesta

Vesta, the second largest Main-Belt Asteroid, will be the first to be explored in 2011 by NASA’s Dawn mission. It is a dry, likely differentiated body with spectrum suggesting that is has been resurfaced by basaltic lava flows, not too different from the lunar maria.Here we present the first disk-resolved spectroscopic observations of an asteroid from the ground. We observed (4) Vesta with the ESO-VLT adaptive optics equipped integral-field near-infrared spectrograph SINFONI, as part of its science verification campaign. The highest spatial resolution of ∼90 km on Vesta’s surface was obtained during excellent seeing conditions (0.5^″) in October 2004.We observe spectral variations across Vesta’ surface that can be interpreted as variations of either the pyroxene composition, or the effect of surface aging. We compare Vesta’s 2 μm absorption band to that of howardite-eucrite-diogenite (HED) meteorites that are thought to originate from Vesta, and establish particular links between specific regions and HED subclasses. The overall composition is found to be mostly compatible with howardite meteorites, although a small area around 180°E longitude could be attributed to a diogenite-rich spot. We finally focus our spectral analysis on the characteristics of Vesta’s bright and dark regions as seen from Hubble Space Telescope’s visible and Keck-II’s near-infrared images.