R. L. Klima

Character and Spatial Distribution of OH/H2O on the Surface of the Moon Seen by M3 on Chandrayaan-1

Lunar Water The Moon has been thought to be primarily anhydrous, although there has been some evidence for accumulated ice in permanently shadowed craters near its poles (see the Perspective by Lucey, published online 24 September). By analyzing recent infrared mapping by Chandrayaan-1 and Deep Impact, and reexamining Cassini data obtained during its early flyby of the Moon, Pieters et al. (p. 568, published online 24 September), Sunshine et al. (p. 565, published online 24 September), and Clark et al. (p. 562, published online 24 September) reveal a noticeable absorption signal for H2O and OH across much of the surface. Some variability in water abundance is seen over the course of the lunar day. The data imply that solar wind is depositing and/or somehow forming water and OH in minerals near the lunar surface, and that this trapped water is dynamic. Space-based spectroscopic measurements provide evidence for water or hydroxyl (OH) on the surface of the Moon The search for water on the surface of the anhydrous Moon had remained an unfulfilled quest for 40 years. However, the Moon Mineralogy Mapper (M3) on Chandrayaan-1 has recently detected absorption features near 2.8 to 3.0 micrometers on the surface of the Moon. For silicate bodies, such features are typically attributed to hydroxyl- and/or water-bearing materials. On the Moon, the feature is seen as a widely distributed absorption that appears strongest at cooler high latitudes and at several fresh feldspathic craters. The general lack of correlation of this feature in sunlit M3 data with neutron spectrometer hydrogen abundance data suggests that the formation and retention of hydroxyl and water are ongoing surficial processes. Hydroxyl/water production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration.

M3 spectral analysis of lunar swirls and the link between optical maturation and surface hydroxyl formation at magnetic anomalies

[1] We examined the lunar swirls using data from the Moon Mineralogy Mapper (M 3 ). The improved spectral and spatial resolution of M 3 over previous spectral imaging data facilitates distinction of subtle spectral differences, and provides new information about the nature of these enigmatic features. We characterized spectral features of the swirls, interswirl regions (dark lanes), and surrounding terrain for each of three focus regions: Reiner Gamma, Gerasimovich, and Mare Ingenii. We used Principle Component Analysis to identify spectrally distinct surfaces at each focus region, and characterize the spectral features that distinguish them. We compared spectra from small, recent impact craters with the mature soils into which they penetrated to examine differences in maturation trends on‐ and off‐swirl. Fresh, on‐swirl crater spectra are higher albedo, exhibit a wider range in albedos and have well‐preserved mafic absorption features compared with fresh off‐swirl craters. Albedoand mafic absorptions are still evident in undisturbed, on‐swirl surface soils, suggesting the maturation process is retarded. The spectral continuum is more concave compared with off‐swirl spectra; a result of the limited spectral reddening being mostly constrained to wavelengths less than ∼1500 nm. Off‐swirl spectra show very little reddening or change in continuum shape across the entire M 3 spectral range. Off‐swirl spectra are dark, have attenuated absorption features, and the narrow range in off‐swirl albedos suggests off‐swirl regions mature rapidly. Spectral parameter maps depicting the relative OH surface abundance for each of our three swirl focus regions were created using the depth of the hydroxyl absorption feature at 2.82 mm. For each of the studied regions, the 2.82 mm absorption feature is significantly weaker on‐swirl than off‐swirl, indicating the swirls are depleted in OH relative to their surroundings. The spectral characteristics of the swirls and adjacent terrains from all three focus regions support the hypothesis that the magnetic anomalies deflect solar wind ions away from the swirls and onto off‐swirl surfaces. Nanophase iron (npFe 0 ) is largely responsible for the spectral characteristics we attribute to space weathering and maturation, and is created by vaporization/deposition by micrometeorite impacts and sputtering/reduction by solar wind ions. On the swirls, the decreased proton flux slows the spectral effects of space weathering (relative to nonswirl regions) by limiting the npFe 0 production mechanism almost exclusively to micrometeoroid impact vaporization/deposition. Immediately adjacent to the swirls, maturation is accelerated by the increased flux of protons deflected from the swirls. Citation: Kramer, G. Y., et al. (2011), M 3 spectral analysis of lunar swirls and the link between optical maturation and surface

Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than ~400 nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

Martian dunite NWA 2737: Integrated spectroscopic analyses of brown olivine

Received 5 October 2007; revised 13 December 2007; accepted 6 March 2008; published 18 June 2008. [1] A second Martian meteorite has been identified that is composed primarily of heavily shocked dunite, Northwest Africa (NWA) 2737. This meteorite has several similarities to the Chassigny dunite cumulate, but the olivine is more Mg rich and, most notably, is very dark and visually brown. Carefully coordinated analyses of NWA 2737 whole-rock and olivine separates were undertaken using visible and near-infrared reflectance,

Spectroscopic characteristics of synthetic olivine: An integrated multi-wavelength and multi-technique approach

Abstract Spectroscopic measurements have been made of two suites of olivine minerals synthesized under slightly different conditions in 5-10 mol% increments across the solid solution from forsterite to fayalite. Here, we present Mössbauer results for the entire Fe-Mg olivine suite, as well as the results for only the fayalite end-member as an introduction to our team’s other diverse spectral-analysis techniques and data that will be presented in forthcoming papers. Experimental methods used to synthesize both suites of samples are discussed here in detail, along with specifics of the analytical techniques used to study them. Electron microprobe data and Mössbauer spectra acquired at 293 K across the solid solution are presented first to characterize and address the presence of impurities in the broad suite of samples that may affect other spectroscopic methods. We then focus specifically on the fayalite end-member to illustrate its properties using multiple techniques. Fayalite is an especially important phase for different types of spectroscopy because, by definition, it contains an equal distribution of Fe2+ cations between the M1 and M2 octahedral sites. Thus, features associated with each of the two sites must represent equal numbers of Fe2+ cations, removing uncertainties associated with assumptions about order/disorder of Fe2+ and other cations. Mössbauer, Raman, thermal emission, attenuated total reflectance (ATR), specular reflectance, and visible to mid-infrared total reflectance studies are presented for fayalite. These include calculation of mid-infrared optical constants (n and k) and fundamental Mössbauer parameters: intrinsic isomer shift (δI), Mössbauer temperature (θM), and recoil-free fraction (f). Data from the different techniques are described and related, demonstrating the importance of multi-wavelength data to provide a complete characterization and understanding of the spectroscopic features in fayalite.

Evidence from MESSENGER for sulfur‐ and carbon‐driven explosive volcanism on Mercury

Targeted MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) X-Ray Spectrometer measurements of Mercury’s largest identified pyroclastic deposit are combined with neutron and reflectance spectroscopy data to constrain the composition of volatiles involved in the eruption that emplaced the pyroclastic material. The deposit, northeast of the Rachmaninoff basin, is depleted in S (relative to Ca and Si) and C, compared with the rest of Mercury’s surface. Spectral reflectance measurements of the deposit indicate relatively high overall reflectance and an oxygen-metal charge transfer (OMCT) absorption band at ultraviolet wavelengths. These results are consistent with oxidation of graphite and sulfides during magma ascent, via reaction with oxides in the magma or assimilated country rock, and the formation of Sand C-bearing volatile species. Consumption of graphite during oxidation could account for the elevated reflectance of the pyroclastic material, and the strength of the OMCT band is consistent with ~0.03–0.1wt% FeO in the deposit.

Link between the potentially hazardous Asteroid (86039) 1999 NC43 and the Chelyabinsk meteoroid tenuous

We explored the statistical and compositional link between Chelyabinsk meteoroid and potentially haz- ardous Asteroid (86039) 1999 NC43 to investigate their proposed relation proposed by Borovicka et al. (Borovicka, J., et al. (2013). Nature 503, 235-237). First, using a slightly more detailed computation we confirm that the orbit of the Chelyabinsk impactor is anomalously close to the Asteroid 1999 NC43. We find � (1-3) � 10 � 4 likelihood of that to happen by chance. Taking the standpoint that the Chelya- binsk impactor indeed separated from 1999 NC43 by a cratering or rotational fission event, we run a for- ward probability calculation, which is an independent statistical test. However, we find this scenario is unlikely at the � (10 � 3 -10

Visible to near-infrared optical properties of pure synthetic olivine across the olivine solid solution

Abstract Olivine exhibits highly diagnostic absorption features across visible to near-infrared (VNIR) wavelengths due to electronic transitions of Fe2+ in its crystal structure. The properties of these absorptions vary with composition, enabling compositional analysis of olivine through VNIR spectroscopy, both in the laboratory and through remote sensing. Previous analyses of these trends have relied on natural olivine samples, which are influenced by the presence of minor cations that can affect the diagnostic absorptions. We conduct a systematic analysis of a suite of synthetic (pure Mg/Fe) olivine samples with VNIR (300-2600 nm) reflectance spectroscopy and quantitative spectral deconvolutions. From the full suite of samples described and characterized by Dyar et al. (2009), we identify a small suite of well-characterized and chemically pure olivine samples that demonstrates consistent and reliable spectral reflectance properties across visible to near-infrared wavelengths. This suite covers the stoichiometric olivine solid solution from x = Mg/(Mg+Fe) = 0 to x = 70 (Fo0 to Fo70). Because of their tight compositional control, these synthetic samples improve on previous analyses of natural samples. The results of this study provide a new standard for spectral reflectance properties of olivine across visible to near-infrared wavelengths for the compositions present in the suite. We present updated data on the trends in olivine band position as a function of olivine composition, which are the basis for remote compositional evaluation of olivine with visible to near-infrared reflectance spectroscopy. For these reasons, these improved olivine band position trends are of major importance to remote compositional analyses of terrestrial planets.

Evaluating an impact origin for Mercury's high‐magnesium region

During its four years in orbit around Mercury, the MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft’s X-Ray Spectrometer revealed a large geochemical terrane in the northern hemisphere that hosts the highest Mg/Si, S/Si, Ca/Si, and Fe/Si and lowest Al/Si ratios on the planet. Correlations with low topography, thin crust, and a sharp northern topographic boundary led to the proposal that this high-Mg region (HMR) is the remnant of an ancient, highly degraded impact basin. Here we use a numerical modeling approach to explore the feasibility of this hypothesis and evaluate the results against multiple mission-wide datasets and resulting maps from MESSENGER. We find that a ~3000-km diameter impact basin easily exhumes Mg-rich mantle material but that the amount of subsequent modification required to hide basin structure is incompatible with the strength of the geochemical anomaly, which is also present in maps of Gamma Ray and Neutron Spectrometer data. Consequently, the high-Mg region is more likely to be the product of high-temperature volcanism sourced from a chemically heterogeneous mantle than the remains of a large impact event. (178 words)

Mineralogical indicators of Mercury's hollows composition in MESSENGER color observations

Early during MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER)’s orbital mission, the Mercury Dual-Imaging System imaged the landform called hollows on the two craters Dominici and Hopper, using its Wide-Angle Camera with eight narrowband color filters ranging from 433 to 996 nm. An absorption feature centered in the MDIS 629 nm filter is evident in reflectance spectra for Dominici’s south wall/rim hollows. A different absorption feature found in photometry of Dominici’s center hollows extends through the MDIS 828 nm filter. Hollows in Hopper exhibit a weaker spectral absorption feature than that observed in Dominici’s center. At Dominici, we postulate that fresher hollows-hosting material in the wall/rim was exposed to the space environment through a process of slumping of the overlying material. With time, local and global processes darken the hollows and change or mix the surface mineralogy, so that the spectral signature evolves. The hollows could contain low-density MgS and an opaque component, potentially derived from background material.