Properties of the hermean regolith: iii. disk-resolved vis-NIR reflectance spectra and implications for the abundance of iron

Disk-resolved reflectance spectra of the surface of Mercury (longitudes 240-300°), obtained in the visual (vis) and near-infrared (NIR) spectral region, are presented and analyzed. The observations were made at the 2.6-m Nordic Optical Telescope with the ALFOSC low-resolution spectrograph on 20 and 22 June 1999 in the wavelength range 520-970 nm with a footprint size of 700 km on the mid-disk of Mercury. A method which enables more accurate correction for telluric line absorptions and atmospheric extinction than that applied on previously published vis-NIR spectra of Mercury is introduced. The resulting reflectance spectra are remarkably linear, lack significant absorption features, and have optical slopes comparable to remotely sensed lunar pure anorthosites. The relation between spectral slope and photometric geometry found by Warell (2002, Icarus 156, 313-317) is confirmed and is explained as caused by strongly backscattering particles with embedded submicroscopic metallic iron in a mature regolith. With the theoretical maturation model of Hapke (2001, J. Geophys. Res. 106 (E5), 10039-10073) an abundance of 0.05-0.3 wt% submicroscopic metallic iron in the regolith for silicate grain sizes in the range 10-80 μm is determined, implying a ferrous iron content in mafic minerals intrinsically lower than that of the lunar highlands. A binary crustal composition model with anorthite linearly mixed with pyroxene provides better spectral fits than a pure anorthitic composition. Comparison with mature lunar pure anorthosite spectra yields a confident upper limit to the FeO content of 3 wt% under the assumption that the surfaces are similarly matured, but this figure probably represents a considerable overestimate. The average mercurian regolith does not seem to be substantially more weathered than the most mature lunar highland soils in terms of abundance of submicroscopic metallic iron, indicating that a steady-state maturation level has been reached. However, the strong relation between optical spectral slope and photometric geometry may imply that the majority of regolith particles are more fine-grained than their lunar counterparts and that the regolith is admixed with complex agglutinate weathering products which are more abundant and more transparent than those of the lunar highlands. This is consistent with more energetic impacts and a higher rate of impact melt production in an iron-poor regolith. An observed relation between the spectral slope and latitude provides evidence that the Ostwald ripening process may be operating at equatorial latitudes on Mercury.     

Reflectance spectra of analog anorthosites: Implications for lunar highland mapping

Lunar highland region and associated craters are mostly composed of anorthosite. In the present study, we studied the reflectance spectra of terrestrial anorthosites collected from Sittampundi Anorthosites Complex, which is considered as equivalent (simulant) of lunar highland anorthosites. The objective of the study is to interpret diagnostic spectral features of analog anorthosite for remotely exploring lunar highland region. Reflectance spectra of anorthosites were measured under two different environments, such as controlled field and laboratory conditions. In these two procedures, the laboratory spectra give clear, diagnostic spectral information in the present study. Reflectance spectra captured under 350-2500 nm covering UV, Visible, NIR, and SWIR part of the electromagnetic spectrum. The spectral characteristics of anorthosites measured under various parts of electromagnetic spectrum have diagnostic absorption features at 380-387, 700-740, 930-1100, 1160-1200, 1415, 1920, 2200 and 2330 nm correspondingly due to plagioclase UV absorption, Fe³⁺ electron transition absorption, Fe²⁺ pyroxene and olivine absorption, OH/Mn³⁺ crystal transition absorption, pyroxene absorption, Al-OH absorption and Mg-OH absorption. Mineralogical and chemical analyses were carried out for four anorthosites and compared with the results of chemical component of lunar anorthosite. The percentage of plagioclase content, relative abundance of low and high calcium pyroxene and olivine in different anorthosite samples are correlated with the albedo range, absorption shape, absorption centers and band depth. The similarity in the diagnostic spectral features of the anolog anorthosite with lunar anorthosites could be effectively utilized for remotely mapping the lunar highland region.     

Lunar pure anorthosite as a spectral analog for Mercury

Abstract— Plans are underway for spacecraft missions to the planet Mercury beginning in the latter part of this decade (NASA's MESSENGER (MErcury, Surface, Space ENvironment, GEochemistry, Ranging) and ESA's BepiColombo). Mercury is an airless body whose surface is apparently very low in ferrous iron. Much of the mercurian surface material is expected to be optically mature, a state produced by the “space weathering” process from direct exposure to the space environment. If appropriate analog terrains can be identified on the Moon, then study of their reflectance spectra and composition will improve our understanding of space weathering of low‐Fe surfaces and aid in the interpretation of data returned from Mercury by the spacecraft. We have conducted a search for areas of the lunar surface that are optically mature and have very low ferrous iron content using Clementine ultraviolet‐visible (UV‐vis) image products. Several regions with these properties have been identified on the farside. These areas, representing mature pure anorthosites (>90% plagioclase feldspar), are of interest because only relatively immature pure anorthosites have previously been studied with Earth‐based spectrometry. A comparison of Mercury with the lunar analogs reveals similarities in spectral characteristics, and there are hints that the mercurian surface may be even lower in FeO content than the lunar pure anorthosites. We also investigate the potential for use of spectral features other than the commonly studied “1 μm” mafic mineral absorption band as tools for compositional assessment when spacecraft spectral measurements of Mercury become available. Most low‐Fe minerals plausibly present on Mercury lack absorption bands, but plagioclase possesses an iron impurity absorption at 1.25 μm. Detection of this diagnostic band may be possible in fresh crater deposits.     

Properties of the Hermean Regolith: II. Disk-Resolved Multicolor Photometry and Color Variations of the “Unknown” Hemisphere

Multicolor photometric observations of the “unknown” hemisphere of Mercury have been performed with the Swedish Vacuum Solar Telescope on La Palma at maximal elongations from the Sun in 1997 and 1998. A set of six interference filters with central wavelengths from 450 to 940 nm were used. Multicolor photometry of Mercury was performed on disk-resolved images of the unknown hemisphere (longitudes 160°-340°) with a highest resolution of ∼200 km (J. Warell and S. Limaye 2001, Planet. Space Sci.49, 1531-1552).Disk-integrated spectrophotometry shows that (1) the spectrum of Mercury displays a linear slope from 650 to 940 nm, indicating that the average mercurian regolith is considerably more mature than relatively immature pure anorthosite regions on the Moon; (2) there is negative evidence for the presence of the putative 1-μm absorption feature near 940 nm due to the presence of ferrous iron (Fe²⁺) in pyroxenes; and (3) no effect of phase reddening of the integrated disk is observed between phase angles of 63° and 84°.For the first time, disk-resolved spectrophotometry of Mercury's surface has been obtained, from which it is inferred that (4) the scattering properties of Mercury's regolith are more homogeneous than for the Moon and that there is no clear relation between reflectance and chemical properties at spatial scales of ∼300 km on the unknown hemisphere and (5) there exists an inverse relation of spectral slope with emission angle which is larger for Mercury than for the Moon, indicating that the average mercurian regolith is more backscattering and that this effect increases with wavelength.Finally, from filter ratio images of Mercury's disk it is found that (6) no color variations larger that 2% with respect to the surroundings are detected at a spatial resolution of ∼300 km.     

Mercury's albedo from Mariner 10: Implications for the presence of ferrous iron

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.     

Release of neutral sodium atoms from the surface of Mercury induced by meteoroid impacts

Meteoroid impact has been shown to be a source of sodium, and most likely of other elements, on the Moon. The same process could be also relevant for Mercury. In this work we calculate the vapor and neutral Na production rates on Mercury due to the impacts of meteoroids in the radius range of 10⁻⁸-10⁻¹ m. We limit our calculations to this size range, because meteoroids with radius larger than 10⁻¹ m have not to be found important for the daily production of the exosphere. This work is based on a new dynamical model of the meteoroid flux at the heliocentric distance of Mercury, regarding objects in the size range 10⁻²-10⁻¹ m. This size range, never investigated before, is not affected by nongravitational forces, such as the Poynting-Robertson effect, which is dominant for particles smaller than 10⁻² m. In order to evaluate the release of neutral sodium atoms also for smaller meteoroids we have used the distribution reported by M.J. Cintala [1992. Impact-induced thermal effects in the lunar and mercurian regoliths. J. Geophys. Res. 97, 947-973] calculated for particle size range 10⁻⁸-10⁻³ m. We have extrapolated this distribution up to 10⁻² m and we have based the impact calculations on a new surface composition assuming 90% plagioclase and 10% pyroxene. The results of our model are that (i) the total mass of vapor produced by the impact of meteoroids in the size range 10⁻⁸-10⁻¹ m is 4.752×¹⁰⁸ g per year, and (ii) the production rate of neutral sodium atoms is 1.5×¹⁰²² s⁻¹.     

Recognizing mercurian meteorites

Abstract— With the recent realization that some meteorites may come from Mars and the Moon, it is worthwhile to consider whether meteorites from Mercury could exist in our collections and, if so, whether they could be recognized. The current state of ignorance about Mercury both increases the potential scientific value of mercurian meteorites and aggravates the problem of identifying them. Here, we review evidence supporting the possibility of impact launch and subsequent orbital evolution that could deliver rocks from Mercury to Earth and suggest criteria that could help identify a mercurian meteorite. Mercurian rocks are probably differentiated igneous rocks or breccias or melt rocks derived therefrom. Solar nebula models suggest that they are probably low in volatiles and moderately enriched in Al, Ti, and Ca oxides. Mercurian surface rocks contain no more than 5% FeO and may contain plagioclase. A significant fraction may be volcanic. They may possess an unusual isotopic composition. Most pristine mercurian rocks should have solidification ages of ～3.7 to ～4.4 Ga, but younger impact-remelted materials are possible. Because we know more about the space environment of Mercury than we do about the planet itself, surface-exposed rocks would be easiest to identify as mercurian. The unique solar-to-galactic cosmic-ray damage track ratio expected in materials exposed near the Sun may be useful in identifying a rock from Mercury. Mercury's magnetic field stands off the solar wind, so that solar-wind implants in mercurian regolith breccias may be scarce or fractionated compared to lunar ones. Mercurian regolith breccias should contain more agglutinates (or their recrystallized derivatives) and impact vapor deposits than any other and should show a higher fraction of exogenic chondritic materials than analogous lunar breccias. No known meteorite group matches these criteria. A misclassified mercurian meteorite would most likely be found among the aubrites or the anorthositic lunar meteorites.     

Crater size-shape profiles for the moon and mercury: Terrain effects and interplanetary comparisons

New crater size-shape data were compiled for 221 fresh lunar craters and 152 youthful mercurian craters. Terraces and central peaks develop initially in fresh craters on the Moon in the 0–10 km diameter interval. Above a diameter of 65 km all craters are terraced and have central peaks. Swirl floor texture is most common in craters in the size range 20–30 km, but it occurs less frequently as terraces become a dominant feature of crater interiors. For the Moon there is a correlation between crater shape and geomorphic terrain type. For example, craters on the maria are more complex in terms of central peak and terrace detail at any given crater diameter than are craters in the highlands. These crater data suggest that there are significant differences in substrate and/or target properties between maria and highlands. Size-shape profiles for Mercury show that central peak and terrace onset is in the 10–20 km diameter interval; all craters are terraced at 65 km, and all have central peaks at 45 km. The crater data for Mercury show no clear cut terrain correlation. Comparison of lunar and mercurian data indicates that both central peaks and terraces are more abundant in craters in the diameter range 5–75 km on Mercury. Differences in crater shape between Mercury and the Moon may be due to differences in planetary gravitational acceleration (gMercury=2.3gMoon). Also differences between Mercury and the Moon in target and substrate and in modal impact velocity may contribute to affect crater shape.     

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.     

The dependence of reflectance spectra of Mercury on surface terrain

Reflectance spectra of Mercury, covering the spectral range of ～0.3–1.1 μm obtained during 1963–1976, were examined for any correlations with surface terrain. Mercury's 6.1385°/day rotational rate, the phases of the planet around maximum elongations, and bidirectional reflectance spectroscopy theory were used to identify the surface area associated with each spectrum. Data from 1974–1975, re-reduced with improved standard star flux ratios, show a weak absorption band in the near infrared not see in earlier analyses. Older spectra suggest that the western longitudes of the unimaged side of Mercury are similar to the rest of the planet. Spectra of the intercrater plains in the 0–90° quadrant suggest a possible absorption band. Spectra of areas dominated by Caloris Basin with the encompassing smooth plains may show Fe²⁺ abundances in the soil comparable to lunar highlands soil. No striking differences between spectra of intercrater plains and spectra of smooth plains are found. The absorption features seen in spectra of Mercury are generally weaker than features seen in lunar spectra.     

The petrology,geochemistry, and age of lunar regolith breccias Miller Range 090036 and 090070: Insights into the crustal history of the Moon

Meteorites ejected from the surface of the Moon as a result of impact events are an important source of lunar material in addition to Apollo and Luna samples. Here, we report bulk element composition, mineral chemistry, age, and petrography of Miller Range (MIL) 090036 and 090070 lunar meteorites. MIL 090036 and 090070 are both anorthositic regolith breccias consisting of mineral fragments and lithic clasts in a glassy matrix. They are not paired and represent sampling of two distinct regions of the lunar crust that have protoliths similar to ferroan anorthosites. ⁴⁰Ar‐³⁹Ar chronology performed on two subsplits of MIL 090070,33 (a pale clast impact melt and a dark glassy melt component) shows that the sample underwent two main degassing events, one at ~3.88 Ga and another at ~3.65 Ga. The cosmic ray exposure data obtained from MIL 090070 are consistent with a short (~8–9 Ma) exposure close to the lunar surface. Bulk‐rock FeO, TiO₂, and Th concentrations in both samples were compared with 2‐degree Lunar Prospector Gamma Ray Spectrometer (LP‐GRS) data sets to determine areas of the lunar surface where the regolith matches the abundances observed on the sample. We find that MIL 090036 bulk rock is compositionally most similar to regolith surrounding the Procellarum KREEP Terrane, whereas MIL 090070 best matches regolith in the feldspathic highlands terrane on the lunar farside. Our results suggest that some areas of the lunar farside crust are composed of ferroan anorthosite, and that the samples shed light on the evolution and impact bombardment history of the ancient lunar highlands.     

Spinel assemblages in lunar meteorites Graves Nunataks 06157 and Dhofar 1528: Implications for impact melting and equilibration in the Moon's upper mantle

Magnesium‐rich spinel assemblages occur in the two lunar vitric breccia meteorites—Dhofar (Dho) 1528 and Graves Nunataks (GRA) 06157. Dho 1528 contains up to ~0.7 mm cumulate Mg‐rich spinel crystals associated with Mg‐rich olivine, Mg‐ and Al‐rich pyroxene, plagioclase, and rare cordierite. Using thermodynamic calculations of these mineral assemblages, we constrain equilibration depths and discuss an origin of these lithologies in the upper mantle of the Moon. In contrast, small, 10 to 20 μm spinel phenocryst assemblages in glassy melt rock clasts in Dho 1528 and GRA 06157 formed from the impact melting of Mg‐rich rocks. Some of these spinel phenocrysts match compositional constraints for spinel associated with “pink spinel anorthosites” inferred from remote sensing data. However, such spinel phenocrysts in meteorites and Apollo samples are typically associated with significant amounts of olivine ± pyroxene that exceed the compositional constraints for pink spinel anorthosites. We conclude that the remotely sensed “pink spinel anorthosites” have not been observed in the collections of lunar rocks. Moreover, we discuss impact‐excavation scenarios for the spinel‐bearing assemblages in Dhofar 1528 and compare the bulk rock composition of Dho 1528 to strikingly similar compositions of Luna 20 samples that contain ejecta from the Crisium impact basin.     

The 0.7-5.3 μm IR spectra of Mercury and the Moon: Evidence for high-Ca clinopyroxene on Mercury

We present infrared spectra of Mercury and the Moon in the wavelength range 0.7-5.3 μm obtained with the SpeX spectrograph at the NASA Infrared Telescope Facility. The spectra were acquired from pole and terminator locations of Mercury's surface and of Mersenius C and the Copernicus central peak on the Moon. Spectra of both bodies were measured in close temporal succession and were reduced in the same manner with identical calibration stars to minimize differences in the reduction process. The Copernicus spectra display the expected absorption features due to mafic minerals in the near infrared and show spectral features in the SiO combination/overtone vibrational band region above 4 μm. The spectra of Mercury from longitude 170° and north and south mid-latitudes display a 1-μm absorption band indicative of high-Ca clinopyroxene, while a spectrum from longitude 260° and northern mid-latitudes does not. The Mercury spectra show a broad feature of low emittance over the full 3-5 μm thermal infrared region, but no narrow features in this spectral range. The longitude 260° spectrum shows excess thermal emission around 5 μm attributable to the existence of a thermal gradient in the insolated dayside regolith. The thermal-IR spectra suggest a significant difference in the compositional and/or structural properties of Mercury and the Moon that may be due to grain size, absorption coefficient, or the magnitude of near-surface thermal gradients. The results indicate that the composition of Mercury's surface is heterogeneous on regional scales, and that the near infrared wavelength range provides more discriminative information on the surface composition than the 2-4 μm region, where the solar reflected and thermally emitted radiation contribute approximately equally to the observed flux of these bodies.     

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.     

Cover

Eucrite Elephant Moraine (EET) 87520 in crossed polars (background) and a rare earth element pattern of lunar ferroan anorthosite 15415 (inset). These two images highlight two areas of Michael J. Drake's fundamental contributions to planetary science: his career‐long interest in the origin of the HED meteorites and their likely parent body asteroid 4 Vesta, and his contribution to understanding the complementary nature of Eu anomalies in lunar basalts and anorthosites.     

Chronology,geochemistry, and petrology of a ferroan noritic anorthosite clast from Descartes breccia 67215: Clues to the age,origin, structure,and impact history of the lunar crust

Abstract— The petrology, major and trace element geochemistry, and Nd‐Ar‐Sr isotopic compositions of a ferroan noritic anorthosite clast from lunar breccia 67215 have been studied in order to improve our understanding of the composition, age, structure, and impact history of the lunar crust. The clast (designated 67215c) has an unusually well preserved igneous texture. Mineral compositions are consistent with classification of 67215c as a member of the ferroan anorthositic suite of lunar highlands rocks, but the texture and mineralogy show that it cooled more rapidly and at shallower depths than did more typical ferroan anorthosites (FANs). Incompatible trace element concentrations are enriched in 67215c relative to typical FANs, but diagnostic signatures such as Ti/Sm, Sc/Sm, plagiophile element ratios, and the lack of Zr/Hf and Nb/Ta fractionation show that this cannot be due to the addition of KREEP. Alternatively, 67215c may contain a greater fraction of trapped liquid than is commonly present in lunar FANs. ¹⁴⁷Sm‐¹⁴³Nd isotopic compositions of mineral separates from 67215c define an isochron age of 4.40 ± 0.11 Gyr with a near‐chondritic initial ε¹⁴³_Nd of +0.85 ± 0.53. The ⁴⁰Ar‐³⁹Ar composition of plagioclase from this clast records a post‐crystallization thermal event at 3.93 ± 0.08 Gyr, with the greatest contribution to the uncertainty in this age deriving from a poorly constrained correction for lunar atmosphere ⁴⁰Ar. Rb‐Sr isotopic compositions are disturbed, probably by the same event recorded by the Ar isotopic compositions. Trace element compositions of FANs are consistent with crystallization from a moderately evolved magma ocean and do not support a highly depleted source composition such as that implied by the positive initial ε¹⁴³_Nd of the ferroan noritic anorthosite 62236. Alternatively, the Nd isotopic systematics of lunar FANs may have been subject to variable degrees of modification by impact metamorphism, with the plagioclase fraction being more strongly affected than the mafic phases. ¹⁴⁷Sm‐¹⁴³Nd isotopic compositions of mafic fractions from the 4 ferroan noritic anorthosites for which isotopic data exist (60025, 62236, 67016c, 67215c) define an age of 4.46 ± 0.04 Gyr, which may provide a robust estimate for the crystallization age of lunar ferroan anorthosites.     

Shocked plagioclase signatures in Thermal Emission Spectrometer data of Mars

The extensive impact cratering record on Mars combined with evidence from SNC meteorites suggests that a significant fraction of the surface is composed of materials subjected to variable shock pressures. Pressure-induced structural changes in minerals during high-pressure shock events alter their thermal infrared spectral emission features, particularly for feldspars, in a predictable fashion. To understand the degree to which the distribution and magnitude of shock effects influence martian surface mineralogy, we used standard spectral mineral libraries supplemented by laboratory spectra of experimentally shocked bytownite feldspar [Johnson, J.R., Hörz, F., Christensen, P., Lucey, P.G., 2002b. J. Geophys. Res. 107 (E10), doi:10.1029/2001JE001517] to deconvolve Thermal Emission Spectrometer (TES) data from six relatively large (>50 km) impact craters on Mars. We used both TES orbital data and TES mosaics (emission phase function sequences) to study local and regional areas near the craters, and compared the differences between models using single TES detector data and 3×2 detector-averaged data. Inclusion of shocked feldspar spectra in the deconvolution models consistently improved the rms errors compared to models in which the spectra were not used, and resulted in modeled shocked feldspar abundances of >15% in some regions. However, the magnitudes of model rms error improvements were within the noise equivalent rms errors for the TES instrument [Hamilton V., personal communication]. This suggests that while shocked feldspars may be a component of the regions studied, their presence cannot be conclusively demonstrated in the TES data analyzed here. If the distributions of shocked feldspars suggested by the models are real, the lack of spatial correlation to crater materials may reflect extensive aeolian mixing of martian regolith materials composed of variably shocked impact ejecta from both local and distant sources.     

The lithologic diversity of the Moon recorded in lunar meteorites Northwest Africa 7611 and 10480

Northwest Africa (NWA) 7611/10480 are lunar regolith breccia meteorites, composed of mineral fragments and various clasts including mare basalts, volcanic glasses, gabbroic lithologies, and a diverse variety of highland materials (ferroan anorthosite, Mg-suite, magnesian anorthosite, and alkali suite rocks) as well as different subvarieties of impact melt breccia. The Apollo two-component mixing model calculation reveals that the NWA 7611 source region contains 58 wt% mare materials and 42 wt% highland components, but the estimated mare components in NWA 10480 have a higher abundance (66 wt%). The predominantly very low-Ti (VLT) composition in both fine-grained basaltic and coarse-grained gabbroic lithologies indicates a provenance associated with a thick lava flow or a single magmatic system. The co-occurrence of zoning patterns and fine-scale exsolution lamellae in pyroxene debris supports a cryptomare deposit as the best candidate source. Phosphate Pb–Pb ages in matrix fragments, impact melt breccia, and basaltic clast indicate that the breccia NWA 7611 records geological events spanning approximately 4305–3769 Ma, which is consistent with the ages of ancient lunar VLT volcanism and the products of basin-forming impacts on the lunar nearside. The youngest reset age at ~3.2 Ga is potentially related to the strong shock lithification process of breccia NWA 7611. Moreover, the similar petrology, texture, geochemistry, cosmic-ray exposure data, and crystallization ages support that basaltic component in Yamato (Y)-793274, and Queen Alexandra Range (QUE) 94281, NWA 4884, and NWA 7611 clan came from the same basalt flow.     

Multiple lithic clasts in lunar breccia Northwest Africa 7948 and implication for the lithologic components of lunar crust

This study presents the petrography, mineralogy, and bulk composition of lunar regolith breccia meteorite Northwest Africa (NWA) 7948. We identify a range of lunar lithologies including basaltic clasts (very low-titanium and low-titanium basalts), feldspathic lithologies (ferroan anorthosite, magnesian-suite rock, and alkali suite), granulites, impact melt breccias (including crystalline impact melt breccias, clast-bearing impact melt breccias, and glassy melt breccias), as well as regolith components (volcanic glass and impact glass). A compositionally unusual metal-rich clast was also identified, which may represent an impact melt lithology sourced from a unique Mg-suite parent rock. NWA 7948 has a mingled bulk rock composition (Al₂O₃ = 21.6 wt% and FeO = 9.4 wt%) and relatively low concentrations of incompatible trace elements (e.g., Th = 1.07 ppm and Sm = 2.99 ppm) compared with Apollo regolith breccias. Comparing the bulk composition of the meteorite with remotely sensed geochemical data sets suggests that the sample was derived from a region of the lunar surface distal from the nearside Th-rich Procellarum KREEP Terrane. Our investigations suggest that it may have been ejected from a nearside highlands-mare boundary (e.g., around Mare Crisium or Orientale) or a cryptomare region (e.g., Schickard-Schiller or Mare smythii) or a farside highlands-mare boundary (e.g., Mare Australe, Apollo basin in the South Pole–Aitken basin). The distinctive mineralogical and geochemical features of NWA 7948 suggest that the meteorite may represent lunar material that has not been reported before, and indicate that the lunar highlands exhibit wide geological diversity.     

New lunar meteorite NWA 10986: A mingled impact melt breccia from the highlands—A complete cross section of the lunar crust

Northwest Africa (NWA) 10986 is a new mingled lunar meteorite found in 2015 in Western Sahara. This impact melt breccia contains abundant impact melt glass and clasts as large as 0.75 mm. Clasts are predominantly plagioclase and pyroxene‐rich and represent both highland and basalt lithologies. Highland lithologies include troctolites, gabbronorites, anorthositic norites, and troctolitic anorthosites. Basalt lithologies include crystalline clasts with large zoned pyroxenes representing very low titanium to low titanium basalts. In situ geochemical analysis of minerals within clasts indicates that they represent ferroan anorthosite, Mg‐suite, and gabbronorite lithologies as defined by the Apollo sample collection. Clasts representing magnesian anorthosite, or “gap” lithologies, are prevalent in this meteorite. Whole rock and in situ impact glass measurements indicate low incompatible trace element concentrations. Basalt clasts also have low incompatible trace element concentrations and lack evolved KREEP mineralogy although pyroxferroite grains are present. The juxtaposition of evolved, basaltic clasts without KREEP signatures and highland lithologies suggests that these basaltic clasts may represent cryptomare. The lithologies found in NWA 10986 offer a unique and possibly a complete cross section view of the Moon sourced outside of the Procellarum KREEP Terrane.