Composition and structures of the subsurface in the vicinity of Valles Marineris as revealed by central uplifts of impact craters

Despite recent efforts from space exploration to sound the martian subsurface with RADAR, the structure of the martian subsurface is still unknown. Major geologic contacts or discontinuities inside the martian crust have not been revealed. Another way to analyze the subsurface is to study rocks that have been exhumed from depth by impact processes. The last martian mission, MRO (Mars Reconnaissance Orbiter), put forth a great deal of effort in targeting the central peaks of impact craters with both of its high resolution instruments: CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) and HiRISE (High Resolution Science Experiment). We analyzed the composition with CRISM and the physical characteristics on HiRISE of the rocks exhumed from depth from 31 impact craters in the vicinity of Valles Marineris. Our analyses revealed the presence at depth of two kinds of material: massive light-toned rocks and intact layers. Exhumed light-toned massive rocks are enriched in low calcium pyroxenes and olivine. Hydrated phases such as smectites and putative serpentine are present and may provide evidence of hydrothermal processes. Some of the rocks may represent portions of the volatile-rich, pre-Noachian martian primitive crust. In the second class of central peaks, exhumed layers are deformed, folded, and fractured. Visible-near infrared (VNIR) spectra suggest that they are composed of a mixture of olivine and high calcium pyroxene associated with hydrated phases. These layers may represent a Noachian volcanic accumulation of up to 18 km due to Tharsis activity. The spatial distribution, as well as the in-depth distribution between the two groups of rocks exhumed, are not random and reveal a major geologic discontinuity below the Tharsis lava plateau. The contact may be vertical over several kilometers depth suggesting the pre-existence of a steep basin (early giant impact or subsidence basin) or sagduction processes.     

Complexities in pyroxene compositions derived from absorption band centers: Examples from Apollo samples,HED meteorites,synthetic pure pyroxenes,and remote sensing data

We reexamine the relationship between pyroxene composition and near‐infrared absorption bands, integrating measurements of diverse natural and synthetic samples. We test an algorithm (PLC) involving a two‐part linear continuum removal and parabolic fits to the 1 and 2 μm bands—a computationally simple approach which can easily be automated and applied to remote sensing data. Employing a suite of synthetic pure pyroxenes, the PLC technique is shown to derive similar band centers to the modified Gaussian model. PLC analyses are extended to natural pyroxene‐bearing materials, including (1) bulk lunar basalts and pyroxene separates, (2) diverse lunar soils, and (3) HED meteorites. For natural pyroxenes, the relationship between composition and absorption band center differs from that of synthetic pyroxenes. These differences arise from complexities inherent in natural materials such as exsolution, zoning, mixing, and space weathering. For these reasons, band center measurements of natural pyroxene‐bearing materials are compositionally nonunique and could represent three distinct scenarios (1) pyroxene with a narrow compositional range, (2) complexly zoned pyroxene grains, or (3) a mixture of multiple pyroxene (or nonpyroxene) components. Therefore, a universal quantitative relationship between band centers and pyroxene composition cannot be uniquely derived for natural pyroxene‐bearing materials without additional geologic context. Nevertheless, useful relative relationships between composition and band center persist in most cases. These relationships are used to interpret M³ data from the Humboldtianum Basin. Four distinct compositional units are identified (1) Mare Humboldtianum basalts, (2) distinct outer basalts, (3) low‐Ca pyroxene‐bearing materials, and (4) feldspathic materials.     

Martian meteorite Dhofar 019: A new shergottite

Abstract— Dhofar 019 is a new martian meteorite found in the desert of Oman. In texture, mineralogy, and major and trace element chemistry, this meteorite is classified as a basaltic shergottite. Olivine megacrysts are set within a groundmass composed of finer grained olivine, pyroxene (pigeonite and augite), and maskelynite. Minor phases are chromite‐ulvöspinel, ilmenite, silica, K‐rich feldspar, merrillite, chlorapatite, and pyrrhotite. Secondary phases of terrestrial origin include calcite, gypsum, celestite, Fe hydroxides, and smectite. Dhofar 019 is most similar to the Elephant Moraine (EETA) 79001 lithology A and Dar al Gani (DaG) 476/489 shergottites. The main features that distinguish Dhofar 019 from other shergottites are lack of orthopyroxene; lower Ni contents of olivine; the heaviest oxygen‐isotopic bulk composition; and larger compositional ranges for olivine, maskelynite, and spinel, as well as a wide range for pyroxenes. The large compositional ranges of the minerals are indicative of relatively rapid crystallization. Modeling of olivine chemical zonations yield minimum cooling rates of 0.5‐0.8 °C/h. Spinel chemistry suggests that crystallization took place under one of the most reduced conditions for martian meteorites, at an fO₂ 3 log units below the quartz‐fayalite‐magnetite (QFM) buffer. The olivine megacrysts are heterogeneously distributed in the rock. Crystal size distribution analysis suggests that they constitute a population formed under steady‐state conditions of nucleation and growth, although a few grains may be cumulates. The parent melt is thought to have been derived from partial melting of a light rare earth element‐ and platinum group element‐depleted mantle source. Shergottites, EETA79001 lithology A, DaG 476/489, and Dhofar 019, although of different ages, comprise a particular type of martian rocks. Such rocks could have formed from chemically similar source(s) and parent melt(s), with their bulk compositions affected by olivine accumulation.     

Magma ocean fractional crystallization and cumulate overturn in terrestrial planets: Implications for Mars

Abstract— Crystallization of a magma ocean on a large terrestrial planet that is significantly melted by the energy of accretion may lead to an unstable cumulate density stratification, which may overturn to a stable configuration. Overturn of the initially unstable stratification may produce an early basaltic crust and differentiated mantle reservoirs. Such a stable compositional stratification can have important implications for the planet's subsequent evolution by delaying or suppressing thermal convection and by influencing the distribution of radiogenic heat sources. We use simple models for fractional crystallization of a martian magma ocean, and calculate the densities of the resulting cumulates. While the simple models presented do not include all relevant physical processes, they are able to describe to first order a number of aspects of martian evolution. The models describe the creation of magma source regions that differentiated early in the history of Mars, and present the possibility of an early, brief magnetic field initiated by cold overturned cumulates falling to the coremantle boundary. In a model that includes the density inversion at about 7.5 GPa, where olivine and pyroxene float in the remaining magma ocean liquids while garnet sinks, cumulate overturn sequesters alumina in the deep martian interior. The ages and compositions of source regions are consistent with SNC meteorite data.     

Age and origin of the lowlands of Mars

One of the many significant findings of the Mars Global Surveyor mission is the presence of hundreds of quasi-circular depressions (QCDs) observed from high-resolution MOLA topography data. Their presence has recently been interpreted to reflect a northern lowlands that archive some of the earliest recorded rocks on Mars, mostly below a veneer of Hesperian and Amazonian materials. Here we analyze these data, coupled with a recent synthesis of geologic, geophysical, geomorphic, topographic, and magnetic information. Such analysis allows us to suggest a potential plate tectonic phase during the recorded Early into Middle Noachian martian history that transitioned into a monoplate world with episodic magmatic-driven activity persisting to present. This working hypothesis is based on: (1) the observation that the basement of the northern plains is younger than the basement of the southern highlands, but older than the material exposures of the cratered highlands, suggesting different formational ages for each one of the three geologic-time units; (2) the observation that parts of the very ancient highland's crust are highly magnetized, thus suggesting that most if not all of the formation of the lowlands basement postdates the shut off of the martian dynamo, some 4 Gyr ago, and so allowing hundreds of millions of years for the shaping of the buried lowlands. Consequently, the role of endogenic processes in the earliest geological evolution of Mars (Early perhaps into Middle Noachian) requires reconsideration, since MOLA topographic and MGS magnetic data afford a temporal window sufficient for very early, non-primordial shaping of the northern lowlands' basement.     

Lithological mapping of central part of Mare Moscoviense using Chandrayaan-1 Hyperspectral Imager (HySI) data

We conducted spectral analysis of central region of the Mare Moscoviense area on the far side of the Moon using the Hyperspectral Imager (HySI) data from the Chandrayaan-1 mission in an effort to identify and map the major lithological units present in the area. Various spectral band parameters, namely, band curvature, band tilt and band strength have been used for lithological discrimination based on the nature of the spectral profile. These band parameters essentially measure the shape, position and strength of the absorption feature near 1000 nm arising due to electronic transition of Fe²⁺ in crystallographic sites of major rock forming silicates. Spectral band parameters have been used for generating rock type composite image. Based on spectral studies and rock type composite image as obtained using band parameters, five major compositional units have been identified: highland basin soils, ancient mature mare, highland contaminated mare, buried unit with abundant low-Ca pyroxene (LCP), and youngest mare unit. In the present study, a multispectral approach in the form of spectral band parameters has been adopted for analysing the HySI hyperspectral data from Chandrayaan-1 mission. Present study clearly shows that the spectral band parameters obtained using selected HySI channels could efficiently be used to discriminate and delineate the major litho-units present across the central part of Mare Moscoviense and the same approach can thus be used for lithological mapping of other parts of lunar surface using HySI data.     

The oxygen fugacity of olivine‐phyric martian basalts and the components within the mantle and crust of Mars

Abstract— The oxygen fugacity of olivine‐phyric martian basalts is estimated using olivine‐pyroxene‐spinel equilibria, supported by detailed petrography. Results are plotted, along with previous oxygen fugacity estimates, against La/Yb, which is used as a proxy for long‐term incompatible‐element depletion or enrichment in martian basalt reservoirs. In general, the correlation between oxygen fugacity and La/Yb observed by Herd et al. (2002a) holds for the olivine‐phyric basalts. The implications of the correlation are re‐evaluated in light of work by Borg et al. (Forthcoming), which indicates that the variations in radiogenic isotopic composition can be modeled by mixing of mantle sources established by 4.5 Ga through crystallization of a magma ocean in lieu of assimilation of crustal material. The results demonstrate that the crust‐like component, interpreted as trapped liquid in a magma ocean cumulate pile, must be oxidized to explain the oxygen fugacity of the martian basalts. Consequently, the pre‐eruptive water contents of the more oxidized basalts are expected to be higher, although water is not called upon as the cause of the oxidation. Unmixing of mantle components provides an important context for the interpretation of oxygen isotopes, demonstrated here, and of samples returned from the martian surface.     

Micro Raman spectroscopy of amphiboles and pyroxenes in the martian meteorites Zagami and Lewis Cliff 88516

Abstract— We studied micro Raman spectroscopy of amphiboles and pyroxenes in the martian meteorites Zagami and Lewis Cliff (LEW) 88516. The obtained Raman spectra of the amphiboles are similar to those of kaersutite, reconfirming the previous studies that they are kaersutitic amphiboles enriched in Ca, Al, and Ti. Even though actinolite belongs to the same amphibole group (calcic amphibole) as kaersutite, the Raman spectra of terrestrial actinolite are distinct from those of kaersutite, probably reflecting complex amphibole crystal structures. The Al‐Ti‐rich pyroxene observed in the magmatic inclusions within LEW 88516 olivine is compositionally similar to kaersutite but shows Raman spectra nearly identical to the regular pyroxene rather than amphibole. In contrast to amphibole, this will be due to relatively simple crystal structures of pyroxene. Thus, the Raman spectra of Al‐Ti‐rich phases in the martian meteorites are distinct between kaersutite and Al‐Ti‐rich pyroxene, and this study demonstrates that micro Raman spectroscopy is one of the best tools to perform mineralogical characterization of mineral phases in martian meteorites.     

Occurrence and implications of secondary olivine veinlets in lunar highland breccia Northwest Africa 11273

Lunar breccias preserve the records of geologic processes on the Moon. In this study, we report the occurrence, petrography, mineralogy, and geologic significance of the observed secondary olivine veinlets in lunar feldspathic breccia meteorite Northwest Africa (NWA) 11273. Bulk‐rock composition measurements show that this meteorite is geochemically similar to other lunar highland meteorites. In NWA 11273, five clasts are observed to host veinlets that are dominated by interconnecting olivine mineral grains. The host clasts are mainly composed of mafic minerals (i.e., pyroxene and olivine) and probably sourced from a basaltic lithology. The studied olivine veinlets (~5 to 30 μm in width) are distributed within the mafic mineral host, but do not extend into the adjacent plagioclase. Chemically, these olivine veinlets are Fe‐richer (Fo_41.4–51.9), compared with other olivine grains (Fo_54.3–83.1) in lithic clasts and matrix of NWA 11273. By analogy with the secondary olivine veinlets observed in meteorites from asteroid Vesta (howardite–eucrite–diogenite group samples) and lunar mare samples, our study suggests that the newly observed olivine veinlets in NWA 11273 are likely formed by secondary deposition from a lunar fluid, rather than by crystallization from a high‐temperature silicate melt. Such fluid could be sulfur‐ and phosphorous‐poor and likely had an endogenic origin on the Moon. The new occurrence of secondary olivine veinlets in breccia NWA 11273 reveals that the fluid mobility and deposition could be a previously underappreciated geological process on the Moon.     

The role of aqueous alteration in the formation of martian soils

Martian equatorial dark regions are dominated by unweathered materials and it has often been assumed that they have not been significantly altered from their source lithology. The suite of minerals present is consistent with a basaltic composition and there has been no need to invoke additional processes to explain the origin of these compositions. We have begun to question this result based on detailed observations using a variety of datasets. Locally derived dark soils have a mineralogy distinct from that of adjacent rocky surfaces; most notably a lower olivine content. This pattern is common for many surfaces across the planet. Previous work using detailed measurements acquired within the Gusev Plains has shown that olivine dissolution via acidic weathering may explain chemical trends observed between rock rinds and interiors. Mineralogical trends obtained from rocks and soils within the Gusev Plains are more prominent than the elemental trends and support previous results that indicate that dissolution of olivine has occurred. However, clear differences are also present in elemental abundances that indicate a variety of inputs and processes are likely responsible for the formation of martian dark soils. Despite the potential complexity of source materials and processes, it appears that most martian dark regions have likely experienced aqueous alteration and chemical weathering appears to be closely linked to the mechanical breakdown of materials. Regardless of the responsible mechanism, there appears to be a general, though not perfect, correlation between elevated olivine abundance and high-thermal inertia surfaces on Mars.     

The rocks of Mars,from far and near

Abstract— The age, structure, composition, and petrogenesis of the martian lithosphere have been constrained by spacecraft imagery and remote sensing. How well do martian meteorites conform to expectations derived from this geologic context? Both data sets indicate a thick, extensive igneous crust formed very early in the planet's history. The composition of the ancient crust is predominantly basaltic, possibly andesitic in part, with sediments derived from volcanic rocks. Later plume eruptions produced igneous centers like Tharsis, the composition of which cannot be determined because of spectral obscuration by dust. Martian meteorites (except Allan Hills 84001) are inferred to have come from volcanic flows in Tharsis or Elysium, and thus are not petrologically representative of most of the martian surface. Remote‐sensing measurements cannot verify the fractional crystallization and assimilation that have been documented in meteorites, but subsurface magmatic processes are consistent with orbital imagery indicating thick crust and large, complex magma chambers beneath Tharsis volcanoes. Meteorite ejection ages are difficult to reconcile with plausible impact histories for Mars, and oversampling of young terrains suggests either that only coherent igneous rocks can survive the ejection process or that older surfaces cannot transmit the required shock waves. The mean density and moment of inertia calculated from spacecraft data are roughly consistent with the proportions and compositions of mantle and core estimated from martian meteorites. Thermal models predicting the absence of crustal recycling, and the chronology of the planetary magnetic field agree with conclusions from radiogenic isotopes and paleomagnetism in martian meteorites. However, lack of vigorous mantle convection, as inferred from meteorite geochemistry, seems inconsistent with their derivation from the Tharsis or Elysium plumes. Geological and meteoritic data provide conflicting information on the planet's volatile inventory and degassing history, but are apparently being reconciled in favor of a periodically wet Mars. Spacecraft measurements suggesting that rocks have been chemically weathered and have interacted with recycled saline groundwater are confirmed by weathering products and stable isotope fractionations in martian meteorites.     

Integration and Comparison of Clementine and Lunar Prospector Data: Global Scale Multielement Analysis1 (Fe,Ti, and Th) of the Lunar Surface

In this paper, we present (1) a statistical analysis, based on a systematic clustering method, of a dataset integrating the global abundance maps of the three elements iron, titanium, and thorium derived from Clementine and Lunar Prospector and (2) a comparison of iron abundances between Clementine and Lunar Prospector. Homogeneous geologic units are compositionally characterized and spatially defined in relation to the major rock types sampled on the Moon. With the lowest abundances of Fe, Ti, and Th found on the Moon, the lunar highland terrains are quite homogeneous with two major large feldspathic units, one being slightly more mafic than the other. Two distinct regions with unique compositions are unambiguously identified: the Procellarum KREEP Terrane (PKT) and the South Pole–Aitken (SPA). The PKT, which includes all the units with Th abundances higher than 3.5 ppm (KREEP-rich materials), is delimited by an almost continuous ringlike unit. In particular, it includes the western nearside maria, except for Mare Humorum. With concentrations in Fe, Ti, and Th enhanced relative to the surrounding highlands, the South Pole–Aitken basin floor represents a large mafic anomaly on the far side, suggesting wide deposits of lower crust and possible mantle materials. However, due to indirect residual latitude effects in the CSR (Clementine spectral reflectance) measurements, iron abundances might have been overestimated in SPA, thus implying that crustal materials, rather than mantle materials, might represent the dominant contributor to the mafic component exposed on the basin floor.     

The origin of selected lunar geochemical anomalies: Implications for early volcanism and the formation of light plains

The Apollo orbital geochemistry, photogeologic, and other remote sensing data sets were used to identify and characterize geochemical anomalies on the eastern limb and farside of the Moon and to investigate the processes responsible for their formation. The anomalies are located in the following regions: (1) Balmer basin, (2) terrain northeast of Mare Smythii, (3) near Langemak crater, (4) Pasteur crater, (5) terrain northwest of Milne basin, (6) northeast of Mendeleev basin, (7) north and northeast of Korolev basin, (8) terrain north of Taruntius crater, and (9) terrain north of Orientale basin. The anomalies are commonly associated with Imbrian- or Nectarian-aged light plains units which exhibit dark-haloed impact craters. The results of recent spectral reflectance studies of dark-haloed impact craters plus consideration of the surface chemistry of the anomalies strongly indicate that those geochemical anomalies associated with light plains deposits which display dark-haloed impact craters result from the presence of basaltic units that are either covered by varying thickness of highland debris or have a surface contaminated with significant amounts of highlands material. The burial or contamination of ancient volcanic surfaces by varying amounts of highland material appears to have been an important (though not the dominant) process in the formation of lunar light plains. Basaltic volcanism on the eastern limb and farside of the Moon was more extensive in both space and time than has been accepted.     

Reflectance Spectral Characteristics of Lunar Surface Materials

Based on a comprehensive analysis of the mineral composition of major lunar rocks (highland anorthosite, lunar mare basalt and KREEP rock), we investigate the reflectance spectral characteristics of the lunar rock-forming minerals, including feldspar, pyroxene and olivine. The affecting factors, the variation of the intensity of solar radiation with wavelength and the reflectance spectra of the lunar rocks are studied. We also calculate the reflectivity of lunar mare basalt and highland anorthosite at 300 nm, 415 nm, 750 nm, 900 nm, 950 nm and 1000 nm. It is considered that the difference in composition between lunar mare basalt and highland anorthosite is so large that separate analyses are needed in the study of the reflectivity of lunar surface materials in the two regions covered by mare basalt and highland anorthosite, and especially in the region with high Th contents, which may be the KREEP-distributed region.     

Space weathering on Eros: Constraints from albedo and spectral measurements of Psyche crater

Abstract— We present combined multi‐spectral imager (MSI) (0.95 μm) and near‐infrared spectrometer (NIS) (0.8–2.4 μm) observations of Psyche crater on S‐type asteroid 433 Eros obtained by the Near‐Earth Asteroid Rendezvous (NEAR)—Shoemaker spacecraft. At 5.3 km in diameter, Psyche is one of the largest craters on Eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material. At spatial scales of 620 m/ spectrum, Psyche crater wall materials exhibit albedo contrasts of 32–40% at 0.946 μm. Associated spectral variations occur at a much lower level of 4–8% (±2–4%). We report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends: grain size differences, olivine, pyroxene, and troilite variations, and optical surface maturation. We find that the albedo contrasts in Psyche crater are not consistent with a cause due solely to variations in grain size, olivine, pyroxene or lunar‐like optical maturation. A grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed. Olivine and pyroxene variations would produce strong band‐correlated variations that are not observed. A simple lunar‐like optical maturation effect would produce strong reddening that is not observed. The contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite (a dark spectrally neutral component simulating optical effects of shock) and lunar‐like optical maturation. These results suggest that space weathering processes may affect the spectral properties of Eros materials, causing surface exposures to differ optically from subsurface bedrock. However, there are significant spectral differences between Eros' proposed analog meteorites (ordinary chondrites and/or primitive achondrites), and Eros' freshest exposures of subsurface bright materials. After accounting for all differences in the measurement units of our reflectance comparisons, we have found that the bright materials on Eros have reflectance values at 0.946 μm consistent with meteorites, but spectral continua that are much redder than meteorites from 1.5 to 2.4 μm. Most importantly, we calculate that average Eros surface materials are 30–40% darker than meteorites.     

Mineralogy and petrology of the Dar al Gani 476 martian meteorite: Implications for its cooling history and relationship to other shergottites

Abstract— Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a finegrained groundmass of pyroxene (59 vol%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessory phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lithology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralogy, but is distinct in several aspects. Low‐Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En₇₆Fs₂₁ Wo₃～En₅₈Fs₃₀Wo₁₂) than those in lithology A of EETA79001 (En₇₃Fs₂₂Wo₅～En₄₅Fs₄₃Wo₁₂), rather similar to pyroxenes in lherzolitic martian meteorites (En₇₆Fs₂₁ Wo₃～En₆₃Fs₂₂Wo₁₅). Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo_76‐58) than EETA79001 olivine (Fo_81‐53), and is also similar to olivines in lherzolitic martian meteorites (Fo_74‐65). The orthopyroxene‐olivine‐chromite xenolith typical in the lithology A of EETA79001 is absent in Dar al Gani 476. It seems that Dar al Gani 476 crystallized from a slightly more primitive mafic magma than lithology A of EETA79001 and several phases (olivine, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites. Olivine megacrysts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg‐rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling. This observation enabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 olivines, giving similar cooling rates of 0.03‐3 °C/h for Dar al Gani 476 and 0.05‐5 °C/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lithology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian lherzolite and other martian rock (Queen Alexandra Range 94201, nakhlites?).     

Petrogenesis of the new lherzolitic shergottite Grove Mountains 99027: Constraints of petrography,mineral chemistry,and rare earth elements

Abstract— We report petrography, mineral chemistry, and microdistribution of rare earth elements (REE) in a new lherzolitic shergottite, Grove Mountains (GRV) 99027. The textural relationship and REE patterns of minerals suggest precipitation of cumulus olivine and chromite, followed by equilibrium crystallization of a closed system with a bulk composition of the inferred intercumulus melt. Subsolidus equilibrium temperatures of pyroxenes and olivine range from 1100 to 1210 °C, based on a two‐pyroxene thermometry and Ca partitioning between augite and olivine. Oxygen fugacity of the parent magma is 1.5–2.5 (av. 2.0 ± 0.4) log units below the quartz‐fayalite‐magnetite (QFM) buffer at 960–1360 °C, according to the olivine‐orthopyroxene‐chromite barometer. The ilmenite‐chromite barometer and thermometer show much wider ranges of oxygen fugacity (1.0–7.0 log unit below QFM) and temperature (1130–480 °C), suggesting subsolidus equilibration of the oxides at low temperatures, probably due to deep burial of GRV 99027 on Mars. The low oxygen fugacity and LREE depletion of the parent magma of GRV 99027 suggest low contamination by martian crust. Characteristics of GRV 99027 demonstrate similarity of lherzolitic shergottites, suggesting a high possibility of launch pairing or a homogeneous upper mantle of Mars if they were ejected by individual impact events. However, GRV 99027 probably experienced severe post‐shock thermal metamorphism in comparison with other lherzolitic shergottites, based on the re‐crystallization of maskelynite, the homogeneity of minerals, and the low subsolidus equilibrium temperatures between chromite and ilmenite.     

Europa’s ridged plains and smooth low albedo plains: Distinctive compositions and compositional gradients at the leading side-trailing side boundary

This investigation uses linear mixture modeling employing cryogenic laboratory reference spectra to estimate surface compositions and water ice grain sizes of Europa’s ridged plains and smooth low albedo plains. Near-infrared spectra for 23 exposures of ridged plains materials are analyzed along with 11 spectra representing low albedo plains. Modeling indicates that these geologic units differ both in the relative abundance of non-ice hydrated species and in the abundance and grain sizes of water ice. The background ridged plains in our study area appear to consist predominantly of water ice (∼46%) with approximately equal amounts (on average) of hydrated sulfuric acid (∼27%) and hydrated salts (∼27%). The solutions for the smooth low albedo plains are dominated by hydrated salts (∼62%), with a relatively low mean abundance of water ice (∼10%), and an abundance of hydrated sulfuric acid similar to that found in ridged plains (∼27%). The model yields larger water ice grain sizes (100 μm versus 50-75 μm) in the ridged plains. The 1.5-μm water ice absorption band minimum is found at shorter wavelengths in the low albedo plains deposits than in the ridged plains (1.498 ± .003 μm versus 1.504 ± .001 μm). The 2.0-μm band minimum in the low albedo plains exhibits a somewhat larger blueshift (1.964 ± .006 μm versus 1.983 ± .006 μm for the ridged plains).The study area spans longitudes from 168° to 185°W, which includes Europa’s leading side-trailing side boundary. A well-defined spatial gradient of sulfuric acid hydrate abundance is found for both geologic units, with concentrations increasing in the direction of the trailing side apex. We associate this distribution with the exogenic effects of magnetospheric charged particle bombardment and associated chemical processing of surface materials (the radiolytic sulfur cycle). However, one family of low albedo plains exposures exhibits sulfuric acid hydrate abundances up to 33% lower than found for adjacent exposures, suggesting that these materials have undergone less processing, thus implying that these deposits may have been emplaced more recently.Modeling identifies high abundances (to 30%) of magnesium sulfate brines in the low albedo plains exposures. Our investigation marks the first spectroscopic identification of MgSO₄ brine on Europa. We also find significantly higher abundances of sodium-bearing species (bloedite and mirabilite) in the low albedo plains. The results illuminate the role of radiolytic processes in modifying the surface composition of Europa, and may provide new constraints for models of the composition of Europa’s putative subsurface ocean.     

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.     

Searching for high alumina mare basalts using Clementine UVVIS and Lunar Prospector GRS data: Mare Fecunditatis and Mare Imbrium

In the context of sample evidence alone, the high-alumina (HA) basalts appear to be an unique, and rare variety of mare basalt. In addition to their distinct chemistry, radiometric dating reveals these basalts to be among the oldest sampled mare basalts. Yet, HA basalts were sampled by four missions spanning a lateral range of ∼2400 km, with ages demonstrating that aluminous volcanism lasted at least 1 billion years. This evidence suggests that HA basalts may be a widespread phenomenon on the Moon. Knowing the distribution of HA mare basalts on the lunar surface has significance for models of the origin and the evolution of the Lunar Magma Ocean. Surface exposures of HA basalts can be detected with compositional remote sensing data from Lunar Prospector Gamma Ray Spectrometer and Clementine. We searched the lunar surface for regions of interest (ROIs) that correspond to the intersection of three compositional constraints taken from values of sampled HA basalts: 12-18 wt% FeO, 1.5-5 wt% TiO₂, and 0-4 ppm Th. We then determined the “true” (unobscured by regolith) composition of basalt units by analyzing the rims and proximal ejecta of small impacts (0.4-4 km in diameter) into the mare surface of these ROIs. This paper focuses on two ROIs that are the best candidates for sources of sampled HA basalts: Mare Fecunditatis, the landing site of Luna 16; and northern Mare Imbrium, hypothesized origin of the Apollo 14 HA basalts. We demonstrate our technique's ability for delineating discrete basalt units and determining which is the best compositional match to the HA basalts sampled by each mission. We identified two units in Mare Fecunditatis that spectrally resemble HA basalts, although only one unit (Iltm) is consistent with the compositional and relative age of the Luna 16 HA samples. Northern Mare Imbrium also reveals two units that are within the compositional constraints of HA basalts, with one (Iltm) best matching the composition of the basalts sampled by Apollo 14.