¹Dwijesh Ray, ¹Anil D.Shukla
Planetary and Space (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.11.005]
¹Physical Research Laboratory, Ahmedabad 380 009, India
Copyright Elsevier

Mukundpura is a new CM chondrite fell near Jaipur, Rajasthan, India on June 6, 2017 at 5:15 IST. The fall was observed by local villager. According to eyewitness, the meteorite was fragmented into several pieces once the object hit the ground. Based on petrography, mineralogy and bulk composition, Mukundpura is classified as CM2 chondrite. The chondrules are mainly similar to type I (Olivine: Fo99). Olivines are often found associated with pyroxene (Wo10-35En62-87Fs2-7) phenocryst. However, occurrences of forsteritic and fayalitic olivine (Fa58-71) as isolated mineral clast in matrix are not uncommon. Other types of chondrules include porphyritic pyroxene (En86Fs14) and barred olivine (Fa32.7±0.3) clast. Chondrules are commonly rimmed by fine-grained accretionary dust mantles. Phyllosilicates are the most dominant secondary mineral in matrix and largely associated with poorly characterised phases (PCP). FeO/SiO2 and S/SiO2 of PCP are 2.7 and 0.4 respectively. Other phases in matrix generally include calcite (pure CaCO3), Fe-Ni metal and sulphides. Spinel and perovskite occur occasionally as inclusions. The spherical or elliptical shaped metals (within chondrule or in isolated grains) are low-Ni type (kamacite <7.5 wt%) and resembles the solar Ni/Co ratio. However, Ni content in metal rarely exceeds 8.5 wt% (up to 23 wt%, taenite). Pyrrhotite (Fe ∼62 wt%; S ∼38 wt%) and pentlandite (Fe ∼31–33 wt%, Ni ∼28–32 wt%, S ∼33 wt%)) are the common sulphides occur as isolated grains within the matrix, however, the former is the most dominant. The bulk chemical composition of Mukundpura is largely similar to other CM type chondrite (e.g. Paris CM). Based on petrography, we infer a modest aqueous alteration stage for Mukundpura while the effect of thermal metamorphism was negligible.

¹O.Tschauner,²C. Ma,^3,4C. Prescher,³V. B. Prakapenka
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13009]
¹Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA
²Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
³Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, USA
⁴Institut für Geologie und Mineralogie, Universität zu Köln, Köln, Germany
Published by arrangement with John Wiley & Sons

Akimotoite (Mg,Fe)SiO₃ is one of the most common mineralogical indicators for high-level shock metamorphism in meteorites. First described 1997, its occurrence has been amply confirmed in a number of highly shocked chondrites. Yet, a thorough structure analysis of natural akimotoite has remained extant. Here we report accurate cell parameters, fractional atomic coordinates, and site occupancies for natural akimotoite from the holotype specimen based on synchrotron microdiffraction. The variation of unit cell shape and volume with Fe content define mixing volumes. Based on the mixing volume relation for akimotoite and hemleyite, we constrain the unit cell volume of endmember hemleyite to 273.8 ± 1.0 Å³. We show that mixing is nearly ideal for low Fe content but evolves to positive excess volume toward the Fe endmember. Based on this finding and the actual composition of akimotoite in Tenham, we show that this mineral has formed by solid–solid transformation prograde from enstatite, not by crystallization from melt.

^1,2Mario Trieloff,^1,2,3Ekaterina V. Korochantseva,^1,2,3Alexei I. Buikin,^1,2Jens Hopp,³Marina A. Ivanova,³Alexander V. Korochantsev
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13012]
¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Heidelberg, Germany
³Vernadsky Institute of Geochemistry, Moscow, Russia
Published by arrangement with John Wiley & Sons

We studied three lithologies (light and dark chondritic and impact melt rock) differing in shock stage from the LL5 chondrite Chelyabinsk. Using the 40Ar-39Ar dating technique, we identified low- and high-temperature reservoirs within all samples, ascribed to K-bearing oligoclase feldspar and shock-induced jadeite–feldspar glass assemblages in melt veins, respectively. Trapped argon components had variable 40Ar/36Ar ratios even within low- and high-temperature reservoirs of individual samples. Correcting for trapped argon revealed a lithology-specific response of the K-Ar system to shock metamorphism, thereby defining two distinct impact events affecting the Chelyabinsk parent asteroid (1) an intense impact event ~1.7 ± 0.1 Ga ago formed the light–dark-structured and impact-veined Chelyabinsk breccia. Such a one-stage breccia formation is consistent with petrological observations and was recorded by the strongly shocked lithologies (dark and impact melt) where a significant fraction of oligoclase feldspar was transformed into jadeite and feldspathic glass; and (2) a young reset event ~30 Ma ago particularly affected the light lithology due to its low argon retentivity, while the more retentive shock-induced phases were more resistant against thermal reset. Trapped argon with 40Ar/36Ar ratios up to 1900 was likely incorporated during impact-induced events on the parent body, and mixed with terrestrial atmospheric argon contamination. Had it not been identified via isochrons based on high-resolution argon extraction, several geochronologically meaningless ages would have been deduced.

¹C. Bu,¹G. Rodriguez Lopez,¹C. A. Dukes,²O. Ruesch,²L. A. McFadden,³J.-Y. Li
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13024]
¹Laboratory for Astrophysics and Surface Physics, University of Virginia, Charlottesville, Virginia, USA
²NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
³Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile-rich protoplanets like Ceres. Sulfates, present on water-bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres’ UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres’ cryogenic, low-pressure environment and the consequent spectral effects, using UV–Vis–IR reflectance spectroscopy. H2O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO4·6H2O by X-ray powder diffraction. Long-term (>40 h), continuous exposure of MgSO4·nH2O (n = 0, 6, 7) to low pressure (10−3–10−6 Torr) causes material decomposition and strong UV absorption below 0.5 μm. Our measurements suggest that MgSO4·6H2O grains (45–83 μm) dehydrate to 2% of the original 1.9 μm water band area over ~0.3 Ma at 200 K on Ceres and after ~42 Ma for 147 K. These rates, inferred from an Avrami dehydration model, preclude MgSO4·6H2O as a component of Ceres’ surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5–17 μm) suggests sulfates cannot be excluded from Ceres’ mineralogy.

¹G. Micca Longo, ^1,2,3S. Longo
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.12.001]
¹Department of Chemistry, University of Bari, via Orabona 4, Bari, 70126, Italy
²CNR-Nanotec, via Amendola 122/D, Bari, 70126, Italy
³INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, Firenze, I-50125, Italy
Copyright Elsevier

Current models allow to reliably simulate mechanical and thermal phenomena associated with a micrometeor passage through the Earth’s atmosphere. However, these models have rarely been applied to materials other than those most common in meteorites, such as silicates and metals. A particular case that deserves attention is the one of micrograins made of minerals, in particular carbonates, which have been associated, in meteorites, with organic molecules. Carbonates are known for their decomposition in vacuum at moderate temperatures, and they might contribute to the thermal protection of organic matter. In this work, a model with non isothermal atmosphere, power balance, evaporation, ablation, radiation losses and stoichiometry, is proposed. This paper includes the very first calculations for meteoroids with a mixed carbonate composition. Results show that the carbonate fraction of these objects always go to zero at high altitudes except for grazing entries, where the reached temperature is lower and some carbonate remains unreacted. For all entry conditions, peculiar temperature curves are obtained due to the decomposition process. Furthermore, a significant impact of decomposition cooling on the temperature peak is observed for some grazing entry cases. Although specific solutions used in these calculations can be improved, this work sets a definite model and a basis for future research on sub-mm grains of relatively volatile minerals entering the Earth’s atmosphere.

¹M. E. Tabetah,¹H. J. Melosh
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13034]
¹EAPS Department, Purdue University, West Lafayette, Indiana, USA
Published by arrangement with John Wiley & Sons

The entry and subsequent breakup of the ~17–20 m diameter Chelyabinsk meteoroid deposited approximately 500 kT of TNT equivalent energy to the atmosphere, causing extensive damage that underscored the hazard from small asteroid impacts. The breakup of the meteoroid was characterized by intense fragmentation that dispersed most of the original mass. In models of the entry process, the apparent mechanical strength of the meteoroid during fragmentation, ~1–5 MPa, is two orders of magnitude lower than the mechanical strength of the surviving meteorites, ~330 MPa. We implement a two-material computer code that allows us to fully simulate the exchange of energy and momentum between the entering meteoroid and the interacting atmospheric air. Our simulations reveal a previously unrecognized process in which the penetration of high-pressure air into the body of the meteoroid greatly enhances the deformation and facilitates the breakup of meteoroids similar to the size of Chelyabinsk. We discuss the mechanism of air penetration that accounts for the bulk fragmentation of an entering meteoroid under conditions similar to those at Chelyabinsk, to explain the surprisingly low values of the apparent strength of the meteoroid during breakup.

¹A.Longobardo et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.12.011]
¹INAF-IAPS, via Fosso del Cavaliere 100, I-00133 Rome, Italy
Copyright Elsevier

We studied the distribution in the Urvara-Yalode region of Ceres (latitudes 21-66°S, longitudes 180-360°E) of main spectral parameters derived from the VIR imaging spectrometer onboard the NASA/Dawn spacecraft, as an overall study of Ceres mineralogy reported in this special issue. In particular, we analyzed the distribution of reflectance at 1.2 μm, band depth at 2.7 and 3.1 μm, ascribed to magnesium and ammoniated phyllosilicates, respectively.

Whereas the average band depths of this region are lower than eastern longitudes, reflecting the E-W dichotomy of abundance of phyllosilicates on Ceres, spectral variations inside this region are observed in the following units: a) the central peak of the Urvara crater (45.9°S, 249.2°E, 170 km in diameter), showing a deep 3.1 μm band depth, indicating an ammonium enrichment; b) the cratered terrain westwards of the Yalode basin (42.3°S, 293.6°E, 260 km in diameter), where absorption bands are deeper, probably due to absence of phyllosilicates depletion following the Yalode impact; c) the hummocky cratered floor of Yalode and Besua (42.4°S, 300.2°E) craters, characterized by lower albedo and band depths, probably due to different roughness; d) Consus (21°S 200°E) and Tawals (39.1°S, 238°E) craters, whose albedo and band depths decreasing could be associated to different grain size or abundance of dark materials. Twenty-two small scale (i.e., lower than 400 m) bright spots are observed: because their composition is similar to the Ceres average, a strong mixing may have occurred since their formation.

¹M. Naito, ^1,2N. Hasebe, ²H. Nagaoka, ²E. Shibamura, ³M. Ohtake, ⁴K.J. Kim, ⁵C. Wöhler, ⁶A.A. Berezhnoy
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.12.005]
¹School of Advanced Science and Engineering, Waseda University, Japan
²Research Institute for Science and Engineering, Waseda University, Japan
³Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Japan
⁴Korea Institute of Geoscience and Mineral Resources, South Korea
⁵Image Analysis Group, TU Dortmund University, Germany
⁶Sternberg Astronomical Institute, Moscow State University, Russia
Copyright Elsevier

In this study we describe the distribution of iron on the Moon as obtained by the Kaguya high energy resolution gamma-ray spectrometer (KGRS). We achieved for the first time the identification of iron based on the fast neutron flux obtained by the KGRS. The iron distribution obtained by KGRS is compared to that of the Lunar Prospector Gamma-Ray Spectrometer (LP GRS), showing that the FeO distributions observed by KGRS and LP GRS, in general, are in good agreement. Furthermore, we compare the iron content data obtained by KGRS and LP GRS to spectral reflectance measurements of the Clementine, Kaguya and Chandrayaan-1 spacecraft as well as those inferred from returned samples. We found differences in FeO concentration and distribution in areas of moderate abundance (6-15 wt%) of the South Pole-Aitken basin, Mare Orientale, and around the crater Tycho crater. It implies that high concentrations of FeO at Mare Ingenii in the South Pole-Aitken basin and Mare Orientale are due to the presence of mare basalts, whereas the enriched FeO content in the central depression of the South Pole-Aitken basin and the Tycho crater indicates the presence of mafic materials such as impact melt breccia.

^1,2A. R. Poppe,^2,3W. M. Farrell,^2,4J. S. Halekas
Journal of Geophysical Research, Planets Link to Article [DOI: 10.1002/2017JE005426]
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA, USA
²Solar System Exploration Research Virtual Institute, NASA Ames Research Center, Moffett Field, CA, USA
³NASA Goddard Space Flight Center, Greenbelt, MD, USA
⁴Dept. of Physics and Astronomy, University of Iowa, Iowa City, IA, USA
Published by arrangement with John Wiley & Sons

The weathering of airless bodies exposed to space is a fundamental process in the formation and evolution of planetary surfaces. At the Moon, space weathering induces a variety of physical, chemical, and optical changes including the formation of nanometer sized amorphous rims on individual lunar grains. These rims are formed by vapor redeposition from micrometeoroid impacts and ion irradiation-induced amorphization of the crystalline matrix. For ion irradiation-induced rims, however, laboratory experiments of the depth and formation timescales of these rims stand in stark disagreement with observations of lunar soil grains. We use observations by the ARTEMIS spacecraft in orbit around the Moon to compute the mean ion flux to the lunar surface between 10 eV and 5 MeV and convolve this flux with ion irradiation-induced vacancy production rates as a function of depth calculated using the Stopping Range of Ions in Matter (SRIM) model. By combining these results with laboratory measurements of the critical fluence for charged-particle amorphization in olivine, we can predict the formation timescale of amorphous rims as a function of depth in olivinic grains. This analysis resolves two outstanding issues: (1) the provenance of >100 nm amorphous rims on lunar grains and (2) the nature of the depth-age relationship for amorphous rims on lunar grains.

¹P.E.Martin et al. (>10)
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005445]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
Published by arrangement with John Wiley & Sons

Following K-Ar dating of a mudstone and a sandstone, a third sample has been dated by the Curiosity rover exploring Gale Crater. The Mojave 2 mudstone, which contains relatively abundant jarosite, yielded a young K-Ar bulk age of 2.57 ± 0.39 Ga (1σ precision). A two-step heating experiment was implemented in an effort to resolve the K-Ar ages of primary and secondary mineralogical components within the sample. This technique involves measurement of 40Ar released in low (500oC) and high (930oC) temperature steps, and a model of the potassium distribution within the mineralogical components of the sample. Using this method, the high-temperature step yields a K-Ar model age of 4.07 ± 0.63 Ga associated with detrital plagioclase, compatible with the age obtained on the Cumberland mudstone by Curiosity. The low-temperature step, associated with jarosite mixed with K-bearing evaporites and/or phyllosilicates, gave a youthful K-Ar model age of 2.12 ± 0.36 Ga. The interpretation of this result is complicated by the potential for argon loss after mineral formation. Comparison with the results on Cumberland and previously published constraints on argon retentivity of the individual phases likely to be present suggests that the formation age of the secondary materials, correcting for plausible extents of argon loss, is still less than 3 Ga, suggesting post-3 Ga aqueous processes occurred in the sediments in Gale Crater. Such a result is inconsistent with K-bearing mineral formation in Gale Lake, and instead suggests post-depositional fluid flow at a time after surface fluvial activity on Mars is thought to have largely ceased.