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.

¹D.S. Vogt, ¹K. Rammelkamp, ¹S. Schröder, ^1,2H.W. Hübers
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.12.006]
¹Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institute of Optical Sensor Systems, Berlin, Germany
²Humboldt-Universität zu Berlin, Department of Physics, Berlin, Germany
Copyright Elsevier

The intensity of the molecular CaCl emission in LIBS spectra is examined in order to evaluate its suitability for the detection of chlorine in a Martian environment. Various mixtures resembling Martian targets with varying Cl content are investigated under simulated Martian conditions. The reactions leading to the formation of CaCl are modeled based on reaction kinetics and are used to fit the measured CaCl band intensities. MgCl bands are also investigated as potential alternatives to CaCl, but no MgCl bands can be identified in samples containing both Mg and Cl. The study confirms that CaCl is well suited for the indirect detection of chlorine, but finds a strong dependence on the concentrations of Ca and Cl in the sample. Spectra from samples with a high chlorine concentration can have low-intensity CaCl emission due to a deficiency of Ca. A qualitative estimate of the sample composition is possible based on the ratio of the band intensity of CaCl to the intensity of Ca emission lines. Time-resolved measurements show that the CaCl concentration in the plasma is highest after about 1 µs.

¹Zoe A. Landsman, ²Joshua P. Emery, ¹Humberto Campins, ³Josef Hanuš, ⁴Lucy F. Lim, ⁵Dale P. Cruikshank
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.11.035]
¹Department of Physics, University of Central Florida, 4111 Libra Drive, PS 430, Orlando, FL, 32826, United States
²Earth and Planetary Science Department, Planetary Geosciences Institute, University of Tennessee, 306 EPS Building, 1412 Circle Dr, Knoxville, TN, 37996, United States
³Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, Prague, 18000, Czech Republic
⁴NASA Goddard Space Flight Center, Mail Code 691, Greenbelt, MD, 20771, United States
⁵NASA Ames Research Center, Mail Stop 245-6, Moffett Field, CA, 94035, United States
Copyright Elsevier

Asteroid (16) Psyche is a unique, metal-rich object belonging to the “M” taxonomic class. It may be a remnant protoplanet that has been stripped of most silicates by a hit-and-run collision. Because Psyche offers insight into the planetary formation process, it is the target of NASA’s Psyche mission, set to launch in 2023. In order to constrain Psyche’s surface properties, we have carried out a mid-infrared (5–14 μm) spectroscopic study using data collected with the Spitzer Space Telescope’s Infrared Spectrograph. Our study includes two observations covering different rotational phases. Using thermophysical modeling, we find that Psyche’s surface is smooth and likely has a thermal inertia Γ = 5–25 J/m2/K/s1/2 and bolometric emissivity ϵ= 0.9, although a scenario with ϵ=0.7 and thermal inertia up to 95 J/m2/K/s1/2 is possible if Psyche is somewhat larger than previously determined. The smooth surface is consistent with the presence of a metallic bedrock, which would be more ductile than silicate bedrock, and thus may not readily form boulders upon impact events. From comparisons with laboratory spectra of silicate and meteorite powders, Psyche’s 7–14μm emissivity spectrum is consistent with the presence of fine-grained ( < 75μm) silicates on Psyche’s surface. We conclude that Psyche is likely covered in a fine silicate regolith, which may also contain iron grains, overlying an iron-rich bedrock.

¹Christopher W. Haberle, ^1,2Laurence A.J. Garvie
The American Mineralogist 102,2415-2421 Link to Article [DOI: https://doi.org/10.2138/am-2017-6180]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
¹Center for Meteorite Studies, Arizona State University, Arizona 85287-6004, U.S.A
Copyright: The Mineralogical Society of America

The CM and CI carbonaceous chondrites are typically dominated by phyllosilicates with variable proportions of tochilinite, anhydrous silicates, carbonates, sulfides, sulfates, oxides, and organic compounds. During thermal metamorphism the phyllosilicates dehydrate and decompose yielding water and olivine/enstatite. The thermal transformation of carbonate is less well understood, especially in the presence of volatile decomposition products, such as CO, CO2, SO2, H2S, and H2O. Here is described the mineralogical transformation of calcite (CaCO3) to oldhamite (CaS) and portlandite [Ca(OH)2] during extraterrestrial thermal metamorphism on the Sutter’s Mill parent body. Sutter’s Mill is a regolith breccia consisting of at least two lithologic components: phyllosilicate-calcite-bearing and anhydrous olivine-rich. Evidence suggests that the anhydrous stones were derived from extraterrestrial heating of the phyllosilicate-calcite-bearing material. One of only three Sutter’s Mill stones (SM3) collected prior to heavy rainfall over the recovery site is the focus of this study. Its powder X-ray diffraction patterns are dominated by olivine, with lesser enstatite, Fe-sulfides, magnetite, and oldhamite. Oldhamite is absent in the rained-on stones reflecting its water sensitivity and the pristine nature of SM3. Optical micrographs show whitish to bluish grains of oldhamite and portlandite embedded in dark, fine-grained matrix. The presence of abundant olivine and absence of phyllosilicates, tochilinite, and carbonate indicates that SM3 underwent heating to ~750 °C. At this temperature, calcite would have decomposed to lime (CaO). Volatilization experiments show that CO, CO2, SO2, and H2S evolve from CM and CI chondrites heated above 600 °C. Lime that formed through calcite decomposition would have reacted with these gases forming oldhamite under reducing conditions. Residual lime not converted to oldhamite, would have readily hydrated to portlandite, possibly through retrograde reactions during cooling on the parent body. These reactions have parallels to those in coal-fired electricity generating plants and provide an analogous system to draw comparison. Furthermore, the identification of these minerals, which are sensitive to terrestrial alteration, and determination of their formation is enabled only by the rapid collection of samples from an observed fall and their subsequent curation.

¹T. Hopp, ¹M. Fischer-Gödde, ¹T. Kleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.11.033]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

Mass-dependent Ru isotope variations (δ102/99Ru) and Ru concentrations were determined for 35 magmatic iron meteorites from the five major chemical groups (IIAB, IID, IIIAB, IVA, IVB). In addition, four equilibrated ordinary chondrites were analyzed. The IIAB, IIIAB and IVB iron meteorites display increasingly heavier Ru isotopic compositions with decreasing Ru content. Modeling demonstrates that the trends for these three iron groups can be reproduced by the incremental extraction of isotopically lighter Ru into solids, which leads to progressively heavier δ102/99Ru in the remaining melt. The modeling further shows that the Ru isotopic variations of the IIAB and IIIAB irons are consistent with derivation from parental melts with an ordinary chondrite-like δ102/99Ru, whereas the IVB irons more likely derive from a melt with heavier δ102/99Ru. This heavy Ru isotopic composition of the IVB parental melt probably results from high-temperature processing of the IVB precursor material. The Ru isotope systematics of the IID and IVA irons are more complex and show no correlation between δ102/99Ru and Ru content. Although most samples exhibit heavy Ru isotopic compositions, especially the late-crystallized irons of these groups deviate from the expected fractional crystallization trends. This deviation most likely results from mixing and re-equilibration of early-crystallized solids and late-stage liquids, followed by further fractional crystallization. The mixing might be related to the migration of liquids through a complex network of dendrites or to the overturn of a cumulate inner core, and bears testimony to the complex solidification history of at least some protoplanetary cores.