^1,2E.Heggy,¹E.M.Palmer,²T.W.Thompson,³B.J.Thomson,⁴G.W.Patterson
Earth and Planetary Science Letters 541, 116274 Link to Article [https://doi.org/10.1016/j.epsl.2020.116274]
¹University of Southern California, Ming Hsieh Department of Electrical and Computer Engineering, 3737 Watt Way, Los Angeles, CA 90089, USA
²Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³Department of Earth and Planetary Sciences, The University of Tennessee Knoxville, 1621 Cumberland Avenue, Knoxville, TN 37996, USA
⁴The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
Copyright Elsevier

Identifying polarimetric radar signatures of ice in smooth regolith fines on the floors of permanently shadowed lunar craters is hindered by uncertainties in their dielectric properties. We address this deficiency through polarimetric radar analysis of surface backscatter to derive the dielectric constant () of smooth, rock-free regolith fines covering brecciated crater floors observed by Mini-RF, which offer ideal locations for unambiguous retrieval of surface from linear polarimetric scattering models and CPR analysis for volatile identification. Specifically, we select fines covering crater fills in north polar and equatorial regions to constrain the range of variability of as a function of latitude and crater diameter, where we hypothesize that the latter is indicative of the excavation depth of these fines. Our observations suggest that there is measurable variability in the dielectric properties of fines on lunar crater floors as a function of crater size and potentially with impact excavation depth, suggesting that small craters <5-km in diameter have ranging from 2.3-to-3, and large ones >5-km have higher values of that range from 3-to-3.8. We find that the most plausible explanation for the observed variability of of regolith fines on crater floors is mineralogical differences, suggesting an increase in metal abundance in the original excavated substrate with depth, i.e., in the uppermost kilometer of the lunar crust. Finally, we suggest that regolith fines on the floors of permanently shadowed craters <5 km in diameter are optimal targets for the unambiguous detection of water-ice enrichment using S-band radar observations.

¹Forrest Gilfoy,¹Jie Li
Earth and Planetary Science Letters 541, 116285 Link to Article [https://doi.org/10.1016/j.epsl.2020.116285]
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, 48109, USA
Copyright Elsevier

A series of multi-anvil experiments have been conducted to define the iron-rich liquidus of the iron-nickel-sulfur (FeNi-S) system at 20 GPa, the estimated pressure of the martian core-mantle boundary (CMB). The liquidus curve of FeNi-S containing about 9 wt.% Ni has a concave up shape, and is as much as 400 K lower than the liquidi previously applied to the martian core with sparse experimental constraints. Unlike existing liquidi of Fe-S and FeNi-S at 23 GPa, which predict a fully molten core for a narrow range of sulfur content between 14 and 15 wt.% S, our results are consistent with a molten state for all proposed core compositions, and establishes a new minimum CMB temperature of 1500 K for 10 wt.% S and 1250 K for 16 wt.% S. Extrapolating our FeNi-S liquidus to high pressures and comparing it to calculated areotherms, we find that three core crystallization regimes are possible. For a martian core with moderate sulfur content (10 to 13 wt.%) or lower, crystallization takes the form of iron snow near the CMB, while for cores with higher sulfur content (15-16 wt.%), solidification occurs near the center of the planet in the form of solid Fe3S. At an intermediate sulfur content of 14 wt.%, Fe3S would precipitate over a broad depth range and may appear fully molten to surface observations.

^1,2Lionel G.Vacher,¹Laurette Piani,¹Thomas Rigaudier,¹Dorian Thomassin,¹Guillaume Florin,¹Maxime Piralla,Yves Marrocchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.007]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy F-54501, France
²Department of Physics, Washington University, St. Louis, St. Louis, MO, USA
Copyright Elsevier

Hydrogen occurs at the near percent level in the most hydrated chondrites (CI and CM) attesting to the presence of water in the asteroid-forming regions. Their H abundances and isotopic signatures are powerful proxies for deciphering the distribution of H in the protoplanetary disk and the origin of Earth’s water. Here, we report H contents and isotopic compositions for a set of carbonaceous and ordinary chondrites, including previously analyzed and new samples analyzed after the powdered samples were degassed under vacuum at 120°C for 48 hours to remove adsorbed atmospheric water. By comparing our results to literature data, we reveal that the H budgets of both H-poor and H-rich carbonaceous chondrites are largely affected by atmospheric moisture, and that their precise quantification requires a specific pre-degassing procedure to correct for terrestrial contamination. Our results show that indigenous H contents of CI carbonaceous chondrites usually considered the most hydrated meteorites might be almost a factor of two lower than those previously reported, with uncontaminated D/H ratios differing significantly from that of Earth’s oceans. Without pre-degassing, the H concentrations of H-poor samples (e.g., CVs chondrites) are also affected by terrestrial contamination. After correction for contamination, it appears that the amount of water in chondrites is not controlled by the matrix modal abundance, suggesting that the different chondritic parent bodies accreted variable amounts of water-ice grains. Our results also imply that (i) thermal metamorphism play an important role in determining the H content of both CV and ordinary chondrites but without affecting drastically their H isotopic composition since no clear D enrichment is observed with the increase of petrographic type and (ii) the D enrichment of ordinary chondrite organics does not result from the loss of isotopically light H₂ induced by metal oxidation but is rather linked to the persistence of a thermally resistant D-rich component.

¹J.R.Lyons
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.001]
¹School of Earth & Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287, United States
Copyright Elsevier

Isotope fractionation due to photochemical self-shielding is believed to be responsible for the enrichment of inner solar system planetary materials in the rare isotopes of carbon, nitrogen, and oxygen relative to the Sun. Self-shielding may also contribute to sulfur isotope mass-independent fractionation in modern atmospheric sulfates, although its role in the early Earth atmosphere has not yet been convincingly established. Here, I present an analytical formulation of isotopic photodissociation rate coefficients that describe self-shielding isotope signatures for 3 and 4-isotope systems broadly representative of O and S isotopes. The analytic equations are derived for idealized molecular spectra, making an analytic formulation tractable. The idealized spectra characterize key features of actual isotopologue spectra, particularly for CO and SO2, but are applicable to many small molecules and their isotopologues. The analytic expressions are convenient for evaluating the magnitude of isotope effects without having to pursue involved numerical solutions. More importantly, the analytic expressions illustrate the origin of particular isotope signatures, such as the previously unexplained large mass-dependent fractionation associated with photodissociation of optically-thick SO2. The formulation presented here elucidates the origin of some of these important isotopic fractionation processes.

¹Rhiannon G. Mayne,²Catherine M. Corrigan,²Timothy J. McCoy,³James M. D. Day,²Timothy R. Rose
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13478]
¹Oscar E. Monnig Meteorite Collection, Texas Christian University, Fort Worth, Texas, 76109 USA
²Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, 20013‐7012 USA
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, 92093‐0244 USA
Published by arrangement with John Wiley & Sons

Qarabawi’s Camel Charm was acquired from Abdullah Qarabawi of the Ababda tribe of eastern Egypt. The charm consists of a chain with four links and an ~6.5 cm diameter flattened disk with the Arabic inscription “Allahu Akbar,” which translates as “God is Greatest.” Belief in the evil eye is prevalent among the Ababda, even to the modern day, and as men identify camels and the cultural objects and activities related to them as one of their most important possessions, charms and amulets are often used to ward off its influence. Nondestructive analyses of the disk and metallographic examination of the distal link reveal a deformed medium octahedral pattern, confirming the meteoritic origin of the Camel Charm. Major, minor, and trace element compositions are consistent with classification as a IIIAB iron. Combined heating to modest temperatures (~600 °C) and cold working were used in the manufacture of the Camel Charm. Although compositionally similar to the Wabar IIIAB irons, chemical differences, the significant distance between Wabar and eastern Egypt, and the lack of established trade routes suggest that the Camel Charm source material was a meteorite unknown as an unworked specimen. This meteorite has been given the name Wadi El Gamal, the name of a National Park in the Ababda homelands.

¹Juraj Beno,¹Robert Breier,¹Jozef Masarik
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13487]
¹Department of Nuclear Physics and Biophysics, Faculty of Mathematics, Physics and Informatics, Commenius University Bratislava, Bratislava, SK‐842 48 Slovakia
Published by arrangement with John Wiley & Sons

The solar activity can be quantified by solar modulation parameter Φ that affects the heliospheric magnetic field. This activity influences the intensity of the galactic cosmic ray (GCR) particle flux within the solar system, and consequently, the differential primary particle spectra depend on the solar modulation parameter Φ (MeV). The modulation parameter Φ shows spatial and temporal variations (Leya and Masarik 2009). Some of the solar activity variations are cyclic and result in measurable effects as for example the 11‐year solar cycle. Variations in solar activity only induce small effects on the production of long‐lived cosmogenic radionuclides. This is due to the fact that activities measured in meteorites usually correspond to saturation values and represent long‐term average values. Long‐lived radionuclides often require millions of years of irradiation by GCR to reach saturation and therefore activity cycles average out. In contrast, one can expect strongly pronounced variations for saturation values caused by primary flux intensity variations, if short‐lived radionuclides with half‐lives ranging from days to a few years are investigated. Short‐lived cosmogenic nuclides were the subject of many experimental and theoretical investigations (e.g., Evans et al. 1982; Spergel et al. 1986; Neumann et al. 1997; Komura et al. 2002; Laubenstein et al. 2012). The aim of this work is to develop formulae for calculating production rates of radionuclides with short half‐life, taking into account temporal variations in the primary cosmic ray intensity. The developed formulae were applied to the Kosice and Chelyabinsk meteorites. The results for the Košice meteorite were already published (Povinec et al. 2015). Here, we give a full explanation of underlying model.

¹Richard C. Greenwood,^1,2Mahesh Anand
Space Science Reviews 216, 52 Link to Article [DOIhttps://doi.org/10.1007/s11214-020-00669-8]
¹Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
²Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Leos Pohl,¹Daniel T. Britt
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13449]
¹University of Central Florida, 4111 Libra Drive, Physical Sciences Bldg. 430, Orlando, Florida, 32816‐2385 USA
Published by arrangement with John Wiley & Sons

We report all available measurements on strength of meteorites, primarily focusing on compressive and tensile strengths and supplementary data such as Young’s modulus, Poisson’s ratio, elastic sound wave velocities, density, porosity, and sample sizes. These data are solely taken from the original papers to avoid misprints and other issues. The data are provided as originally presented by the authors with the exception of standardization of units to the SI system. A brief overview of methods for each original work is also provided as a guide to “data quality” since individual papers go to varying levels of detail on their experimental setup and procedures. From this data set, we confirm that the compressive strength of ordinary chondrites (varying in the range of 10s to 100s of MPa) is about an order of magnitude larger compared to their tensile strength and the difference increases with iron content. For carbonaceous chondrites, the tensile strength seems to be about an order of magnitude below the tensile strength of ordinary chondrites and at least an order of magnitude below their compressive strength. We also provide a statistical relation between the strength of meteorites and their densities and porosities and discuss the role of strain rate and sample size on the resultant measured strength. Finally, the data do not provide sufficient statistics to support a size scale effect of strength of meteorites.

¹N. Mari,¹L. J. Hallis,¹L. Daly,¹M. R. Lee
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13488]
¹School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ UK
Published by arrangement with John Wiley & Sons

The Tissint Martian meteorite is an unusual depleted olivine‐phyric shergottite, reportedly sourced from a mantle‐derived melt within a deep magma chamber. Here, we report major and trace element data for Tissint olivine and pyroxene, and use these data to provide new insights into the dynamics of the Tissint magma chamber. The presence of irregularly spaced oscillatory phosphorous (P)‐rich bands in olivine, along with geochemical evidence indicative of a closed magmatic system, implies that the olivine grains were subject to solute trapping caused by vigorous crystal convection within the Tissint magma chamber. Calculated equilibration temperatures for the earliest crystallizing (antecrystic) olivine cores suggest a Tissint magma source temperature of 1680 °C, and a local Martian mantle temperature of 1560 °C during the late Amazonian—the latter being consistent with the ambient mantle temperature of Archean Earth.

^1,2,3Nicholas E. Timms et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.04.031]
¹The Institute for Geoscience Research (TIGeR), Curtin University, Perth, GPO Box U1987, WA 6845, Australia
²Space Science and Technology Centre, Curtin University, Perth, GPO Box U1987, WA 6845, Australia
³School of Earth and Planetary Sciences, Curtin University, Perth, GPO Box U1987, WA 6845, Australia
Copyright: Elsevier

Hydrothermal activity is a common phenomenon in the wake of impact events, yet identifying and dating impact hydrothermal systems can be challenging. This study provides the first detailed assessment of the effects of shock microstructures and impact-related alteration on the U-Pb systematics and trace elements of titanite (CaTiSiO5), focusing on shocked granite target rocks from the peak ring of the Chicxulub impact structure, Mexico. A >1 mm long, shock-twinned titanite grain preserves a dense network of irregular microcracks, some of which exploit shock twin interfaces. Secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks. In situ laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICPMS) analysis reveals a mixture of three end-member Pb components. The Pb components are: 1) common Pb, consistent with the Pb isotopic signature of adjacent alkali feldspar; 2) radiogenic Pb accumulated since magmatic crystallization; and 3) a secondary, younger Pb signature due to impact-related complete radiogenic Pb loss. The youngest derived ages define a regression from common Pb that intersects Concordia at 67 ± 4 Ma, in agreement with the established age of 66.04 ± 0.05 Ma for the Chicxulub impact event. Contour maps of LA-MC-ICPMS data reveal that the young ages are spatially restricted to microstructurally-complex domains that correlate with significant depletion in trace elements (REE-Y-Zr-Nb-Mo-Sn-Th) and reduction in magnitude of the Eu/Eu* anomaly. Mapping by time-of-flight secondary ion mass spectrometry (ToF-SIMS) show that patterns of localised element depletion in titanite are spatially related to microcracks, which are enriched in Al. The spatial correlation of ages and trace element abundance is consistent with localised removal of Pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid-mediated element transport process associated with the impact event. Here we interpret the 67 ± 4 Ma U-Pb age to represent hydrothermal Pb-loss in the Chicxulub peak ring in the wake of the impact event. These results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post-impact hydrothermal activity in impact structures elsewhere.