Selection of chemical species for Europa’s surface using Galileo/NIMS

1G. Cruz Mermy,1,2F. Schmidt,3T. Cornet4, I. Belgacem,4N. Altobelli
Icarus (in Press) Link to Article []
1Université Paris-Saclay, CNRS, GEOPS, 91405, Orsay, France
2Institut Universitaire de France (IUF), France
3Aurora Technology BV for ESA, Netherlands
4European Space Agency (ESA), European Space Astronomy Centre (ESAC), Spain
Copyright Elsevier

Europa’s surface shows evidence of active resurfacing which can be explained by either endogenic or exogenic processes. Apart from water ice, several compounds have been proposed to account for the potential complex chemistry that could take place if a connection with the subsurface salty ocean would occur. Previous spectroscopic studies that investigated the surface composition were limited by the number of compounds to consider due to the unavailability of laboratory measurements. We now have access to optical constants and laboratory spectra of synthetic chemical compounds such as hydrated sulfates and chlorinated salts under Europa’s surface conditions.

In this study, we test for the first time the relevance of 15 potential endmembers on a Galileo/NIMS observation of a dark lineament of the Trailing Anti-Jovian hemisphere using a realistic radiative transfer modelling and a robust Bayesian inference framework. We consider an intimate mixture of 3, 4 and 5 endmembers and perform a fitting procedure on all possible combinations among the list of 15 compounds in the 1.0 – 2.4

m spectral range. At the end, 5000 combinations have been tested and analyzed. The parameters we are fitting for are the surface roughness, the volumetric abundances and the grain size. Given the fact that NIMS noise level and absolute calibration are uncertain, we have distinguished two scenarios: assuming a pessimistic signal-to-noise ratio (SNR) of 5 and an optimistic SNR of 50. For each scenario, we use several criteria to highlight the contribution of each endmember and thus identify the most relevant compounds.

We first show that no combinations using only 3 endmembers are able to accurately reproduce the observation. Then, when considering 4 and 5 endmembers together and assuming the SNR=50, there is 21 and 153 combinations that evenly reproduce the observation. From this result, we discuss the relevance of each endmember using criteria such as the Root-Mean-Square (RMS) deviation of the best-fits, the spectral contribution at each wavelength, the occurrences distribution of the selected best-fits and the numerical abundances retrieved by the fitting procedure. By grouping criteria together, we show that only sulfuric acid octahydrate and water ice appear as essential compounds. Then, hydrated sulfates are in general preferred over others compounds. This result is only valid for a lineament of the Trailing Anti-Jovian hemisphere and can be possibly extended to other lineaments in the same hemisphere. However, we show that chlorinated salts but also ammonium sulfate mascagnite ((NH4)2SO4), sodium chloride (NaCl) and even magnetite (Fe3O4) are not excluded, their contributions from the selected criteria are just less pronounced than hydrated sulfates. Nevertheless, it is clear that it is not possible to correctly distinguish between these endmembers, and even less between endmembers with similar chemical composition, at these wavelengths. We therefore propose for future investigation to either consider a mixture that would include only one representative of each chemical compound, rather than using all possible endmembers and overfitting the data.

Askival: An altered feldspathic cumulate sample in Gale crater

1Donald Lewis Bowden et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article []
1School of Physics and Astronomy, University of Leicester, LE1 7RH Leicester, UK
Published by arrangement with John Wiley & Sons

Askival is a light-toned, coarsely crystalline float rock, which was identified near the base of Vera Rubin Ridge in Gale crater. We have studied Askival, principally with the ChemCam instrument but also using APXS compositional data and MAHLI images. Askival and an earlier identified sample, Bindi, represent two rare examples of feldspathic cumulate float rocks in Gale crater with >65% relict plagioclase. Bindi appears unaltered whereas Askival shows textural and compositional signatures of silicification, along with alkali remobilization and hydration. Askival likely experienced multiple stages of alteration, occurring first through acidic hydrolysis of metal cations, followed by deposition of silica and possible phyllosilicates at low T and neutral-alkaline pH. Through laser-induced breakdown spectroscopy compositional analyses and normative calculations, we suggest that an assemblage of Fe-Mg silicates including amphibole and pyroxene, Fe phases, and possibly Mg-rich phyllosilicate are present. Thermodynamic modeling of the more pristine Bindi composition predicts that amphibole and feldspar are stable within an upper crustal setting. This is consistent with the presence of amphibole in the parent igneous rocks of Askival and suggests that the paucity of amphiboles in other known Martian samples reflects the lack of representative samples of the Martian crust rather than their absence on Mars.

Quantification of Impact-Induced Melt Production in Numerical Modeling Revisited

1,2Lukas Manske,1,2Kai Wünnemann,3Kosuke Kurosawa
Journal of Geophysical Research (Planets) (in Press) Open Access Link to Article []
1Museum für Naturkunde Berlin, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
2Department of Earth Sciences, Freie Universität Berlin, Berlin, Germany
3Planetary Exploration Research Center, Chiba Institute of Technology, Narashino, Japan
Published by arrangement with John Wiley & Sons

Melting and vaporization of rocks in impact cratering is mostly attributed to be a consequence of shock compression. However, other mechanism such as plastic work and decompression by structural uplift also contribute to melt production. In this study we expand the commonly used method to determine shock-induced melting in numerical models from the peak shock pressure by a new approach to account for additional heating due plastic work and internal friction. We compare our new approach with the straight-forward method to simply quantify melting from the temperature relative to the solidus temperature at any arbitrary point in time in the course of crater formation. This much simpler method does account for plastic work but suffers from reduced accuracy due to numerical diffusion inherent to ongoing advection in impact crater formation models. We demonstrate that our new approach is more accurate than previous methods in particular for quantitative determination of impact melt distribution in final crater structures. In addition, we assess the contribution of plastic work to the overall melt volume and find, that melting is dominated by plastic work for impacts at velocities smaller than 7.5–12.5 km/s in rocks, depending on the material strength. At higher impact velocities shock compression is the dominating mechanism for melting. Here, the conventional peak shock pressure method provides similar results compared with our new model. Our method serves as a powerful tool to accurately determine impact-induced heating in particular at relatively low-velocity impacts.

Vesicular Olivines and Pyroxenes in Shocked Kamargaon L6 Chondrite: Implications for Primary Volatiles and Its Multiple Impacts History

1Kishan Tiwari,1Sujoy Ghosh,2Masaaki Miyahara,3Dwijesh Ray
Journal of Geophysical Research (Planets) (in Press) Link to Article []
1Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, India
2Graduate School of Advanced Science and Engineering, Hiroshima University, Higashihiroshima, Japan
3Planetary Sciences Division, Physical Research Laboratory, Ahmedabad, India
Published by arrangement with John Wiley & Sons

Abundant vesicles are found in terrestrial rocks, basaltic meteorites, and carbonaceous chondrites which testify to the presence of significant quantities of volatiles and favorable conditions for vesiculation. Furthermore, vesicular olivine has been reported in terrestrial rocks and carbonaceous chondrites. However, vesicles in the ordinary chondrites are rare due to their low volatile content and obliteration by the late impact events. Here, we report the first evidence of vesicular olivine (Fo76) and pyroxene (En73–81Fs17–26Wo01–02) in an ordinary chondrite. The vesicular texture in the shocked Kamargaon L6 chondrite possibly formed due to localized melting during a shock event and subsequent degassing of volatiles after decompression. We propose three possible mechanisms for vesicle formation in the Kamargaon meteorite: (a) liberation of S2 vapor, (b) evaporation of moderately volatile elements (MVEs) like Na and Mn, and (c) vaporization of olivine and pyroxene, by constraining the primary abundance of volatiles based on the observed volume of vesicles. We suggest that all three or any combination of these mechanisms could be responsible for vesicle formation. Segmented texture in olivine is also observed in the shock vein (SV) of the Kamargaon chondrite. The segmentation has developed due to the formation of sub-grain boundaries during the recovery process when grains were subjected to localized shear stress. Average shock pressure and temperature conditions in the SV are ∼24–25 GPa and ∼2310–2633K, respectively. The thermal model of the SV cooling gives the crystallization time of ∼50 ms and shock pulse duration of ∼2s.

Insights into the petrogenetic history of the Northwest Africa 7635 augite-rich shergottite

1Christopher D. K. Herd
Meteoritics & Planetary Science (in Press) Link to Article []
1Department of Earth and Atmospheric Sciences, University of Alberta, 1–26 Earth Sciences Building, Edmonton, Alberta, T6G 2E3 Canada
Published by arrangement with John Wiley & Sons

The petrogenesis of the Northwest Africa (NWA) 7635 Martian meteorite involved the entrainment of xenocrystic olivine grains into a relatively magnesian and oxidized melt, followed by a redox-dependent reaction between olivine and melt that resulted in the crystallization of orthopyroxene and magnetite. Subsequent crystallization of the melt began with augite, plagioclase, and magnetite phenocrysts, and was followed by crystallization of augite, plagioclase, magnetite, ilmenite, and pyrrhotite in the groundmass, which took place under more rapid conditions of cooling, as reflected in the groundmass grain size. The petrogenetic history of NWA 7635 is similar in many ways to that of NWA 8159; this observation, coupled with similarities in geochemical and isotopic characteristics from other studies, suggests that the parent melts of the two rocks—as represented by all minerals except the xenocrystic olivine—were one and the same. The main distinctions between the two rocks are that their parent melts entrained xenocrystic olivine of different composition, and the cooling rate of the groundmass of NWA 7635 was more rapid than that of NWA 8159. The conclusion that the redox reaction took place between olivine and melt is in contrast to other work that suggests the reaction took place in the subsolidus, and has implications for the nature of the reaction in both NWA 7635 and NWA 8159.

Lithium and boron isotopic compositions of olivine in chondrules from carbonaceous and ordinary chondrite meteorites: implications for the origin of solar 11B/10B ratio

1,3Ming-Chang Liu,2Marc Chaussidon,1Nozomi Matsuda
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles, CA, USA
2Université de Paris Cité , Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, Paris 75005 France
3Lawrence Livermore National Laboratory, Livermore, CA, USA
Copyright Elsevier

The origin of solar 11B/10B ratio ∼4 remains an open question. It has been thought that a significant portion of boron in the Solar System was derived from continuous spallation nucleosynthesis during interactions between Galactic Cosmic Rays and C-N-O nuclei in the interstellar medium. However, because GCR-produced boron is characterized by 11B/10B ∼2.5, an endmember with 11B/10B > 4 is required to account for the solar 11B/10B ratio. Two leading hypotheses for the sources of 11B-rich components include low energy spallation in the Sun’s parental molecular cloud and the neutrino process during the supernova explosions. In this study, lithium and boron elemental and isotopic compositions of seven porphyritic olivine chondrules and one isolated olivine crystal from four meteorites (Allende, Yamato 81020, Asuka 12236, and QUE 97008) were determined in-situ to help constrain which of the two nucleosynthetic mechanisms has most likely supplied the forming Solar System with extra 11B. Apparent isotopic variations in Li/Si, B/Si, δ7Li and δ11B were found in chondrule olivine crystals, but only three chondrules exhibit statistically resolved δ11B heterogeneities. Using these three chondrules as a basis for discussion, we evaluated the processes that could potentially cause elemental and isotopic variations of Li and B. We argue that the data can be best understood in the context of condensation of lithium-boron-rich material onto chondrule precursors in (an) initially heterogeneous gaseous reservoir(s), which could be realized if the Solar System derived 11B-rich components from a supernova or supernovae.

Ar-Ar and U-Pb ages of Chelyabinsk and a re-evaluation of its impact chronology

1,2,3,4Sky P. Beard,2,3Timothy D. Swindle,5Thomas J. Lapen,2,3David A. Kring
Meteoritics & Planetary Science (in Press) Link to Article []
1State Key Laboratory in Lunar and Planetary Science, Macau University of Science and Technology, Avenida Wai Long, Macau, Taipa, 999078 P.R. China
2NASA Solar System Exploration Research Virtual Institute, Moffett Field, California, 94035 USA
3Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, 85721 USA
4CNSA Macau Center for Space Exploration and Science, Macau, P.R. China
5University of Houston, Houston, Texas, 77004 USA
Published by arrangement with John Wiley & Sons

The LL5 chondrite Chelyabinsk has had numerous isotopic studies since its fall in 2013. These data have been used to suggest ~8 impact events recorded from multiple isotopic systems (e.g., Ar-Ar, U–Pb, Sm-Nd, Rb-Sr, among others). We report details of Ar-Ar and U-Pb results and re-evaluate the geochronology of Chelyabinsk. Argon has the youngest Ar-Ar age recorded in meteorites, 25 ± 11 Ma, and an older resetting event at ~2550 Ma. The U-Pb analysis has an upper concordia age of 4456 ± 23 Ma and a lower concordia age of 184 ± 200 Ma. The lower concordia intercept represents a later thermal event (e.g., an impact), the most recent time that lead loss occurred, and could represent resetting by the youngest event recorded by Ar-Ar. Combining our data with literature results, we find strong evidence of at least four impact events (~4450, 2550, 1700, 25 Ma), with some evidence for two additional impacts (~3700, 1000 Ma).

Analysis of the daylight fireball of July 15, 2021, leading to a meteorite fall and find near Antonin, Poland, and a description of the recovered chondrite

1Lukáš Shrbený,2Agata M. Krzesińska,1Jiří Borovička,1Pavel Spurný,3Zbigniew Tymiński,4Kryspin Kmieciak
Meteoritics & Planetary Science (in Press) Open Access Link to Article []
1Astronomical Institute of the Czech Academy of Sciences, Ondřejov, 251 65 Czech Republic
2Centre for Earth Evolution and Dynamics, Department of Geosciences, University of Oslo, Oslo, N-0371 Norway
3National Centre for Nuclear Research, POLATOM Radioisotope Centre, Otwock, 05-400 Poland
4Meteorite Finder
Published by arrangement with John Wiley & Sons

We present the description of an observation of a fireball recorded during the sunrise on July 15, 2021. Atmospheric trajectory, impact area, and heliocentric orbit were determined on the basis of three instrumental video records. The terminal part of the fireball was not instrumentally recorded due to clouds. Based on our computations, one meteorite was found in the predicted impact area by Polish searchers. The specimen was, soon after recovery, analyzed for the presence of short-lived radionuclides and the measurement confirms a very fresh fall, coinciding with the time of the fireball event. The recovered meteorite, Antonin, is an unbrecciated L5 chondrite with shock stage S3, weathering grade W0, and bulk density of 3.42 g cm−3. Unusual for L chondrites, it contains assemblages composed of metal and two sulfides, troilite and mackinawite. We interpret these assemblages to have been formed as products of shock metamorphism and post-shock annealing on the parent body. This suggests that the thermal and collisional history of the Antonin parent body was complex.

Birth and decline of magma oceans in planetesimals. Part 2: Structure and thermal history of early accreted small planetary bodies

1Cyril Sturtz,1Angela Limare,1Stephen Tait,1Édouard Kaminski
Journal of Geophysical Research (Planets) (in Press) Link to Article []
1Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, F-75005 France
Published by arrangement with John Wiley & Sons

This is the second of two companion papers that present a theoretical and experimental study of the thermal history of planetesimals in which heating by short-lived radioactive isotopes generates an internal magma ocean and the subsequent cooling and crystallization thereof. We study the conditions required to form and preserve basal cumulates and flotation crusts, and the implications for the thermal evolution of planetary bodies. Our model predicts that planetesimals larger than 30km can reach 1300oC and a melt fraction of 40 vol%, producing a solid-like to liquid-like rheological transition that triggers an internal magma ocean. In the magma ocean regime core-mantle differentiation occurs very quickly and the mantle convects under a relic of chondritic material whose thickness is controlled by the temperature of rheological transition. We show that the magma ocean episode is associated with time-dependent crystal segregation and no re-entrainment. Segregation of crystals is essentially constrained by their size and by their density difference with respect to the melt, the latter being fully determined by the planetesimal’s initial composition. Olivine cumulates are likely to form at the core-mantle boundary. Under certain particular conditions, a flotation crust can also form, which reduces the efficiency of heat evacuation by convection, thereby enhancing the magma ocean’s lifetime and the efficiency of crystal segregation. Two types of large-scale mantle structure are possible outcomes: a well-mixed upper mantle above an olivine cumulate, or a more finely layered ”onion-shell” structure.

Hydrogen solubility in FeSi alloy phases at high pressures and temperatures

1Suyu Fu,2Stella Chariton,2Vitali B. Prakapenka,3Andrew Chizmeshya,1Sang-Heon Shim
American Mineralogist 107, 2307-2314 Link to Article []
1School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
2Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.
Copyright: The Mineralogical Society of America

Light elements alloying with metallic Fe can change the properties and therefore play a key role
in the structure and dynamics of planetary cores. Hydrogen and silicon are possible light elements in
the rocky planets’ cores. However, hydrogen storage in Fe-Si alloy systems remains unclear at high
pressures and high temperatures because of experimental difficulties. Taking advantage of pulsed laser
heating combined with high-energy synchrotron X‑ray diffraction, we studied reactions between FeSi
and H in laser-heated diamond-anvil cells (LHDACs) up to 61.9 GPa and 3500 K. We found that under
H-saturated conditions the amount of H alloying with FeSi (0.3 and <0.1 wt% for the B20 and B2 structures, respectively) is much smaller than that in pure Fe metal (>1.8 wt%). Our experiments also
suggest that H remains in the crystal structure of FeSi alloy when recovered to 1 bar. Further density
functional theory (DFT) calculations indicate that the low-H solubility likely results from the highly
distorted interstitial sites in the B20 and B2 structures, which are not favorable for H incorporation.
The recovery of H in the B20 FeSi crystal structure at ambient conditions could open up possibilities
to understand geochemical behaviors of H during core formation in future experiments. The low-H
content in FeSi alloys suggests that if a planetary core is Si-rich, Si can limit the ingassing of H into
the Fe-rich core.