Bidirectional reflectance spectroscopy 8. The angular width of the opposition effect in regolith-like media

1Bruce Hapke
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114105]
1Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, PA, USA
Copyright Elsevier

The opposition effect is the sharp, narrow surge observed in the reflectance of a scattering medium near zero phase angle. Numerous observations and experiments have shown that the primary cause of the phenomenon in particulate media is coherent backscattering, in which wavelets traveling in opposite directions along chains of scatterers interfere constructively and generate the peak. A broader opposition surge caused by shadow hiding and preferential escape is also present, but is entangled with the incoherent continuum reflectance on which the coherent peak is superposed, making it difficult to identify and isolate. Theoretical models of media of independent scatterers predict that the angular width and shape of the coherent backscatter peak depend on the wavelength, porosity and particle size. It was hoped that remote measurements of the opposition effect would give information on the latter two quantities in planetary regoliths. However, observations and laboratory studies of media of large particles in contact with one another find little dependence on any of these quantities. Instead, these studies imply that the opposition effect in regolith-like media comes from reflection by short chains only a few scatterers long located on the surfaces of the particles of the medium, and that the lengths of these chains are proportional to the wavelength. Since the angular width of the peak is controlled by the ratio of the wavelength to the mean scattering chain length, the width is independent of wavelength. Because the wavelets never enter a particle, low albedo media can exhibit a strong coherent backscatter peak. Opposition effect peaks less than a degree wide on solar system bodies can imply an immature regolith; peaks several degrees wide imply a mature regolith.

Detecting Ce3+ as a biosignature mimicker using UV time-resolved laser-induced fluorescence and Raman spectroscopy: Implications for planetary missions

1,2,3S.Shkolyar,4E.Lalla,4,5M.Konstantindis,6K.Cote,4M.G.Daly,7A.Steele
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114093]
1Universities Space Research Association, Columbia, MD 21046, USA
2NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
3Blue Marble Space Institute of Science, Seattle, WA 98154, USA
4Centre for Research in Earth and Space Science, York University, Toronto M3J 1P3, ON, Canada
5Department of Mathematics and Statistics, York University, Toronto M3J 1P3, ON, Canada
6Department of Physics, University of Toronto, Toronto M5S 1A7, ON, Canada
7Earth and Planets Laboratory, Carnegie Institution of Washington, Washington, D.C 20015, USA
Copyright Elsevier

Combined UV Raman and laser-induced fluorescence (LIF) spectroscopy instruments will soon be launched onboard missions to planetary surfaces, including Mars, to search for biosignatures. However, the rare earth element Ce3+, found in many common and Mars-relevant minerals, can produce fluorescence features within the spectral window usually attributed to organic compounds in a LIF spectrum. This study explored the detection of Ce3+ as a biosignature mimicker using UV Raman-LIF mission instruments. We assessed how LIF spectra of a suite of synthetic CePO4 samples compare with those of organics, how varying concentrations of both Ce3+ and organics in Martian regolith simulant affect this comparison, and whether two additional data sets obtainable by combined UV Raman-LIF instruments, including time-resolved fluorescence decay lifetimes and Raman spectra, can distinguish Ce3+-containing samples from organics. Results showed that the dominant LIF features of Ce3+ (320 and 338 nm) are similar to those of the aromatic amino acid tryptophan (325 and 340 nm), even when Ce3+ samples were mixed in a Martian regolith simulant at a range of concentrations. Lifetimes were revealed to be 2–9 ns in Ce3+-containing samples, typical for organic fluorophores. These results support the erroneous interpretation that LIF spectra and lifetime values obtained on these samples constitute potential organic signatures. Raman spectroscopy results suggested that with UV laser excitation, Raman is unlikely to identify Ce-bearing samples due to strong absorption of Raman scattered energy by Ce3+. We conclude that biosignature searches using UV LIF and Raman spectroscopy instrumentation may encounter challenges with unambiguously distinguishing spectra of organic compounds from Ce-bearing compounds.

Shock compaction heating and collisional processes in the production of type 3 ordinary chondrites: Lessons from the (nearly) unique L3 chondrite melt breccia Northwest Africa 8709

1Alex M. Ruzicka,2,3Jon M. Friedrich,1Melinda L. Hutson,2Juliette W. Strasser,4Robert J. Macke,5Mark L. Rivers,6Richard C. Greenwood,7Karen Ziegler,1Richard N. Pugh
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13567]
1Department of Geology and Cascadia Meteorite Laboratory, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, Oregon, 97201 USA
2Department of Chemistry, Fordham University, 441 East Fordham Road, Bronx, New York, 10458 USA
3Department of Earth and Planetary Sciences, American Museum of Natural History, 79th Street at Central Park West, New York City, New York, 10024 USA
4Vatican Observatory, Vatican City, V‐00120 Italy
5Center for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, 60439 USA
6Planetary Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
7Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 8709 is a rare example of a type 3 ordinary chondrite melt breccia and provides critical information for the shock compaction histories of chondrites. An L3 protolith for NWA 8709 is inferred on the basis of oxygen isotope composition, elemental composition, diverse mineral chemistry, and overall texture. NWA 8709 is among the most strongly shocked type 3 chondrites known, and experienced complete melting of the matrix and partial melting of chondrules. Unmelted phases underwent FeO reduction and partial homogenization, with reduction possibly occurring by reaction of olivine and low‐Ca pyroxene with an S‐bearing gas that was produced by vaporization. Chondrules and metal grains became foliated by uniaxial compaction, and during compression, chondrules and fragments became attached to form larger clumps. This process, and possibly also melt incorporation into chondrules to cause “inflation,” may have contributed to anomalously large chondrule sizes in NWA 8709. The melt breccia character is attributed to strong shock affecting a porous precursor. Data‐model comparisons suggest that a precursor with 23% porosity that was impacted by a 3 km/s projectile could have produced the meteorite. The rarity of other type 3 ordinary chondrite melt breccias implies that the immediate precursors to such chondrites were lower in porosity than the NWA 8709 precursor, or experienced weaker shocks. Altogether, the data imply a predominantly “quiet” dynamical environment to form most type 3 ordinary chondrites, with compaction occurring in a series of relatively weak shock events.

Chondrules reveal large-scale outward transport of inner Solar System materials in the protoplanetary disk

1Curtis D. Williams,1Matthew E. Sanborn,2Céline Defouilloy,1Qing-Zhu Yin,2Noriko T. Kita,3Denton S. Ebel,1Akane Yamakawa,4Katsuyuki Yamashita
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [DOI:
https://doi.org/10.1073/pnas.2005235117]
1Department of Earth and Planetary Sciences, University of California, Davis, CA 95616;
2WiscSIMS, Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706;
3Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024;
4Graduate School of Natural Science and Technology, Okayama University, Kita-ku, 700-8530 Okayama, Japan

Dynamic models of the protoplanetary disk indicate there should be large-scale material transport in and out of the inner Solar System, but direct evidence for such transport is scarce. Here we show that the ε50Ti-ε54Cr-Δ17O systematics of large individual chondrules, which typically formed 2 to 3 My after the formation of the first solids in the Solar System, indicate certain meteorites (CV and CK chondrites) that formed in the outer Solar System accreted an assortment of both inner and outer Solar System materials, as well as material previously unidentified through the analysis of bulk meteorites. Mixing with primordial refractory components reveals a “missing reservoir” that bridges the gap between inner and outer Solar System materials. We also observe chondrules with positive ε50Ti and ε54Cr plot with a constant offset below the primitive chondrule mineral line (PCM), indicating that they are on the slope ∼1.0 in the oxygen three-isotope diagram. In contrast, chondrules with negative ε50Ti and ε54Cr increasingly deviate above from PCM line with increasing δ18O, suggesting that they are on a mixing trend with an ordinary chondrite-like isotope reservoir. Furthermore, the Δ17O-Mg# systematics of these chondrules indicate they formed in environments characterized by distinct abundances of dust and H2O ice. We posit that large-scale outward transport of nominally inner Solar System materials most likely occurred along the midplane associated with a viscously evolving disk and that CV and CK chondrules formed in local regions of enhanced gas pressure and dust density created by the formation of Jupiter.

The Cl isotope composition and halogen contents of Apollo-return samples

1,2Anthony Gargano,1,2Zachary Sharp,3Charles Shearer,4Justin I. Simon,5Alex Halliday,6Wayne Buckley
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [DOI:https://doi.org/10.1073/pnas.2014503117]
1Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-0001;
2Center for Stable Isotopes, University of New Mexico, Albuquerque, NM 87131-0001;
3Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131-0001;
4Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science Division, The Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, Houston, TX 77058;
5The Earth Institute, Columbia University, New York, NY 10025;
6Jacob–Johnson Space Center Engineering, Technology and Science Contract, The Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, Houston, TX 77058

Lunar mare basalts are depleted in F and Cl by approximately an order of magnitude relative to mid-ocean ridge basalts and contain two Cl-bearing components with elevated isotopic compositions relative to the bulk-Earth value of ∼0‰. The first is a water-soluble chloride constituting 65 ± 10% of total Cl with δ37Cl values averaging 3.0 ± 4.3‰. The second is structurally bound chloride with δ37Cl values averaging 7.3 ± 3.5‰. These high and distinctly different isotopic values are inconsistent with equilibrium fractionation processes and instead suggest early and extensive degassing of an isotopically light vapor. No relationship is observed between F/Cl ratios and δ37Cl values, which suggests that lunar halogen depletion largely resulted from the Moon-forming Giant Impact. The δ37Cl values of apatite are generally higher than the structurally bound Cl, and ubiquitously higher than the calculated bulk δ37Cl values of 4.1 ± 4.0‰. The apatite grains are not representative of the bulk rock, and instead record localized degassing during the final stages of lunar magma ocean (LMO) or later melt crystallization. The large variability in the δ37Cl values of apatite within individual thin sections further supports this conclusion. While urKREEP (primeval KREEP [potassium/rare-earth elements/phosphorus]) has been proposed to be the source of the Moon’s high Cl isotope values, the ferroan anorthosites (FANs) have the highest δ37Cl values and have a positive correlation with Cl content, and yet do not contain apatite, nor evidence of a KREEP component. The high δ37Cl values in this lithology are explained by the incorporation of a >30‰ HCl vapor from a highly evolved LMO.

Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites

1,2Frédéric Moynier,3,1Jiubin Chen,3Ke Zhang,3Hongming Cai,1Zaicong Wang,4Matthew G.Jackson,5James M.D.Day
Earth and Planetary Science Letters 551, 116544 Link to Article [https://doi.org/10.1016/j.epsl.2020.116544]
1State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
2Institut de Physique du Globe de Paris, Université de Paris, CNRS, 1 rue Jussieu, Paris 75005, France
3Institute of Surface-Earth System Science, Tianjin University, China
4Department of Earth Science, University of California, Santa Barbara, CA 93106, USA
5Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA
Copyright Elsevier

Variations in the abundances of moderately volatile elements (MVE) are one of the most fundamental geochemical differences between the terrestrial planets. Whether these variations are the consequence of nebular processes, planetary volatilization, differentiation or late accretion is still unresolved. The element mercury is the most volatile of the MVE and is a strongly chalcophile element. It is one of the few elements that exhibit large mass-dependent (MDF) and mass-independent (MIF) isotopic fractionations for both odd (odd-MIF, Hg and Hg) and even (even-MIF, Hg) Hg isotopes in nature, which is traditionally used to trace Hg biogeochemical cycling in surface environments. However, the Hg isotopic composition of Earth and meteorites is not well constrained. Here, we present Hg isotopic data for terrestrial basaltic, trachytic and granitic igneous samples. These rocks are isotopically lighter (delta202Hg = −3.3 ± 0.9‰; 1 standard deviation) than sedimentary rocks that have previously been considered to represent the terrestrial Hg isotope composition (delta202Hg=-0.7±0.5‰
; 1 standard deviation). We show degassing during magma emplacement induces MIF that are consistent with kinetic fractionation in these samples. Also presented is a more complete dataset for chondritic (carbonaceous, ordinary and enstatite) meteorites, which are consistent with previous work for carbonaceous chondrites (positive odd-MIF) and ordinary chondrites (no MIF), and demonstrate that some enstatite chondrites exhibit positive odd-MIF, similar to carbonaceous chondrites. The terrestrial igneous rocks fall within the range of chondritic compositions for both MIF and MDF. Given the fact that planetary differentiation (core formation, evaporation) would contribute to Hg loss from the silicate portion of Earth and would likely fractionate Hg isotopes from chondritic compositions, we suggest that the budget of the mantle Hg is dominated by late accretion of chondritic materials to Earth, as also suggested for other volatile chalcophile elements (S, Se, Te). Considering the Hg isotopic signatures, materials with compositions similar to CO chondrites or ordinary chondrites are the most likely late accretion source candidates. Finally, eucrite meteorites, which are highly depleted in volatile elements, are isotopically heavier than chondrites and exhibit negative odd-MIF. The origin of volatile depletion in eucrites has been vigorously debated. We show that Hg versus Hg relationships point toward an equilibrium nuclear field shift effect, suggesting that volatile loss occurred during a magma ocean phase at the surface of the eucrite parent body, likely the asteroid 4-Vesta.

On the petrogenesis of lunar troctolites: New insights into cumulate mantle overturn & mantle exposures in impact basins

1,2,3Tabb C.Prissel,1,3,4JulianeGross
Earth and Planetary Science Letters 551, 116531 Link to Article [https://doi.org/10.1016/j.epsl.2020.116531]
1Lunar & Planetary Institute, Universities Space Research Association, 3600 Bay Area Blvd., Houston, TX 77058, United States
2Astromaterials Research & Exploration Science Division, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, United States
3Department of Earth & Planetary Sciences Rutgers, the State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854, United States
4Department of Earth & Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, United States
Copyright Elsevier

We investigate lunar troctolite petrogenesis with a series of forward models. We simulate the cumulate mantle overturn hypothesis by modeling the adiabatic ascent and decompression melting of primary mantle cumulates produced during differentiation of a lunar magma ocean (LMO). Combined equilibrium and fractional crystallization of candidate liquids generated by the melting model can reproduce the predominant constituents of the lunar magnesian-suite (Mg-suite: troctolites and norites), contrary to previous hypotheses. Model results are consistent with previous studies challenging the proposed and long-standing genetic relationship between Mg-suite and gabbronorites.
Our Mg-suite petrogenetic model validates a direct temporal and chemical link between Mg-suite melt production and pressure-release melting of primary LMO cumulates. If so, Mg-suite crystallization ages (4345 ± 15 Ma) can be used to constrain the onset and duration of melting associated with mantle overturn. Based on our model results, we propose an alternative mantle overturn hypothesis whereby upwelling olivine-dominated cumulates experience decompression melting to produce the Mg-suite primary melt (∼1.9% melt at ∼2.1 GPa), but that this melt was extracted from depth akin to lunar picritic glass magmas (low-degree partial melts at depths corresponding to ∼1.3–2.5 GPa). Thus, our revised mantle overturn hypothesis reconciles Mg-suite petrogenesis without the expanse of an olivine-dominated upper mantle (as suggested by the current paradigms, but contradicted by orbital data). This hypothesis supports the presence of a low-Ca pyroxene dominated upper mantle, consistent with mantle stratigraphy constrained by experimental and numerical simulations of LMO differentiation and proposed mantle exposures within impact basins.

Cosmic ray effects on the isotope composition of hydrogen and noble gases in lunar samples: Insights from Apollo 12018

1Evelyn Füri,1Laurent Zimmermann,1Etienne Deloule,2Reto Trappitsch
Earth and Planetary Science Letters 550, 116550 Link to Article [https://doi.org/10.1016/j.epsl.2020.116550]
1Centre de Recherches Pétrographiques et Géochimiques, Université de Lorraine, CNRS, F-54000 Nancy, France
2Lawrence Livermore National Laboratory, Nuclear and Chemical Sciences Division, 7000 East Ave, L-231, Livermore, CA 94550, USA
Copyright Elsevier

Exposure of rocks and regolith to solar (SCR) and galactic cosmic rays (GCR) at the Moon’s surface results in the production of ‘cosmogenic’ deuterium and noble gas nuclides at a rate that depends on a complex set of parameters, such as the energy spectrum and intensity of the cosmic ray flux, the chemical composition, size, and shape of the target as well as the shielding depth. As the effects of cosmic rays on the D production in lunar samples remain poorly understood, we determine here the D content and noble gas (He-Ne-Ar) characteristics of nominally anhydrous mineral (olivine and pyroxene) grains and rock fragments, respectively, from different documented depths (0 to ≥4.8 cm) within Apollo olivine basalt 12018. Deuterium concentrations, determined by secondary ion mass spectrometry, and cosmogenic 3He, 21Ne, and 38Ar abundances, measured by CO2 laser extraction static mass spectrometry, are constant over the depth range investigated. Neon isotope ratios (20Ne/22Ne ≈0.86 and 21Ne/22Ne ≈0.85) of the cosmogenic endmember are comparable to the theoretical signature of GCR-produced neon. These observations indicate that the presence of significant amounts of SCR nuclides in the studied sub-samples can be ruled out. Hence, D within the olivines and pyroxenes must have been predominantly produced in situ by GCR-induced spallation reactions during exposure at the lunar surface. Comparison of the amount of D with the 21Ne (184 ± 26 Ma) or 38Ar (193 ± 25 Ma) exposure ages yields a D production rate that is in good agreement with the value of mol(g rock)−1Ma−1 from Füri et al. (2017). These results confirm that cosmic ray effects can substantially alter the hydrogen isotope (D/H) ratio of indigenous ‘water’ in returned extraterrestrial samples and meteorites with long exposure ages.

Sulfide‐dominated partial melting pathways in brachinites

1Samuel D. Crossley,1Richard D. Ash,1,2Jessica M. Sunshine,3Catherine M. Corrigan,3Timothy J. McCoy,4David W. Mittlefehldt,1Igor S. Puchtel
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13558]
1Department of Geology, University of Maryland, College Park, Maryland, 20742 USA
2Department of Astronomy, University of Maryland, College Park, Maryland, 20742 USA
3Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, 20560‐0119 USA
4Mail Code SR, NASA/Johnson Space Center, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

Petrogenetic relationships among members of the brachinite family were established by analyzing major and trace element concentrations of minerals for 9 representative specimens: Al Huwaysah 010, Eagles Nest, Northwest Africa (NWA) 4882, NWA 5363, NWA 7297, NWA 7299, NWA 11756, Ramlat as Sahmah (RaS) 309, and Reid 013. The brachinite family, which includes brachinites and ungrouped achondrites with compositional and isotopic similarities to brachinites, comprises FeO‐rich, olivine‐dominated achondrites whose compositional and mineralogic variability is correlated with oxidation state. Most classical brachinites are derived from precursors that were more oxidized and sulfur‐rich than those of ungrouped “brachinite‐like” achondrites. This is manifest in the distinct Fe‐Ni‐S systems among brachinite family precursors, which were sulfide‐dominated for the most oxidized brachinites and metal‐dominated for the least oxidized brachinite‐like achondrites. Consequently, highly siderophile element behavior was controlled through melting and removal of their dominant host phase in the precursor, which was likely pentlandite in sulfide‐dominated systems and kamacite/taenite in metal‐dominated systems. Anomalous Ir/Os and Pt/Os ratios of oxidized brachinites may be attributed to selective complexing during melting of As‐rich pentlandite, consistent with our observations of impact‐melted sulfides in R chondrite NWA 11304, although further experimental work is needed to model this process. The apparent redox trend among the brachinite family is consistent with silicate FeO content and Fe/Mn ratios, which may be used as a proxy for determining the relative oxidation state of brachinite family members. Based on our analyses, we make several recommendations for reclassification of samples into a continuum of oxidized to reduced endmembers for the brachinite family. Along with a common range of Δ17O, this evidence is consistent with either formation on a common heterogeneous parent body, or at least from the same nebular reservoir, with variable O and S fugacities, resulting in mineralogically distinct igneous products for oxidized and reduced endmembers. Sulfur‐bearing, oxidized differentiation may extend to other bodies that formed at or beyond the snow line in the early solar system, and should be considered when interpreting observational data for asteroids in upcoming missions.

The surprising thermal properties of CM carbonaceous chondrites

1C. P. Opeil,2,3D. T. Britt,4R. J. Macke,4G. J. Consolmagno
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13556]
1Department of Physics, Boston College, 140 Commonwealth Ave., Chestnut Hill, Massachusetts, 02467 USA
2Department of Physics, University of Central Florida, 4111 Libra Dr., Orlando, Florida, 32816 USA
3The Center of Lunar and Asteroid Surface Science, 12354 Research Pkwy Suite 214, Orlando, Florida, 32826 USA
4Vatican Observatory, V‐00120 Vatican City State
Published by arrangement with John Wiley & Sons

Measurements of the low‐temperature thermodynamic and physical properties of meteorites provide fundamental data for the study and understanding of asteroids and other small bodies. Of particular interest are the CM carbonaceous chondrites, which represent a class of primitive meteorites that record substantial chemical information concerning the evolution of volatile‐rich materials in the early solar system. Most CM chondrites are petrographic type 2 and contain anhydrous minerals such as olivine and pyroxene, along with abundant hydrous phyllosilicates contained in the meteorite matrix interspersed between the chondrules. Using a Quantum Design Physical Property Measurement System, we have measured the thermal conductivity, heat capacity, and thermal expansion of five CM2 carbonaceous chondrites (Murchison, Murray, Cold Bokkeveld, Northwest Africa 7309, Jbilet Winselwan) at low temperatures (5–300 K) which span the range of possible surface temperatures in the asteroid belt and outer solar system. The thermal expansion measurements show a substantial and unexpected decrease in CM2 volume as temperature increases from 210 to 240 K followed by a rapid increase in CM2 volume as temperature rises from 240 to 300 K. This transition has not been seen in anhydrous CV or CO carbonaceous chondrites. Thermal diffusivity and thermal inertia as a function of temperature are calculated from measurements of density, thermal conductivity, and heat capacity. Our thermal diffusivity results compare well with previous estimates for similar meteorites, where conductivity was derived from diffusivity measurements and modeled heat capacities; our new values are of higher precision and cover a wider range of temperatures.