¹Gordon Peter R., ¹Sephton Mark A.
Astrobiology 18, 5 Link to Article [https://doi.org/10.1089/ast.2017.1744]
¹Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, UK

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¹Jennifer L. Eigenbrode et al. (>10)
Science 360, 1096-1101 Link to Article [DOI: 10.1126/science.aas9185]
¹1Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
Reprinted with permission from AAAS

Establishing the presence and state of organic matter, including its possible biosignatures, in martian materials has been an elusive quest, despite limited reports of the existence of organic matter on Mars. We report the in situ detection of organic matter preserved in lacustrine mudstones at the base of the ~3.5-billion-year-old Murray formation at Pahrump Hills, Gale crater, by the Sample Analysis at Mars instrument suite onboard the Curiosity rover. Diverse pyrolysis products, including thiophenic, aromatic, and aliphatic compounds released at high temperatures (500° to 820°C), were directly detected by evolved gas analysis. Thiophenes were also observed by gas chromatography–mass spectrometry. Their presence suggests that sulfurization aided organic matter preservation. At least 50 nanomoles of organic carbon persists, probably as macromolecules containing 5% carbon as organic sulfur molecules.

¹Edward A.Cloutis, ²Vishnu Reddy, ³David T.Blewett
Icarus 311, 384-393 Link to Article [https://doi.org/10.1016/j.icarus.2018.04.027]
¹Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB R3B 2E9, Canada
²Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
³Planetary Exploration Group, Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA
Copyright Elsevier

We have measured reflectance spectra (0.35–25.0 µm) of different size powders of the ungrouped achondrite NWA 7325 in order to facilitate spectroscopic identification of its parent body. Previous work has suggested that the meteorite may have come from the planet Mercury based on its oxidation state. The 0.35–2.5 µm reflectance spectra of NWA 7325 exhibit absorption bands that can be attributed to the presence of chromium-bearing diopside and possibly to Ca-rich plagioclase. Spectral evidence for olivine is generally lacking, likely due to interference from stronger diopside absorption bands. With increasing grain size, albedo decreases while absorption band depths increase. The absorption bands are unique in the sense that they allow for the identification of the Cr-rich diopside in NWA 7325. The mid-infrared spectra are similar to those measured by previous investigators, and enable detection of the major silicates in NWA 7325, including more robust identification of olivine and plagioclase feldspar. We find no spectroscopic or compositional evidence supporting a link to Mercury as a possible parent body, even accounting for plausible spectrum-altering processes. In terms of a link to an asteroidal parent body, the most confident link would be made based on the unique Cr-diopside-related absorption bands in the 0.65, 1.05, and 2.3 µm regions. At present, the closest spectral match we have found is with asteroid 10,537 (1991 RY16).

¹Yves Marrocchi, ¹Johan Villeneuve, ²Valentina Batanova, ¹Laurette Piani, ³Emmanuel Jacquet
Earth and Planetary Science Letters 496, 132-141 Link to Article [https://doi.org/10.1016/j.epsl.2018.05.042]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
²Université Grenoble Alpes, ISTerre, CNRS, UMR 5275, Grenoble, F-38000, France
³IMPMC, CNRS & Muséum national d’Histoire naturelle, UMR 7590, CP52, 57 rue Cuvier, 75005 Paris, France
Copyright Elsevier

FeO-poor (type I) porphyritic chondrules formed by incomplete melting of solid dust precursors via a yet-elusive mechanism. Two settings are generally considered for their formation: (i) a nebular setting where primordial solids were melted, e.g. by shock waves propagating through the gas and (ii) a collisional planetary setting. Here we report a method combining high-current electron microprobe X-ray mapping and quantitative measurements to determine the chemical characteristics of relict olivine grains inherited from chondrule precursors. We find that these olivine crystals are Ca–Al–Ti-poor relative to host olivine crystals. Their variable Δ17Δ17O, even in individual chondrule, is inconsistent with derivation from planetary interiors as previously argued from 120 ° triple junctions also exhibited by the chondrules studied herein. This indicates that chondrule precursors correspond to solid nebular condensates formed under changing physical conditions.
We propose that porphyritic chondrules formed during gas-assisted melting of nebular condensates comprising relict olivine grains with varying Δ17Δ17O values and Ca–Al–Ti-rich minerals such as those observed within amoeboid olivine aggregates. Incomplete melting of chondrule precursors produced Ca–Al–Ti-rich melts (CAT-melts), allowing subsequent crystallization of Ca–Al–Ti-rich host olivine crystals via epitaxial growth on relict olivine grains. Incoming MgO and SiO from the gas phase induced (i) the dilution of CAT-melts, as attested by the positive Al–Ti correlation observed in chondrule olivine crystals, and (ii) buffering of the O-isotope compositions of chondrules, as recorded by the constant Δ17Δ17O values of host olivine grains. The O-isotopic compositions of host olivine grains are chondrule-specific, suggesting that chondrules formed in an array of environments of the protoplanetary disk with different Δ17Δ17O values, possibly due to variable solid/gas mixing ratios.

¹Gioacchino Tempesta, ²Giorgio S.Senesi, ³Paola Manzari, ¹Giovanna Agrosì
Spectrochimica Acta Part B: Atomic Spectroscopy 144, 75-81 Link to Article [https://doi.org/10.1016/j.sab.2018.03.014]
¹Dipartimento di Scienze della Terra e Geoambientali (DiSTeGeo), University of Bari, Via E. Orabona 4, 70125 Bari, Italy
²CNR – Istituto di Nanotecnologia (NANOTEC), PLasMI Lab, Via Amendola 122/D, 70126 Bari, Italy
³Istituto Nazionale di Astrofisica, Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS), via Fosso del Cavaliere 100, Roma, Italy

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¹Manuel Raith,²Matthias Ebert,¹Katja Pinkert,¹Christian U. Grosse
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13113]
¹Chair of Non‐destructive Testing, Technical University of Munich, Munich, Germany
²Institute of Earth and Environmental Sciences, Geology, University of Freiburg, Freiburg, Germany
Published by arrangement with John Wiley & Sons

Since the 1960s, hypervelocity impact experiments have been conducted to study the complex deformation mechanisms which occur in the subsurface of meteorite craters. Here, we present ultrasound tomography measurements of the damage zone underneath seven experimentally produced impact craters in sandstone cubes. Within the framework of the Multidisciplinary Experimental and Modeling Impact Research Network and the NEOShield Project, decimeter‐sized sandstone targets were impacted by aluminum and steel projectiles with radii of 2.5, 4, and 5 mm at velocities between ~3.0 and ~7.4 km s−1. The 2‐D ultrasound tomography clearly shows a correlation between impact energy and the damaged volume within the target blocks. When increasing impact energies from 805 to 2402 J, a corresponding increase in the damage radius from ~13.1 cm to ~17.6 cm was calculated. p‐Wave velocity reductions up to 18.3% (for the highest impact energy) were observed in the vicinity of the craters. The reduction in seismic velocity decreased uniformly and linearly with increasing distance from the impact point. The damage intensities correspond to peak damage parameters of 0.4–0.51 compared to undamaged target blocks. In addition to the damage zone below the crater, we could identify weakened zones at the sandstone walls which represent precursors of spalling. The volume of the damaged subsurface beneath experimentally produced craters determined through ultrasound tomography is larger than that obtained from previously reported p‐wave velocity reductions or to microscopic and microcomputed tomography observations of crack densities in experimentally produced craters.

^1,2Atsushi Takenouchi,^1,3Takashi Mikouchi,^4,5Akira Yamaguchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13120]
¹Department of Earth and Planetary Science, Graduate School of Arts and Sciences, The University of Tokyo, Bunkyo‐ku, Tokyo, Japan
²Department of Basic Sciences, Graduate School of Science, The University of Tokyo, Meguro‐ku, Tokyo, Japan
³The University Museum, The University of Tokyo, Bunkyo‐ku, Tokyo, Japan
⁴National Institute of Polar Research, Tachikawa‐shi, Tokyo, Japan
⁵Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies)Tachikawa‐shi, Tokyo, Japan
Published by arrangement with John Wiley & Sons

Shergottite Martian meteorites are known to contain brown‐colored olivine (brown olivine), which is considered to form during a shock event on Mars. In order to constrain the formation conditions of brown olivine, four shergottites with brown olivine and four shergottites without brown olivine are analyzed in this study. Based on our observations, brown olivine is often accompanied by thin (<10 μm) melt veins indicating local temperature increase (1750–1870 K). Even in shergottites without brown olivine, olivine around shock melt veins/pockets is partly darkened and shows similar features to those of brown olivine. These observations support that brown olivine is formed under conditions similar to those around shock melt veins/pockets. Components of shock melt veins (vesicles, quench crystals, etc.) and the absence of high‐pressure phases in shergottites with brown olivine indicate that they have high postshock temperature (>1200–1170 K). Such high postshock temperature may indicate that shergottites with brown olivine experienced high pressure (around 55 GPa), while shergottites without brown olivine experienced lower shock pressure (>20–35 GPa). Therefore, brown olivine may be a good indicator for strong shock events (peak shock pressure: ~55 GPa; postshock temperature: >1200–1170 K) and such shock events could be induced by small but rapid projectiles onto Mars.

¹E. A. MacLagan,^1,2E. L. Walton,¹C. D. K. Herd,³M. Dence
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13122]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
²Department of Physical Sciences, MacEwan UniversityEdmonton, Alberta, Canada
³2602 – 38 Metropole Pvt.Ottawa, ON, Canada
Published by arrangement with John Wiley & Sons

The Steen River impact structure (SRIS) formed in mixed target rocks, with Devonian carbonates, shales, and evaporites overlying granitic basement rocks of the Canadian Shield. A detailed study of impact melt phases within a continuous sequence of polymict impact breccia, as intersected by drill core, evaluated the relationship of impact melt to the breccia, identified the target rocks that contributed to the melt, and calculated the amount of melt within the breccia. Impact melt in the SRIS breccia occurs in three main forms (1) as individual matrix‐supported clasts, (2) as rims enveloping granitic clasts, and (3) as layers of agglomerated melt. Major and minor element abundances of large impact melt clasts (>1 mm) are similar to granitic basement, aside from elevated CaO and K2O wt% oxides in these melt clasts from incorporation of carbonates and calcareous shales. In contrast, submillimeter‐sized impact melt clasts have a composition derived almost exclusively from melting of shales. The small size of the shale‐derived melt clasts is attributed to increased fragmentation and a wider dispersion due to the volatile‐rich nature of the shale protolith. The wide compositional range of impact‐melted target lithologies documented at the SRIS contradicts breccia clast formation by impact melts that merged into larger bodies but were subsequently disrupted. Our observations are consistent with disruption of impact melt early in its formation history, and the volatile‐rich nature of the target materials likely contributed to this disruption. Bimodal thin section scans provide an estimate of the proportion of impact melt phases in the SRIS breccias (~19 vol%). When compared to similarly sized, mixed‐target impact structures, our results are consistent with the estimated volume of impact melt clasts from Ries, Germany (21 vol%), but are roughly twice that observed at Haughton, Canada (<10 vol%).

^1,2,3Steven B. Simon,^1,4Stephen R. Sutton
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13123]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA
²The Field Museum of Natural History, Chicago, Illinois, USA
³Institute of Meteoritics, University of New Mexico Albuquerque, New Mexico, USA
⁴Center for Advanced Radiation Sources (CARS), The University of Chicago, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

To assess the variability of redox states among mare basalt source regions, investigation of the valence of Ti, Cr, and V and the coordination environment of Ti in pyroxene and olivine in lunar rocks via XANES (X‐ray absorption near‐edge structure) spectroscopy has been extended to Apollo 17 basalts: two high‐Ti (70017 and 74275) hand samples, and three very low‐Ti (70006,371, 70007,289B, and 70007,296) basalt fragments from the Apollo 17 deep drill core. Valences of Ti in pyroxene of both suites range from 3.6 to 4, or from 40% to 0% Ti3+, averaging 15–20% Ti3+. Assuming Ti3+ is more compatible in pyroxene than Ti4+, then even lower Ti3+ proportions are indicated for the parental melts. The VLT pyroxene exhibits a slightly wider range of V valences (2.57–2.96) than the high‐Ti pyroxene (2.65–2.86) and a much wider range of Cr valences (2.32–2.80 versus 2.68–2.86); Cr is generally reduced in VLT pyroxene compared to high‐Ti pyroxene. Valences of Ti and Cr in VLT pyroxene become less reduced with increasing FeO contents, possibly indicating change in oxygen fugacity during crystallization. Olivine in all samples has very low (<20%) proportions of Ti3+, with no Ti3+ and higher proportions of Ti in tetrahedral coordination in the VLTs than in the high‐Ti basalts. Olivine in 74275, including that in a dunite clast, has much higher proportions of Cr2+ than the pyroxene in that sample, consistent with previous studies indicating that the olivine grains in this sample are xenocrysts and possibly indicating oxidation just prior to pyroxene crystallization. Results for this sample, the VLTs, and previously studied Apollo 14 and 15 basalts all indicate that mare magmas were in reducing environments at depth, as recorded in early crystallization products, and that later, presumably shallower environments, were relatively oxidizing; single, characteristic fO2s of formation cannot be assigned to these samples. A process likely to account for this feature seen in multiple samples is loss by degassing of a reducing, H‐rich vapor (probably H2) during ascent and/or eruption, causing oxidation of the residual melt, recorded in relatively late‐crystallized pyroxene.

¹Tasha L. Dunn,¹Oriana K. Battifarano,^2,3Juliane Gross,¹Emma J. O’Hara
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13118]
¹Department of Geology, Colby College Waterville, Maine, USA
²Department of Earth and Planetary Sciences, Rutgers University Piscataway, New Jersey, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History New York, New York, USA
Published by arrangement with John Wiley & Sons

Based on the chemical heterogeneity of chondrule and matrix olivine, Northwest Africa (NWA) 5343 is the least metamorphosed CK chondrite reported so far. To better constrain the lower limit of metamorphism in the CK chondrites, we performed a detailed analysis of matrix material in NWA 5343, including characterization of the texture and bulk composition and analyses of individual silicate minerals. Results suggest that NWA 5343 is petrologic type 3.6 or 3.7. Although silicate minerals in the matrix seem to be equilibrated to roughly the same extent throughout the sample, there are recognizable differences in grain size and shape. These textural differences may be the result of transient heating events during impacts, which would be likely on the CK chondrite parent body. The difference between the extent of chemical equilibration and texture may also suggest that grain size and shape are still sensitive to metamorphism at petrologic subtypes where silicate mineral equilibration is nearly complete (e.g., >3.7). Carbonate material present in NWA 5343 is a product of terrestrial weathering; however, infiltration of a Ca‐bearing fluid did not influence the composition of silicate minerals in the matrix. To evaluate the possibility of a continuous metamorphic sequence between the CV and CK chondrites, the bulk matrix composition of NWA 5343 is compared to the CVred chondrite, Vigarano. Although the matrix composition of NWA 5343 could be derived by secondary processing of a Vigarano‐like precursor, porosity and texture of matrix olivine in NWA 5343 are hard to reconcile with a continuous metamorphic sequence.