^1,2Tianqi Xie,^1,2Gordon R. Osinski,^1,2Sean R. Shieh
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13523]
¹Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7 Canada
²Institute for Earth and Space Exploration, University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

Plagioclase feldspar is one of the most abundant minerals on the surface of the Earth, the Moon, and Mars, and is also commonly found in meteorites. Studying shock effects in feldspar thus provides us with fundamental information about impact cratering processes on planetary bodies. In this study, plagioclase from monomict and polymict breccias, impact melt rocks, and shock‐metamorphosed target rocks, from throughout the Mistastin Lake impact structure, Canada, was examined using 514 nm laser Raman spectroscopy. As one of the very few impact structures with anorthosite in the target rocks, the Mistastin Lake impact structure provides a unique opportunity to study shocked plagioclase displaying progressive shock metamorphic features. A series of microscopic features was observed within plagioclase, including twins, needle‐like inclusions, planar features, and alteration. The lack of planar deformation features is notable. Raman spectra of these features suggest that this technique is capable of differentiating and classifying shock features in low to moderately shocked rocks. Caution should be exercised, however, as Raman spectra collected from unshocked plagioclase references with known compositions indicate that peak width and peak ratio of the Raman peaks in lower wave number region (<350 cm−1) and the main signature peaks around 500 cm−1 vary with chemical composition and crystal orientation. Data collected from diaplectic glass suggest that Raman features are efficient in distinguishing crystalline plagioclase and diaplectic glass. We also observed significant variations in the Raman intensities collected from diaplectic glass, which we ascribe to the localized disorder or inhomogeneity of shock pressure and temperature throughout the target.

¹Balz S.Kamber,^2,3Ronny Schoenberg
Earth and Planetary Science Letters 544, 116356 Link to Article [https://doi.org/10.1016/j.epsl.2020.116356]
¹School of Earth and Atmospheric Sciences, Queensland University of Technology, Australia
²Isotope Geochemistry, Department of Geosciences, Eberhard-Karls University of Tuebingen, Germany
³Department of Geology, University of Johannesburg, South Africa
Copyright Elsevier

The retention of moderately volatile elements on the growing Earth remains a major uncertainty in models of terrestrial accretion. Large impactors were the main carriers of accreted material but their mutual energetic collisions and impacts onto the Earth also caused chemical fractionation for which limited experimental data exist. The objective of this work was to study several moderately volatile elements in the third-largest impact basin preserved on Earth at Sudbury, Ontario. We conducted a new chemostratigraphic transect (
) of Zn isotope ratios and concentrations by analysing melt sheet and basin fill samples. The data were compared to common Pb, Cs, Cd and Sb concentration systematics. Within the crystallised melt sheet there are strong trends in the extent of moderately volatile element deficits, Zn isotope composition (
ZnJMC-L from 0.18 to 0.47‰) and initial Pb isotope composition. The combined evidence suggests that these trends reflect footwall contamination of a melt sheet that had experienced evaporative Zn-loss of up to 75–80%. Accounting for plausible isotopic signatures of target rocks, the maximum mass-dependent Zn isotope fractionation ε was 0.29 ± 0.04‰ (1 s.d.), which translates to modest fractionation factors
to 0.99975. This is comparable to melt fallout-glass and fused sands from nuclear detonation sites. We attribute the observed Zn loss and isotope fractionation to the formation of the impact melt. The rapid formation of a solid lid of breccias upon seawater ingress may have prevented stronger evaporative loss and isotope fractionation. Within the crater fill, there is an up-stratigraphy increase in Zn isotope variability (
ZnJMC-L from 0.29 to 1.05‰). Combined with evidence for biogenic reduced C, this suggests sedimentation of authigenic particulates within an enclosed crater sea.

In the melt sheet, the Zn-Pb and Rb-Cs pairs experienced different extents of maximum evaporative loss (Pb up to 98.4% vs. Zn 78%; and Cs ∼90% vs. Rb ∼30%). The relative loss pattern could reflect evaporation from superheated silicate melt at ∼1,450 °C and 1 atm. Loss from super-liquidus melts formed by bolide impacts could have been a significant process shaping the Earth’s volatile and moderately volatile inventory.

^1,2Sheng Gou et al. (>10)
Earth and Planetary Science Letters 544, 116378 Link to Article [https://doi.org/10.1016/j.epsl.2020.116378]
¹State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100101, China
²State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
Copyright Elsevier

Chang’e-4 rover discovered a dark greenish and glistening impact melt breccia in a crater during its traverse on the floor of Von Kármán crater within the South Pole Aitken (SPA) basin on the lunar farside. The discovered breccia, being 52 × 16 cm, resembles the lunar impact melt breccia samples 15466 and 70019 that returned by the Apollo missions. It was formed by impact-generated welding, cementing and agglutinating of lunar regolith and breccia. Clods surrounds the breccia-hosting crater were crushed into regolith powders by the rover’s wheels, indicating the regolith may be compacted slightly and becomes blocky and friable. Relative mineral fractions are estimated from the in situ measured spectra by a Hapke model-based unmixing algorithm. Unmixing reveals that plagioclase (PLG, 45 ± 6%) is dominant in the regolith, followed by almost equal fractions of pyroxene (PYX, 7 ± 1%) and olivine (OL, 6 ± 2%), indicating the regolith is likely related to noritic rocks. The regolith measured by Chang’e-4 rover was actually a highly mixture of multiple sources, with ejecta from Finsen crater being primary and possible contributions from Alder crater. Finsen and Alder craters are on the margin of the proposed impact melt pool produced by the SPA basin-forming event. Therefore, the ultimate source of the regolith might originate from a differentiated melt pool or from a suite of igneous rocks.

^1,2Ekaterina Chornaya,¹Alexander M.Zakharenko,³Evgenij Zubko,^1,4Aleksandr Kuchmizhak,¹Kirill S.Golokhvast,^5,6Gorden Videen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113907]
¹Far Eastern Federal University, 8 Sukhanova St., Vladivostok 690950, Russia
²Institute of Applied Astronomy of RAS, 10 Kutuzova Emb., Saint-Petersburg 191187, Russia
³Humanitas College, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, South Korea
⁴Institute of Automation and Control Processes FEB RAS, 5 Radio St., Vladivostok 690041, Russia
⁵Space Science Institute, 4750 Walnut Street, Boulder, Suite 205, CO 80301, USA
⁶Department of Astronomy and Space Science, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, South Korea
Copyright Elsevier

We experimentally measure the mid-IR spectra of half-millimeter-sized olivine particles with very compact morphology and smooth surfaces. We find that the 10-μm silicate feature is present in these large, irregularly shaped samples having surface roughness that is much smaller than optical wavelengths. Based on Mie theory, which assumes the particles are spherical, this feature should not exist for such large particles. As a consequence, its presence has been taken as an indicator that the particles or grains composing such particles were micron-sized or smaller. The measurement of this feature in real, irregularly shaped particles, suggests that the assumption of sphericity may severely limit our interpretations of remote-sensing data.

¹Pei Ma et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113901]
¹Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan, China
Copyright Elsevier

The Chang’E-4 spacecraft successfully landed in Von Kármán crater inside the South Pole-Aitken basin on the lunar far side on January 3, 2019 and the Yutu-2 Rover has performed explorations on the lunar surface for nine lunar days as of September 2019. Our earlier analysis of the visible and near-infrared spectrometer measurements made by the Yutu-2 rover during the first two lunar days shows that the regolith of the landing site may have come from the nearby Finsen crater and is dominated by plagioclase with lesser amount of mafic minerals. During its third lunar day explorations, the Yutu-2 photographed a small piece of lunar rock and measured its reflectance spectra. Compared with the spectra of its surrounding regolith, this rock’s spectra have deeper absorption features, indicating its fresher nature. To obtain the mineralogy of the rock, we compared the rock’s spectra with the spectral library data of NASA’s reflectance experiment laboratory of returned lunar rocks and lunar meteorites. We found that this rock is also plagioclase-rich with a possible plagioclase abundance of 60–80 vol%. A source region analysis using the Moon Mineralogy Mapper’s remote sensing observations indicates this rock was ejected from the Zhinyu crater, about 30 km west of the landing site, rather than directly from the Finsen crater. Numerical simulations of the Zhinyu crater on the impact cratering process and ejecta thickness distribution confirmed our findings and imply that the surficial materials at the CE-4 landing site experienced a complicated evolution rather than simply retaining the pristine or primordial ejecta directly from the Finsen crater.

¹Roger C. Wiens et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113897]
¹Los Alamos National Laboratory, Los Alamos, NM, USA
Copyright Elsevier

Heterolithic, boulder-containing, pebble-strewn surfaces occur along the lower slopes of Aeolis Mons (“Mt. Sharp”) in Gale crater, Mars. They were observed in HiRISE images acquired from orbit prior to the landing of the Curiosity rover. The rover was used to investigate three of these units named Blackfoot, Brandberg, and Bimbe between sols 1099 and 1410. These unconsolidated units overlie the lower Murray formation that forms the base of Mt. Sharp, and consist of pebbles, cobbles and boulders. Blackfoot also overlies portions of the Stimson formation, which consists of eolian sandstone that is understood to significantly postdate the dominantly lacustrine deposition of the Murray formation. Blackfoot is elliptical in shape (62 × 26 m), while Brandberg is nearly circular (50 × 55 m), and Bimbe is irregular in shape, covering about ten times the area of the other two. The largest boulders are 1.5–2.5 m in size and are interpreted to be sandstones. As seen from orbit, some boulders are light-toned and others are dark-toned. Rover-based observations show that both have the same gray appearance from the ground and their apparently different albedos in orbital observations result from relatively flat sky-facing surfaces.

Chemical observations show that two clasts of fine sandstone at Bimbe have similar compositions and morphologies to nine ChemCam targets observed early in the mission, near Yellowknife Bay, including the Bathurst Inlet outcrop, and to at least one target (Pyramid Hills, Sol 692) and possibly a cap rock unit just north of Hidden Valley, locations that are several kilometers apart in distance and tens of meters in elevation. These findings may suggest the earlier existence of draping strata, like the Stimson formation, that would have overlain the current surface from Bimbe to Yellowknife Bay. Compositionally these extinct strata could be related to the Siccar Point group to which the Stimson formation belongs.

Dark, massive sandstone blocks at Bimbe are chemically distinct from blocks of similar morphology at Bradbury Rise, except for a single float block, Oscar (Sol 516). Conglomerates observed along a low, sinuous ridge at Bimbe consist of matrix and clasts with compositions similar to the Stimson formation, suggesting that stream beds likely existed nearly contemporaneously with the dunes that eventually formed the Stimson formation, or that they had the same source material. In either case, they represent a later pulse of fluvial activity relative to the lakes associated with the Murray formation.

These three units may be local remnants of infilled impact craters (especially circular-shaped Brandberg), decayed buttes, patches of unconsolidated fluvial deposits, or residual mass-movement debris. Their incorporation of Stimson and Murray rocks, the lack of lithification, and appearance of being erosional remnants suggest that they record erosion and deposition events that post-date the exposure of the Stimson formation.

¹Dominik Spahr et al (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113898]
¹Institute of Geosciences, Goethe University Frankfurt, Altenhöferallee 1, 60438 Frankfurt, Germany
Copyright Elsevier

The formation of chondrules in the solar nebula is still an enigmatic process. In order to reconstruct this process, we performed a chondrule formation experiment aboard the International Space Station (ISS), called EXCISS. The purpose of the EXCISS experiment is to acquire new insights in chondrule formation by “nebular lightnings”. During the experiment forsterite (MgSiO₄) particles were repeatedly exposed to arc discharges under long-term micro-gravity conditions. The purpose of this paper is to describe the experimental set-up and establish and benchmark approaches for the analysis of the samples once they are returned from the ISS. We produced aggregates of fused MgSiO₄ particles in our Earth-based experiment which we studied by SEM and X-ray microtomography. The results of our preliminary Earth-based experiments indicate that the chosen experimental parameters will likely lead to melting of MgSiO₄ particles and the formation of aggregates. The results from the first experiments aboard the ISS revealed that the EXCISS experiment is fully operational.

¹Jeff A.Berger,²Sherry L.Cady,³Victoria E.Hamilton
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113868]
¹NASA Johnson Space Center, Houston, TX, USA
²Pacific Northwest National Laboratory, USA
³Southwest Research Institute, Boulder, CO, USA
Copyright Elsevier

Surface roughness is known to decrease thermal infrared (TIR) absorption band intensity, but studies of the effect on geologically relevant samples are relatively limited. To determine the effect of surface roughness (with features smaller than ~2/3 of the wavelength) on TIR spectra, we investigated two glass compositions with prepared roughened surfaces: (1) high purity fused silica and (2) soda-lime glass (73 wt% SiO2). We roughened the surfaces of the glasses by sandblasting and polishing with grit paper. The surfaces were characterized with scanning electron microscopy and stylus profilometry. We then analyzed the roughened glasses with TIR emittance spectroscopy. Micrometer-scale roughness causes a decrease in TIR absorption band intensity, relative to a specular surface. No significant changes in band shape or shifts in wavelength were detected. As roughness increases, empirical results show a logarithmic decrease in TIR absorption band intensity. The logarithmic trends of the two glass compositions are different; empirical roughness calibrations do not translate across different compositions. A linear, least-squares spectral deconvolution using two endmembers, specular and blackbody, predicts model spectra of roughened glass surfaces with relatively low error. This is of consequence to orbital TIR measurements of poorly constrained targets, such as the Martian surface, because micrometer-scale roughness is adequately modeled by the addition of a blackbody spectrum to the deconvolution endmember matrix.

^1,2Jessica A.Watkins,^2,3Bethany L.Ehlmann,¹An Yin
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113836]
¹Department of Earth, Planetary, and Space Sciences and Institute of Planets and Exoplanets (iPLEX), University of California, Los Angeles, CA 90095-1567, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Copyright Elsevier

Long-runout landslides with transport distances of >50 km are ubiquitous in Valles Marineris (VM), yet the transport mechanisms remain poorly understood. Four decades of studies reveal significant variation in landslide morphology and emplacement age, but how these variations are related to landslide transport mechanisms is not clear. In this study, we address this question by conducting systematic geological mapping and compositional analysis of VM long-runout landslides using high-resolution Mars Reconnaissance Orbiter imagery and spectral data. Our work shows that: (1) a two-zone morphological division (i.e., an inner zone characterized by rotated blocks and an outer zone expressed by a thin sheet with a nearly flat surface) characterizes all major VM landslides; (2) landslide mobility is broadly dependent on landslide mass; and (3) the maximum width of the outer zone and its transport distance are inversely related to the basal friction that was estimated from the surface slope angle of the outer zone. Our comprehensive Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) compositional analysis indicates that hydrated silicates are common in landslide outer zones and nearby trough-floor deposits. Furthermore, outer zones containing hydrated minerals are sometimes associated with longer runout and increased lateral spreading compared to those without detectable hydrated minerals. Finally, with one exception we find that hydrated minerals are absent in the inner zones of the investigated VM landslides. These results as whole suggest that hydrated minerals may have contributed to the magnitude of lateral spreading and long-distance forward transport of major VM landslides.

¹Jens Barosch,^1,2Dominik C. Hezel,³Yves Marrocchi,³Andrey Gurenko,^1,4Christoph Lenting
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13496]
¹Department of Geology and Mineralogy, University of Cologne, Zülpicher Str. 49b, 50674 Köln, Germany
²Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD UK
³CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre‐lès‐Nancy, 54501 France
⁴Institute of Geoscience, University of Bonn, Meckenheimer Allee 169, 53111 Bonn, Germany
Published by arrangement with John Wiley & Sons

We found a large (~2 mm) compound object in the primitive Yamato 793408 (H3.2‐an) chondrite. It consists mostly of microcrystalline material, similar to chondrule mesostasis, that hosts an intact barred olivine (BO) chondrule. The object contains euhedral pyroxene and large individual olivine grains. Some olivine cores are indicative of refractory forsterites with very low Fe‐ and high Ca, Al‐concentrations, although no ¹⁶O enrichment. The entire object is most likely a new and unique type, as no similar compound object has been described so far. We propose that it represents an intermediate stage between compound chondrules and macrochondrules, and formed from the collision between chondrules at low velocities (below 1 m s⁻¹) at high temperatures (around 1550 °C). The macrochondrule also trapped and preserved a smaller BO chondrule. This object appears to be the first direct evidence for a genetic link between compound chondrules and macrochondrules. In accordance with previous suggestions and studies, compound chondrules and macrochondrules likely formed by the same mechanism of chondrule collisions, and each represents different formation conditions, such as ambient temperature and collision speed.