¹William Abbey et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113885]
¹Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, United States of America
Copyright Elsevier

The Mars Science Laboratory (MSL) rover, Curiosity, completed its second Martian year, 1337 sols (1374 Earth days), of operation on May 11, 2016, and its third Martian year, 2006 sols (2061 Earth days), of operation on March 28, 2018. During this time the rover successfully drilled twelve full depth drill holes into the Martian surface and analyzed the recovered material using onboard instruments, giving us new insights into the potential habitability and geologic diversity of ancient Mars. During the second Martian year, four holes were drilled into the mudstones of the Murray formation: ‘Confidence Hills’ (Sol 759), ‘Mojave 2’ (Sol 882), ‘Telegraph Peak’ (908) & ‘Buckskin’ (Sol 1060); while four more holes were drilled into the sandstones of the Stimson formation: ‘Big Sky’ (Sol 1119), ‘Greenhorn’ (Sol 1137), ‘Lubango’ (Sol 1320) & ‘Okoruso’ (Sol 1332). During the third Martian year, four additional holes were drilled into the Murray formation: ‘Oudam’ (Sol 1361), ‘Marimba’ (Sol 1422), ‘Quela’ (Sol 1464) & ‘Sebina’ (Sol 1495). In this paper, we will give a brief overview of the rover sampling hardware and nominal drilling protocols, followed by a discussion of how these protocols were refined and altered early during the course of Curiosity’s second year on Mars. In addition, we will describe the ‘Bonanza King’ (Sol 724) drill campaign, the mission’s first ‘successful failure’, and how it influenced these changes. We will also briefly discuss the events leading up to the drill feed fault on Sol 1536, which resulted in suspension of all drill activities for the remainder of the third Martian year. Finally, we will present scientific highlights obtained from each drill site utilizing MSL’s onboard instrumentation (SAM & CheMin), results enabled by the drill’s ability to excavate sample at depth and transfer it to these instruments.

¹Lidia Pittarello,¹Ludovic Ferrière,¹Jean‐Guillaume Feignon,¹Gordon R. Osinski,¹Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13490]
¹Natural History Museum Vienna, Burgring 7, A‐1010 Vienna, Austria
²Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A‐1090 Vienna, Austria
Published by arrangement with John Wiley & Sons

Shocked quartz and feldspar grains commonly exhibit planar microstructures, such as planar fractures, planar deformation features, and possibly microtwins, which are considered to have formed by shock metamorphism. Their orientation and frequency are typically reported to be randomly distributed across a sample. The goal of this study is to investigate whether such microstructures are completely random within a given sample, or whether their orientation might also retain information on the direction of the local shock wave propagation. For this work, we selected samples of shatter cones, which were cut normal to the striated surface and the striation direction, from three impact structures (Keurusselkä, Finland, and Charlevoix and Manicouagan, Canada). These samples show different stages of pre‐impact tectonic deformation. Additionally, we investigated several shocked granite samples, selected at different depths along the drill core recovered during the joint IODP‐ICDP Chicxulub Expedition 364 (Mexico). In this case, thin sections were cut along two orthogonal directions, one parallel and one normal to the drill core axis. All the results refer to optical microscopy and universal‐stage analyses performed on petrographic thin sections. Our results show that such shock‐related microstructures do have a preferred orientation, but also that relating their orientation with the possible shock wave propagation is quite challenging and potentially impossible. This is largely due to the lack of dedicated experiments to provide a key to interpret the observed preferred orientation and to the lack of information on postimpact orientation modifications, especially in the case of the drill core samples.

¹Nan Liu,¹Ryan C.Ogliore,¹Lionel G.Vacher
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.026]
¹Laboratory for Space Sciences and the Department of Physics, Washington University in St. Louis, St. Louis, MO 63130, USA
Copyright Elsevier

We report on the investigation of the inventories of presolar grains and organic matter (OM) in 14 xenolithic carbonaceous clasts (C-clasts) identified in the Kapoeta howardite based on high-resolution NanoSIMS hydrogen, carbon, nitrogen, and oxygen isotopic imaging data. The 14C-clasts are ∼50–200 μm in size and consisted of one CM-like and 13 CI-like clasts, according to the mineralogy-based classification scheme adopted in the literature. All of the 14 C-clasts are located along one edge of the thin section. In two CI-like clasts, embayments of magnetite grains between the C-clast and the host howardite point to aqueous alteration occurring on Vesta as a result of melting the ice embedded in the C-clasts. It also strongly suggests that all of the C-clasts, especially the 13 CI-like clasts, are originated from the same parent body, because of their localized distribution across the thin section and the much higher ratio of CI-like to CM-like clasts with respect to the ratios reported in the literature. Thus, taking the two pieces of evidence together implies that the clasts from this study are sourced from an ice-bearing parent body, either an icy asteroid or a comet, originated from the outer solar system. Four presolar silicon carbide (SiC) and two presolar silicate grains were identified in the C-clasts. In addition, all the C-clasts contain moderate bulk D- and 15N-enrichments with the presence of micron to submicron-sized D and 15N hotspots, indicating the presence of primitive organic material. Comparison of our data with the literature data for a wide range of extraterrestrial materials for their inventories of presolar grains and OM, provides support to (1) the genetic linkage of xenolithic C-clasts to highly aqueously altered but minimally heated carbonaceous chondritic materials and (2) homogeneous distribution of circumstellar and interstellar materials in the protoplanetary disk. The low amounts of heat experienced by the C-clasts suggest their rather late arrival at Vesta and/or Vestoids at low speeds after the occurrence of late heavy bombardment in the inner solar system during ∼3.5–4.0 Gyr ago.

¹Kathleen E. Vander Kaaden,²Francis M. McCubbin,^1,3Amber A. Turner,^1,4D. Kent Ross
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006239]
¹Jacobs, NASA Johnson Space Center, Houston, TX, USA
²NASA Johnson Space Center, Houston, TX, USA
³Department of Geoscience, University of Las Vegas, Las Vegas, NV, USA
⁴University of Texas at El Paso‐CASSMAR, El Paso, TX, USA
Published by arrangement with John Wiley & Sons

The composition of a planet’s core has important implications for the thermal and magmatic evolution of that planet. Here, we conducted carbon (C) solubility experiments on iron‐silicon (Fe‐Si) metal mixtures (up to 35 wt% [~52 atom%] Si) at 1 GPa and 800–1800°C to determine the carbon concentration at graphite saturation (CCGS) in metallic melt and crystalline metal with varying proportions of Fe and Si to constrain the C content of Mercury’s core. Our results, combined with those in the literature, show that composition is the major controlling factor for carbon solubility in Fe‐rich metal with minimal effects from temperature and pressure. Moreover, there is a strong anticorrelation between the abundances of carbon and silicon in iron‐rich metallic systems. Based on the previous estimates of <1–25 wt% Si in Mercury’s core, our results indicate that a carbon‐saturated Mercurian core has 0.5–6.4 wt% C, with 6.4 wt% C corresponding to an Si‐free, Fe core and 0.5 wt% C corresponding to an Fe‐rich core with 25 wt% Si. The upper end of estimated FeO abundances in the mantle (up to 2.2 wt%) are consistent with a core that has <1 wt% Si and up to 6.4 wt% C, which would imply that bulk Mercury has a superchondritic Fe/Si ratio. However, the lower end of estimated FeO (≤0.05 wt%) supports CB chondrite‐like bulk compositions of Mercury with core Si abundances in the range of 5–18.5 wt% and C abundances in the range of 0.8–4.0 wt%.

¹B. S. Prince,¹M. P. Magnuson,²L. C. Chaves,²M. S. Thompson,^3,4M. J. Loeffler
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006242]
¹Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
³Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA⁴Center for Materials Interfaces in Research and Applications, Northern Arizona University, Flagstaff, AZ, USA
Published by arrangement with John Wiley & Sons

Here we present results from pulsed laser irradiation of troilite samples in an effort to simulate space weathering on airless bodies via micrometeorite impacts. We find that the spectral trends observed in directly irradiated samples and samples with a vapor‐deposited coating are different than those found in silicate minerals previously studied. For instance, direct laser irradiation causes our troilite samples to initially brighten, but continued irradiation causes darkening and a decrease in spectral slope. In contrast, our samples with a vapor‐deposited coating show a continuous increase in spectral slope and overall albedo as the deposit thickness increases. Observation using both digital imaging and electron microscopy of our directly irradiated samples leads us to conclude that topography effects likely become important after a relatively high number of laser pulses in our directly irradiated samples, causing the apparent darkening and decrease in spectral slope. Thus, we conclude that the spectral changes observed relevant to space weathering via micrometeorite impacts are an increase in spectral slope and an increase in the albedo of troilite. Future studies will investigate whether these trends are generally representative of other sulfide‐bearing minerals and of weathering trends in other components found in the asteroid regolith.

¹Roger R. Fu,^1,2Pauli Kehayias,¹Benjamin P. Weiss,³Devin L. Schrader,⁴Xue‐Ning Bai,^5,6,7Jacob B. Simon
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2019JE006260]
¹Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
²Sandia National Laboratories, Albuquerque, NM, USA
³Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
⁴Institute for Advanced Study, Tsinghua University, Beijing, China
⁵Department of Physics and Astronomy, Iowa State University of Science and Technology, Ames, IA, USA
⁶JILA, University of Colorado Boulder and NIST, Boulder, CO, USA
⁷Department of Space Studies, Southwest Research Institute, Boulder, CO, USA
Published by arrangement with John Wiley & Sons

Theoretical investigations suggest that magnetic fields may have played an important role in driving rapid stellar accretion rates and efficient planet formation in protoplanetary disks. Experimental constraints on magnetic field strengths throughout the solar nebula can test the occurrence of magnetically driven disk accretion and the effect of magnetic fields on planetary accretion. Here we conduct paleomagnetic experiments on chondrule samples from primitive CR (Renazzo type) chondrites GRA 95229 and LAP 02342, which likely originated in the outer solar system between 3 and 7 AU approximately 3.7 million years after calcium aluminum‐rich inclusion formation. By extracting and analyzing 18 chondrule subsamples that contain primary, igneous ferromagnetic minerals, we show that CR chondrules carry internally non‐unidirectional magnetization that requires formation in a nebular magnetic field of ≤8.0 ± 4.3 μT (2σ ). These weak magnetic fields may be due to the secular decay of nebular magnetic fields by 3.7 million years after calcium aluminum‐rich inclusions, spatial heterogeneities in the nebular magnetic field, or a combination of both effects. The possible inferred existence of spatial variations in the nebular magnetic field would be consistent with a prominent role for disk magnetism in the formation of density structures leading to gaps and planet formation.

^1,2Lidia Pittarello,^3,4,5Luke Daly,³Annemarie E. Pickersgil,¹Ludovic Ferrière,³Martin R. Lee
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13494]
¹Department of Mineralogy and Petrography, Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
²Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A‐1090 Vienna, Austria
³School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow, G12 8QQ UK
⁴Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, GPO Box U 1987, Perth, Western Australia, 6845 Australia
⁵Australian Centre for Microscopy and Microanalysis, University of Sydney, Sydney, 2006 New South Wales, Australia
Published by arrangement with John Wiley & Sons

Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals.

¹David A. Kring et al. (>10)
Science Advances 22, eaaz3053 Link to Article [DOI: https://doi.org/10.1126/sciadv.aaz3053]
¹Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, TX 77058, USA.

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

^1,2Lee F. White,^1,2Kimberly T. Tait,³Brian Langelier,⁴Elizabeth A. Lymer,^1,2Ana Černok,^1,2 Tanya V. Kizovski,⁵Chi Ma,⁶Oliver Tschauner,¹Richard I. Nicklin
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.2003276117]
¹Department of Natural History, Royal Ontario Museum, Toronto, ON M5S 2C6, Canada;
²Department of Earth Sciences, University of Toronto, Toronto, ON M5S 3B1, Canada;
³Canadian Centre for Electron Microscopy, McMaster University, Hamilton, ON L8S 4M1, Canada;
⁴Lassonde School of Engineering, York University, Toronto, ON M3J 1P3, Canada;
⁵Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125;
⁶Department of Geoscience, University of Nevada, Las Vegas, NV 89154

Understanding the timing and mechanisms of amino acid synthesis and racemization on asteroidal parent bodies is key to demonstrating how amino acids evolved to be mostly left-handed in living organisms on Earth. It has been postulated that racemization can occur rapidly dependent on several factors, including the pH of the aqueous solution. Here, we conduct nanoscale geochemical analysis of a framboidal magnetite grain within the Tagish Lake carbonaceous chondrite to demonstrate that the interlocking crystal arrangement formed within a sodium-rich, alkaline fluid environment. Notably, we report on the discovery of Na-enriched subgrain boundaries and nanometer-scale Ca and Mg layers surrounding individual framboids. These interstitial coatings would yield a surface charge state of zero in more-alkaline fluids and prevent assimilation of the individual framboids into a single grain. This basic solution would support rapid synthesis and racemization rates on the order of years, suggesting that the low abundances of amino acids in Tagish Lake cannot be ascribed to fluid chemistry.

^1,2,3Y. Li,^3,4A. E. Rubin,¹W. Hsu,⁴K. Ziegler
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2019JE006360]
¹Center for Excellence in Comparative Planetology, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
²Macau University of Science and Technology, Macau, China
³Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA
⁴Maine Mineral and Gem Museum, Bethel, ME, USA
⁵Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA
Published by arrangement with John Wiley & Sons

The NWA 11004 ordinary chondrite (OC) can provide insights into the complex petrogenetic processes of the early solar system. Although originally classified as an L7 chondrite, it is reclassified as LL based on kamacite Ni (4.9 ± 0.3 wt.%) and Co (3.6 ± 0.5 wt.%) and bulk O‐isotopic composition (δ¹⁷O = 3.76‰; δ¹⁸O = 5.39‰). NWA 11004 is characterized by (1) the occurrence of 3‐ to 5‐mm‐sized poikilitic pyroxene, (2) scattered low‐Ca pyroxene data in a TiO₂ versus Al₂O₃ diagram, (3) relatively magnesian olivine and low‐Ca pyroxene (Fa_25.4, Fs_21.3), (4) low abundances of high‐Ca pyroxene, plagioclase, troilite and Ca‐phosphate, and (5) low rare earth element contents in low‐Ca pyroxene. The geochemical features of olivine and low‐Ca pyroxene in NWA 11004 differ from literature data for grains that crystallized from a melt in an OC impact melt breccia. We suggest that in NWA 11004, a plagioclase‐phosphate high‐Ca pyroxene‐troilite melt migrated away during partial melting. Some high‐Ca pyroxene grains crystallized from the residual melt, as indicated by a positive linear trend in a TiO₂ versus Al₂O₃ diagram. Whereas poikilitic low‐Ca pyroxene in NWA 11004 exhibits undulose‐to‐weak mosaic extinction, the olivine chadacrysts exhibit sharp optical extinction; this implies that NWA 11004 experienced a late‐stage shock event (S4) followed by annealing. The Ca‐phosphate ²⁰⁷Pb/²⁰⁶Pb age of 4546 ± 34 Ma most likely dates this late‐stage shock event. We suggest that the presence of type 7 OC in the early solar system may be attributable to impacts on warm chondritic asteroids that were initially heated by the decay of ²⁶Al.