A possible explanation for the blue spectral slope observed on B-type asteroids

Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114881]
1Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ 86011, United States of America
2Center for Materials Interfaces in Research and Applications, Northern Arizona University, Flagstaff, AZ 86011, United States of America
3Department of Applied Physics and Materials Science, Northern Arizona University, Flagstaff, AZ 86011, United States of America
Copyright Elsevier

In an effort to better understand the role dark material plays in the reflectance spectrum of carbonaceous asteroids, we performed laboratory studies focusing on quantifying how the addition of relevant dark material (graphite, magnetite and troilite) can alter the ultraviolet-visible and near-infrared spectrum of a neutral silicate mineral. We find that addition of graphite, magnetite and troilite all darken the reflectance spectrum of our forsterite samples and cause the spectral slope to decrease (become blue). These spectral changes can be caused by both nm- and μm-sized grains. In the ultraviolet-visible region, we find that graphite is most efficient at altering the spectral slope, while in the near-infrared, magnetite is the most efficient. At all wavelengths studied, graphite is the most efficient at darkening our sample spectrum. However, the observation that troilite also alters the slope and albedo of our samples suggests that the spectral changes caused by magnetite and graphite may not be unique. In addition, we find that the spectral slopes in our mixtures compare generally well to what has been observed on Bennu suggesting that a significant portion of fine-grained dark material, including sulfides, present in the regolith can cause the observed negative (blue) slope found on B-type asteroids.

Blaubeuren, Cloppenburg, and Machtenstein—Three recently recognized H-group chondrite finds in Germany with distinct terrestrial ages and weathering effects

1Addi Bischoff,1Jakob Storz,2,3Jean-Alix Barrat,4Dieter Heinlein,5,6A. J. Timothy Jull,7,8Silke Merchel,9Andreas Pack,7Georg Rugel
Meteorotics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13779]
1Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm Str. 10, Münster, D-48149 Germany
2CNRS, IRD, Ifremer, LEMAR, University of Brest, Plouzané, F-29280 France
3Institut Universitaire de France, Paris, 75005 France
4German Fireball Network, Lilienstraße 3, Augsburg, D-86156 Germany
5University of Arizona AMS Laboratory, 1118 East Fourth St, Tucson, Arizona, 85721 USA
6Isotope Climatology and Environmental Research Centre (ICER), Institute for Nuclear Research, Hungarian Academy of Sciences, Bem ter 18/c, Debrecen, 4026 Hungary
7Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstr. 400, Dresden, D-01328 Germany
8Faculty of Physics, Isotope Physics, VERA Laboratory, University of Vienna, Währinger Str. 17, Vienna, 1090 Austria
9Geowissenschaftliches Zentrum, Universität Göttingen, Goldschmidtstr. 1, Göttingen, D-37077 Germany
Published by Arrangement with John Wiley & Sons

In the last 7 years, three meteorites (Blaubeuren, Cloppenburg, and Machtenstein) found in Germany were identified as chondrites. Two of these rocks had been recovered from the impact sites decades ago but not considered to be meteorites. The aim of this study is to fully characterize these three meteorites. Based on the compositional data on the silicates, namely olivine and low-Ca pyroxene, these meteorites fit nicely within the H-group ordinary chondrites. The brecciated texture of Blaubeuren and Cloppenburg (both H4-5) is perfectly visible, whereas that of Machtenstein, officially classified as an H5 chondrite, is less obvious but was detected and described in this study. Considering chondrites in general, brecciated rocks are very common rather than an exception. The bulk rock degree of shock is S2 for Blaubeuren and Machtenstein and S3 for Cloppenburg. All samples show significant features of weathering. They have lost their original fusion crust and more than half (W3) or about half (W2-3) of their original metal abundances. The oxygen isotope compositions of the three chondrites are consistent with those of other H chondrites; however, the Cloppenburg values are heavily disturbed and influenced by terrestrial weathering. This is supported by the occurrence of the very rare hydrated iron phosphate mineral vivianite (Fe2+Fe2+2[PO4]2·8H2O), which indicates that the chondrite was weathered in a very wet environment. The terrestrial ages of Blaubeuren (~9.2 ka), Cloppenburg (~5.4 ka), and Machtenstein (~1.8 ka) show that these chondrites are very similar in their degree of alteration and terrestrial age compared to meteorite finds from relatively wet terrestrial environments. They still contain abundant metal, although, as noted, the oxygen isotope data indicate substantial weathering of Cloppenburg. The bulk compositions of the three meteorites are typical for H chondrites, although terrestrial alteration has slightly modified the concentrations, leading in general to a loss of Fe, Co, and Ni due to preferential alteration of metals and sulfides. As exceptions, Co and Ni concentrations in Machtenstein, which has the shortest terrestrial age, are typical for H chondrites. The chemical data show no enrichments in Ba and Sr, as is often observed in different meteorite groups of desert finds.

Has the impact flux of small and large asteroids varied through time on Mars, the Earth and the Moon?

1Anthony Lagain,2Mikhail Kreslavsky,3,4David Baratoux,5Yebo Liu,1Hadrien Devillepoix,1Philip Bland,6,7Gretchen K.Benedix,5Luc S.Doucet,8Konstantinos Servish
Earth and Planetary Science Letters 579, 117362 Link to Article [https://doi.org/10.1016/j.epsl.2021.117362]
1Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Kent St, Bentley, 6102, WA, Australia
2Earth and Planetary Sciences, University of California – Santa Cruz, CA, USA
3Géosciences Environnement Toulouse, University of Toulouse, 14, Avenue Edouard Belin, Toulouse, 31400, France
4University Félix Houphouët-Boigny, UFR des Scinces de la Terre et des Ressoures Minières, Cocody, Abidjan, Côte d’Ivoire
5Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), Department of Earth and Planetary Sciences, Curtin University, Kent St, Bentley, 6102, WA, Australia
6Planetary Sciences Institute, Tucson, AZ, USA
7Department of Earth and Planetary Sciences, Western Australian Museum, WA, Australia
8CSIRO – Pawsey Supercomputing Centre, WA, Australia
Copyright Elsevier

The impact flux over the last 3 Ga in the inner Solar System is commonly assumed to be constant through time due to insufficient data to warrant a different choice for this range of time. However, asteroid break-up events in the main belt may have been responsible for cratering spikes over the last ∼2 Ga on the Earth-Moon system. Due to its proximity with the main asteroid belt, i.e., the main impactors reservoir, Mars is at the outpost of these events with respect to the other inner planets. We investigate here, from automatic crater identification, the possible variations of the size frequency distributions of impactors from the record of small craters of 521 impact craters larger than 20 km in diameter. We show that 49 craters (out of the 521) correspond to the complete crater population of this size formed over the last 600 Ma. Our results on Mars show that the flux of both small (> 5 m) and large asteroids (> 1 km) are coupled, does not vary between each other over the last 600 Ma. Existing data sets for large craters on the Earth and the Moon are analyzed and compared to our results on Mars. On Earth, we infer the formation location of a set of impact craters thanks to plate tectonic reconstruction and show that a cluster of craters formed during the Ordovician period, about 470 Ma ago, appears to be a preservation bias. On the Moon, the late increase seen in the crater age signal can be due to the uncertain calibration method used to date those impacts (i.e. rock abundance in lunar impact ejecta), and other calibrations are consistent with a constant crater production rate. We conclude to a coupling of the crater production rate between kilometer-size craters (∼100 m asteroids) and down to ∼100 m in diameter (∼5 m asteroids) in the inner Solar System. This is consistent with the traditional model for delivering asteroids to planet-crossing obits: the Yarkovsky effect slowly pushes the large debris from asteroid break-ups towards orbital resonances while smaller debris are grinded through collisional cascades. This suggests that the long-term impact flux of asteroids > 5 m is most likely constant over the last 600 Ma, and that the influence of past asteroid break-ups in the cratering rate for D > 100 m is limited or inexistent.

Noble gases in Dome C micrometeorites – An attempt to disentangle asteroidal and cometary sources

1,2Bastian Baecker,1,2,3Ulrich Ott,2Mario Trieloff,4Cécile Engrand,5Jean Duprat
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114884]
1Max-Planck Institut für Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
2Klaus-Tschira-Labor für Kosmochemie, Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
3MTA Atomki, Bem tér 18/C, 4026 Debrecen, Hungary
4IJCLab, CNRS-Paris-Saclay, Orsay, France
5IMPMC, CNRS-MNHN, Paris, France
Copyright Elsevier

We have performed a comprehensive noble gas study, including the isotopes of krypton and xenon, on a set of micrometeorites (MMs) collected from surface snow at Dome C (DC) on the Antarctic plateau. He and Ne are generally dominated by a solar component, with lower 4He concentrations and 4He/20Ne ratios in crystalline (Xtal) compared to fine-grained carbonaceous (FgC) MMs. Concentrations of (surface-correlated) solar wind (SW) He and Ne in FgC MMs are at the high end of what has been seen in earlier work, whereas the abundances of (volume-correlated) Kr and Xe are similar to what has been found in previous studies of MMs. In most samples, isotopic ratios for Kr and Xe are in the usual range of Q-Kr and single bondXe (the Q component is the dominating component in primitive macroscopic meteorites) and air. When quantifiable, cosmic ray exposure (CRE) ages based on cosmogenic 21Ne and 3He, in combination with the Poynting-Robertson effect, are broadly consistent with an origin of the MMs from the asteroid belt. An exception is an Xtal MM, which exhibits a cosmogenic 21Ne concentration in agreement with an origin from beyond Saturn, consistent with a possible cometary origin. In addition, data for trapped noble gases in three (out of ten analyzed) DC MMs provide hints that these may be related to a cometary source. One sample, a fragment of a FgC MM, is of particular interest. This fragment exhibits a Xe composition, although with large analytical uncertainties, deficient in the heavy isotopes 134Xe and 136Xe. This is similar to the Xe isotopic pattern, probably related to cometary ice, measured by Rosetta in the coma of comet 67P/Churyumov-Gerasimenko. The same MM also has an unusually high 36Ar/38Ar ratio, consistent with Rosetta’s Ar measurement (in this case the latter having a large uncertainty). The other hints are for two MMs, of crystalline (Xtal) type, that show Ne similar to that found in laboratory analysis of refractory grains captured from comet 81P/Wild 2 by the Stardust mission. Additionally, a FgC/Xtal MM may contain excess 3He, similar to what has been seen in some cluster interplanetary dust particles (cluster IDPs).

Controls on S mineral formation and preservation in hydrothermal sediments: Implications for the volcanic, aqueous, and climatic history of Gusev crater, Mars

1Rhianna D.Moore,1Anna Szynkiewicz
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114880]
1Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, United States of America
Copyright Elsevier

Acidic hydrothermal and fumarolic surface deposits within the Columbia Hills in Gusev crater on Mars were found to have elevated concentrations of Fe-Mg-Ca-sulfate minerals. However, this is inconsistent with analogous terrestrial hydrothermal settings that are usually enriched in elemental sulfur (S). Consequently, this raises questions about the origin and hydrothermal history of the Gusev sediments. To address this discrepancy, we analyzed quantities and S isotope compositions of S-bearing minerals in hydrothermal sediment samples from acidic hot springs, mud pots, and fumaroles with elevated H2S emissions in Iceland and the United States (e.g., Valles Caldera, Lassen, and Yellowstone). Our results indicate that the typical concentrations (e.g., inter-quartile range) of elemental S and sulfide minerals (0.3 to 10.5 wt% S, but as high as ~75 wt% S; and 0.1 to 1.7 wt% S, but as high as ~10 wt% S) are significantly higher compared to sulfate (0.1 to 1.1 wt% S, but as high as ~4.5 wt% S) in the surface hydrothermal deposits. In most cases, the concentrations of elemental S, sulfides, and sulfates in the sediments decreased with increasing hydrological connectivity and in wetter climates. Similar δ34S values between sulfate (−0.1 to +1.4‰) and elemental S (−0.4 to +1.6‰) compared to lower δ34S of sulfide (−2.4 to +0.4‰) suggest that more sulfate is likely derived from the subsequent oxidation of elemental S than sulfide. Conversely, minor amounts of sulfate are formed via direct oxidation of H2S which had higher δ34S values (+1.1 to +5.9‰). Our laboratory experiments carried over a wide range of temperatures (25, 65, and 85 °C) and low pH (~2) indicate that elemental S and pyrite undergo subsequent oxidation to sulfate via both ferric iron (Fe3+) and O2. While the amount of sulfate increased with increasing temperature in the presence of both Fe3+ and O2, Fe3+ appears to be a more efficient oxidizer than O2. For example, pyrite oxidation by only Fe3+ resulted in ~1.5× more sulfate (~80 to 180 mg/L SO42−) than by only O2 (~40 to 140 mg/L SO42−). In contrast, considerably less sulfate was formed during the oxidation of elemental S, although in the presence of O2 ~ 10× more sulfate (~0.1 to 45 mg/L SO42−) was formed than when Fe3+ was present (~0.3 to 7.5 mg/L SO42−).

Despite the prevalence of sulfate minerals rather than elemental S and sulfides in the hydrothermal Gusev deposits on Mars, the total S concentrations measured by the Spirit rover (2.9 to 9.3 wt% S) are highly comparable to the total S in hydrothermal sediments formed in colder and moderately wet climates such as coastal Iceland (1.8 to 10.7 wt% S). This contrasts with sediments formed in the high-altitude and drier climate of Valles Caldera (9.9 to 37.6 wt% S), or the wetter climates of Yellowstone (4.1 to 17.3 wt% S) and Lassen (0.5 to 3.5 wt% S). Because water is needed to further oxidize the hydrothermal elemental S and sulfide to sulfate, we infer that the aqueous conditions must have persisted in Gusev crater for a period of time after the main hydrothermal activity ceased. Later, under low water-to-rock conditions with little (or no) H2S emission, complete oxidation of the Gusev hydrothermal deposits likely took place and led to the formation of the sulfate minerals that were identified by the Spirit rover.

Fayalite formation through hydrothermal experiments: Insights into early fluid-assisted aqueous alteration processes on asteroids

1E. Dobrică,2J. A. Nuth,3A. J. Brearley
Meteoritics&Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13765]
1Hawai’i Institute of Geophysics and Planetology, School of Ocean, Earth Science, and Technology, University of Hawai’i at Mānoa, Honolulu, Hawaii, 96822 USA
2Solar System Exploration Division, Code 690, NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771 USA
3Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, New Mexico, 87131–0001 USA
Published by arrangemengt with Jophn Wiley & Sons

In order to understand the effects of the earliest fluid-assisted hydration processes on asteroids, we performed one hydrothermal experiment using three different reactants (FeO-rich amorphous silicates, iron metal powder, and water) at conditions informed by our current state of knowledge of asteroidal alteration. This experiment provides, for the first time, clear evidence that the growth of fayalite can occur during hydrothermal alteration, as described previously in meteorites. These newly formed fayalite crystals are elongated and porous, similar to the ones described in CV3, CK, and ordinary chondrites. The results show that (1) fayalite could form even if chemical equilibrium was not reached in the experiment, at a water to rock mass ratio (0.4 W/R at the beginning of the experiment) higher than the values calculated to be thermodynamically viable at equilibrium (W/R > 0.2); (2) the composition and the texture of the reactants changed during the hydrothermal alteration process, suggesting that the reactants, especially the amorphous silicates, underwent dissolution and reprecipitation; (3) fayalite can form at low temperature (220 °C), which is at the transition between hydrothermal alteration and fluid-assisted metamorphism in chondrites. The results are consistent with previous mineralogical observations and thermodynamic models, which suggest that fayalite crystals are formed on asteroidal parent bodies by the interaction between a hydrothermal fluid and disequilibrium assemblages that compose the pristine materials that condensed in the early solar nebula. This experiment suggests that two variables play a very important role in the formation of fayalite during the hydrothermal growth (W/R mass ratio and the fluid composition). These results are similar to the recent observations of the fine-grained matrix of ordinary chondrites.

Northwest Africa 6486: Record of large impact events and fluid alteration on the L chondrite asteroid

1C. A. Lorenz,1E. V. Korochantseva,1M. A. Ivanova,2J. Hopp,3I. A. Franchi,4M. Humayun,1M. O. Anosova,1S. N. Teplyakova,2M. Trieloff
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13774]
1Vernadsky Institute RAS, Kosygin St. 19, Moscow, 119991 Russia
2Institut für Geowissenschaften, Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 234-236, Heidelberg, 69120 Germany
3Planetary & Space Sciences, School of Physical Sciences, Open University, Milton Keynes, MK7 6AA UK
4National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, Florida, 32310 USA
Published by arrangement with John Wiley & Sons

We report the results of petrological, geochemical, and geochronological investigations of the unusual K-rich L chondrite melt rock Northwest Africa 6486 (NWA 6486). The rock has slightly fractionated siderophile elements and a mostly unfractionated L chondrite pattern of lithophile elements with the exceptions of enrichments in K and Rb and chondritic Sr abundance similar to the K-rich inclusions found in the ordinary chondrites and indicating a fractionation of alkaline elements through the vapor. We suggest that NWA 6486 and related K-rich chondritic inclusions were formed in situ on the OC parent bodies and that K and Rb enrichment of these rock most probably is a result of the selective impact evaporation of volatile alkali elements followed by the reaction of a vapor with shock melt. NWA 6486 recorded a breakup event of the L chondrite parent asteroid at 470 Ma during which it was formed. Unusual veins, depleted in K, Na, Ca, and Al relative to the host rock were found in NWA 6486. We suggest that NWA 6486 was affected by aqueous fluids that produced alteration zones depleted in a feldspar component on the walls of opened fractures. The melt veins could be formed during a subsequent impact event by in situ melting of the fracture walls or due to decomposition of an injected supercritical aqueous silicate fluid. The aqueous alteration and the second impact event had no detectable effect on Ar and oxygen isotopic systems. Cosmic ray exposure ages indicate that NWA 6486 was ejected from its parent asteroid ~3–4 Ma ago.

The Formation of Type B CAIs: Evolution from Type A CAIs

Geochimica et Cosmochimica Acta (in Press) lIk to Article [https://doi.org/10.1016/j.gca.2021.12.033]
1Dept. of Mineral Sciences, Museum of Natural History, Smithsonian Institution, Washington, DC, USA 20560
2Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
3WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706
4Carnegie Institution for Science, Earth and Planets Laboratory, 5241 Broad Branch Rd., N.W., Washington, DC 20015, USA
5Kochi Institute for Core Sample Research, JAMSTEC, Nankoku, Kochi 783-8502, Japan
6Vernadsky Institute, Kosygin St. 19, Moscow, Russia
Copyright Elsevier

Five Type A CAIs from three CV3 chondrites (Vigarano, Northwest Africa 3118, Allende), which differ in age by no more than ∼105 years, show mineralogical and textural evidence of gradual transition into Type Bs, indicating that Type B inclusions formed by evolution of Type A CAIs in the solar nebula. This model differs from the conventional condensation model in which aggregates of condensate grains form different kinds of CAIs depending on the relative populations of different kinds of grains. In our model the pyroxene forms nearly isochemically by reaction of perovskite with melilite under highly reducing conditions, and the reaction may be triggered by influx of hydrogen from the gas. Anorthite requires the addition of silica from the gas, and originally forms as veins and reaction rims on gehlenitic melilite within Fluffy Type As. Later partial re-melting of these assemblages results in the formation of poikilitic pyroxene and anorthite that enclose rounded (partially melted) tablets of melilite. Oxygen isotopes in four of the CAIs support the formation of Ti-rich 16O-depleted pyroxene from 16O-depleted perovskite, but not in the fifth CAI. An alternative possibility is that Ti-rich 16O-depleted pyroxene is the result of later solid-state exchange that preferentially affects the most Ti-rich pyroxene. Regardless of the origin of the 16O-depleted pyroxene, we give a model for nebular reservoir evolution based on sporadic FU-Orionis flare-ups in which the 16O-rich region near the proto-Sun fluctuated in size depending on whether the proto-Sun was in flare-up stage or quiescent.

Three-dimensional observation of GEMS grains: Their high-temperature condensation origin

Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.12.031]
1Research Organization of Science and Technology, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-7, Japan
2CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), Guangzhou 510640, China
3CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
4Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo, Kyoto 606-8502, Japan
5Johnson Space Center, NASA, Houston, TX 77058, United States
6Present address: Blue 22 Software, Houston, TX
Copyright Elsevier

GEMS (Glass with Embedded Metal and Sulfides) grains found in interplanetary dust particles are considered one of the most primitive materials in the Solar System, yet questions remain on how they formed. It has been suggested that GEMS grains are products of radiation processing and amorphization of sulfide and silicate mineral grains in the interstellar medium. Alternatively, GEMS grains are proposed to be disequilibrium condensation products in late-stage protosolar disks. We examined the 3D distributions of elements and inclusions within GEMS grains using TEM (transmission electron microscopic)-tomography to better constrain their possible formation processes. We found some core-shell particles composed of metals and amorphous silicates and observed a binary distribution of Mg/Si in amorphous silicates of GEMS grains. These properties are highly similar to the features of experimental condensation products. Furthermore, the location of sulfides only on the surface of GEMS and their larger sizes than metals are also consistent with the condensation experiments, where sulfides formed by sulfidation of metal grains with S-bearing gas species. Textures showing aggregation and possible coalescence of primary grains were also observed. Therefore, we conclude that GEMS grains are condensates from gas at high temperatures and some of them were aggregated.

Nanoscale Infrared Characterization of Dark Clasts and Fine-Grained Rims in CM2 Chondrites: Aguas Zarcas and Jbilet Winselwan

1Mehmet Yesiltas,2Thimothy D. Glotch,3,4Melike Kaya
ACS Earth and Space Chemistry 5, 3281-3296 Link to Article [https://doi.org/10.1021/acsearthspacechem.1c00290]
1Faculty of Aeronautics and Space Sciences, Kirklareli University, Kirklareli 39100, Turkey
2Department of Geosciences, Stony Brook University, Stony Brook, New York 11794, United States
3Institute of Acceleration Technologies, Ankara University, Ankara 06830, Turkey
4Turkish Accelerator and Radiation Laboratory (TARLA), Ankara 06830, Turkey

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