Geochemistry of the Von Kármán crater floor and thickness of the non-mare ejecta over the Chang’e-4 landing area

1Dijun Guo,1,2,3Wenzhe Fa,4Xiaojia Zeng,1Jun Du,4,3Jianzhong Liu
Icarus (in Press) Link to Article []
1Institute of Remote Sensing and Geographical Information System, School of Earth and Space Sciences, Peking University, Beijing, China
2State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
3CAS Center for Excellence in Comparative Planetology, Hefei, China
4Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
Copyright Elsevier

China’s Chang’e-4 mission has carried out the first ever lunar farside landing exploration on the floor of the Von Kármán crater, a geologically complex region located in the most ancient and deepest South Pole-Aitken (SPA) basin. In order to demonstrate the characteristics of materials in the landing area, we investigated the regional geochemistry and thickness of non-mare ejecta overlaying the mare basalts. Comparative analyses of FeO, TiO2 and Th concentrations suggest that the landing site surface is dominated by non-mare ejecta from nearby craters (e.g., Finsen crater) with part of basaltic materials. The ejecta thickness is estimated based on the excavation depth of dark-haloed and non-dark-haloed craters by using support-vector machine, a supervised machine learning method for classification. The results show that the ejecta thickness in the region of 40 km across the landing site varies from near zero to ~80 m with a mean value of ~41 m. The ejecta at the CE-4 landing site is ~40 m thick, which is comparable to the in situ observations by the Lunar Penetrating Radar onboard the Yutu-2 rover. Our results provide valuable information for interpretation of the on-going returned data and geologic analysis of the Chang’e-4 exploration region.

Salt – A critical material to consider when exploring the solar system

Icarus (in Press) Link to Article []
1Institute for Planetary Materials, Okayama University – Misasa, 827 Yamada, Misasa, Tottori 682-0122, Japan
2Research School of Earth Sciences, Australian National University Canberra, ACT 2601, Australia
3Inst. Meteoritics, Univ. New Mexico, Albuquerque, NM 87131, USA
4Lab. d’Astrophys. de Marseille, Pôle de l’Etoile Site de Château-Gombert 38, Marseille cedex 13, France
5NASA Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, United States
Copyright Elsevier

Salt-rich deposits may be more widespread on planetary surfaces than is generally appreciated. Remote observations, laboratory studies of meteorites, and cosmochemical constraints all point towards widespread occurrences of salts (including halides, sulfates, and (bi)carbonates) on asteroids, icy bodies, Mars, and elsewhere. We have investigated the mid-infrared (1.8–25 μm) reflectance spectral properties of mixtures of chondritic (ordinary, enstatite and carbonaceous) meteorites with potassium bromide; a mid-infrared transmissive salt like all halides. Our results demonstrate that halide-chondrite mixtures provide spectral signatures that either reveal the presence of transmissive materials or provide evidence for highly porous regolith. Previously, the nature of the surfaces of the asteroids 624 Hektor and 21 Lutetia was inferred using a limited range of spectra from halide-chondrite mixtures. Here, we provide an extensive dataset of halide-chondrite mixtures to encompass a wider set of possible surface compositions.

Study of Bursa L6 ordinary chondrite by X‐ray diffraction, magnetization measurements, and Mössbauer spectroscopy

Alevtina A. Maksimova et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article []
1Institute of Physics and Technology, Ural Federal University, Ekaterinburg, 620002 Russian Federation
2The Zavaritsky Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg, 620016 Russian Federation
Published by arrangement with John Wiley & Sons

We report the results of the complex study of the bulk interior of Bursa L6 ordinary chondrite using optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, electron microprobe analysis (EMPA), X‐ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. The main and minor iron‐bearing phases and their chemical compositions were determined by these techniques. The detected iron‐bearing phases in the bulk interior of Bursa L6 are the following: olivine; orthopyroxene; Ca‐rich clinopyroxene; troilite; chromite; hercynite; ilmenite; the α2‐Fe(Ni, Co), α‐Fe(Ni, Co), and γ‐Fe(Ni, Co) phases; and ferrihydrite resulting from meteorite terrestrial weathering. Using the EMPA, the values of fayalite and ferrosilite were obtained as ~25.2% and ~21.4%, respectively. The unit cell parameters for silicate crystals were determined from XRD, then the Fe2+ and Mg2+ occupations of the M1 and M2 sites in these crystals were estimated. Further calculations of the ratios of the Fe2+ occupancies in the M1 and M2 sites in olivine and orthopyroxene based on XRD and Mössbauer spectroscopy appeared to be in a good agreement. The temperatures of equilibrium cation distributions for olivine and orthopyroxene obtained from these techniques are consistent: 623 K (XRD) and 625 K (Mössbauer spectroscopy) for olivine and 1138 K (XRD) and 1122 K (Mössbauer spectroscopy) for orthopyroxene.

Study of the Composition of ABA Panu (L3) Meteorite Degassing Products

1A. V. Stennikov,1V. S. Fedulov,1S. G. Naimushin,1N. V. Dushenko,1S. A. Voropaev
Solar System Research 54, 150-154 Link to Article [DOI
1Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 119991, Moscow, Russia

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Halo Meteors

1Siraj, A.,1Loeb, A.
New Astronomy 84, 101545 Link to Article [DOI: 10.1016/j.newast.2020.101545]
1Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, MA 02138, United States

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Mineralogy and Spectroscopy (Visible Near Infrared and Fourier Transform Infrared) of Mukundpura CM2: Implications for asteroidal aqueous alteration

Chemie der Erde (Geochemistry)(in Press) Link to Article []
1Physical Research Laboratory, Ahmedabad, 380009, India
2Indian Institute of Technology, Kharagpur, 721302, India
3Space Application Centre, Ahmedabad, 380015, India
Copyright Elsevier

We report the textures, mineralogy and mineral chemistry of the Mukundpura matrix component, a clast-bearing, brecciated, new CM2 carbonaceous chondrite. Like other CMs, Mukundpura is matrix-enriched and has experienced different degrees of aqueous alteration with evidences of fracturing and compaction of clasts due to the impact. A few relict chondrule clasts and CAIs (diopside and spinel) survived despite of the alteration amidst accessory phases of olivine, magnetite, sulphides and calcite. X-Ray Diffraction (XRD), Visible Near Infrared (VNIR) and Fourier Transform Infrared (FTIR) spectroscopic studies reveal higher phyllosilicate content (∼90%) comprising of both Mg and Fe-serpentine and abundant serpentine-sulphide intergrowths. Even then, the presence of accessory olivine as relict clasts can be interpreted from the presence of certain typical olivine absorptions in the FTIR spectra. The non-stoichiometric, Tochilinite-Cronstedtite occurrences probably relate to broadening of XRD and FTIR spectra and can be explained by coupled Al–Si and Mg–Al substitutions in talc and serpentine. The FTIR spectra suggest widespread transformation of olivine to serpentine, unlike the largely unaltered chondrules. The correlations of mineralogical alteration index with FeO/SiO2 and S/SiO2 in different domains of matrix suggest different extent of alterations. Thus, the aqueous alteration is extensive but not pervasive. The majority of alteration seems to have occurred within the asteroidal parent body. The Mukundpura CM2 thus preserves a unique combination of relict chondrules and highly aqueous altered variegated matrix clasts, although the surface mineralogy resembles the C-type asteroids recently probed by OSIRIS-REx and Hayabusa-2 missions.

CM carbonaceous chondrite falls and their terrestrial alteration

1Martin R. Lee,1,2,3Luke Daly,1Cameron Floyd,1Pierre‐Etienne Martin
Meteoritics & Planetary Science (in Press) Link to Article []
1School of Geographical & Earth Sciences, University of Glasgow, Glasgow, G12 8QQ UK
2Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, GPO BOC U1987, Perth, Western Australia, 6845 Australia
3Australian Centre for Microscopy and Microanalysis, The University of Sydney, Camperdown, New South Wales, 2006 Australia
Published by arrangement with John Wiley & Sons

The CM carbonaceous chondrites provide unique insights into the composition of the protoplanetary disk, and the accretion and geological history of their parent C‐complex asteroid(s). Of the hundreds of CMs that are available for study, the majority are finds and so may have been compromised by terrestrial weathering. Nineteen falls have been recovered between 1838 and 2020, and there is a hint of two temporal clusters: 1930–1942 and 2009–2020. Falls are considered preferable to finds to study because they should be near pristine, and here this assumption is tested by investigating their susceptibility to alteration before recovery and during curation. CMs falling on the land surface are prone to contamination by organic compounds from soil and vegetation. Where exposed to liquid water prior to collection, minerals including oldhamite can be dissolved and most fluid mobile elements leached. Within days of recovery, CMs adsorb water from the atmosphere and are commonly contaminated by airborne hydrocarbons. Interaction with atmospheric water and oxygen during curation over year to decadal timescales can produce Fe‐oxyhydroxides from Fe,Ni metal and gypsum from indigenous gypsum and oldhamite. Relationships between the petrologic (sub)types of pre‐1970 falls and their terrestrial age could be due to extensive but cryptic alteration during curation, but are more likely a sampling bias. The terrestrial history of a CM fall, including circumstances of its collection and conditions of its curation, must be taken into account before it is used to infer processes on C‐complex parent bodies such as Ryugu and Bennu.

Fe‐redox changes in Itokawa space‐weathered rims

1L. J. Hicks,1J. C. Bridges,2T. Noguchi,3A. Miyake,1J. D. Piercy,1S. H. Baker
Meteoritics & Planetary Science (in Press) Link to Article []
1Space Research Centre, School of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH UK
2Faculty of Arts and Science, Kyushu University, 744 Motooka, Nishi‐ku, Fukuoka, 819‐0395 Japan
3Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawaoiwake‐cho, Kyoto, 606‐8502 Japan
Published by arrangement with John Wiley & Sons

Synchrotron Fe‐K X‐ray absorption spectroscopy and transmission electron microscopy have been used to investigate the mineralogy and Fe‐redox variations in the space‐weathered (SW) rims of asteroidal samples. This study focuses on the FIB lift‐out sections from five Itokawa grains, returned by the Hayabusa spacecraft, including samples RB‐QD04‐0063, RB‐QD04‐0080, RB‐CV‐0011, RB‐CV‐0089, and RB‐CV‐0148. Each of the samples featured partially amorphized SW rims, caused by irradiation damage from implanted low mass solar wind ions, and the impacting of micrometeorites. Using bright‐field and HAADF‐STEM imaging, vesicular blistering and nanophase Fe metal (npFe0) particles were observed within grain rims, and solar flare tracks were observed in the substrate host grain, confirming the presence of SW zones. We use Fe‐K XANES mapping to investigate Fe‐redox changes between the host mineral and the SW zones. All SW zones measured show some increases in the ferric‐ferrous ratio (Fe3+/ΣFe) relative to their respective host grains, likely the result of the implanted solar wind H+ ions reacting with the segregated ferrous Fe in the surface material.

Presolar stardust in highly pristine CM chondrites Asuka 12169 and Asuka 12236

1Larry R. Nittler,1Conel M. O’D. Alexander,1,2Andrea Patzer,1,3Maximilien J. Verdier‐Paoletti
Meteoritics & Planetary Science (in Press) Link to Article []
1Earth and Planets Laboratory, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington, District of Columbia, 20015 USA
2Geosciences Center Göttingen, University of Göttingen, Goldschmidtstr. 1, 37077 Göttingen, Germany
3Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum national d’Histoire naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD, UMR 206, 75005 Paris, France
Published by arrangement with John Wiley & Sons

We report a NanoSIMS search for presolar grains in the CM chondrites Asuka (A) 12169 and A12236. We found 90 presolar O‐rich grains and 25 SiC grains in A12169, giving matrix‐normalized abundances of 275 (+55/−50, 1σ) ppm or, excluding an unusually large grain, 236 (+37/−34) ppm for O‐rich grains and 62 (+15/−12) ppm for SiC grains. For A12236, 18 presolar silicates and 6 SiCs indicate abundances of 58 (+18/−12) and 20 (+12/−8) ppm, respectively. The SiC abundances are in the typical range of primitive chondrites. The abundance of presolar O‐rich grains in A12169 is essentially identical to that in CO3.0 Dominion Range 08006, higher than in any other chondrites, while in A12236, it is higher than found in other CMs. These abundances provide further strong support that A12169 and A12236 are the least‐altered CMs as indicated by petrographic investigations. The similar abundances, isotopic distributions, silicate/oxide ratios, and grain sizes of the presolar O‐rich grains found here to those of presolar grains in highly primitive CO, CR, and ungrouped carbonaceous chondrites (CCs) indicate that the CM parent body(ies) accreted a similar population of presolar oxides and silicates in their matrices to those accreted by the parent bodies of the other CC groups. The lower abundances and larger grain sizes seen in some other CMs are thus most likely a result of parent‐body alteration and not heterogeneity in nebular precursors. Presolar silicates are unlikely to be present in high abundances in returned samples from asteroids Ryugu and Bennu since remote‐sensing data indicate that they have experienced substantial aqueous alteration.