¹G. J. MacPherson,²K. Nagashima,¹A. N. Krot,³S. M. Kuehner,³A. J. Irving,⁴K. Ziegler,⁵L. Mallozzi,⁶C. Corrigan,⁷D. Pitt
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13943]
¹Smithsonian Institution, Washington, District of Columbia, 20560 USA
²University of Hawai‘i, Mānoa, Honolulu, Hawaii, USA
³University of Washington, Seattle, Washington, 98195 USA
⁴Institute for Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA
⁵Stony Brook University, Stony Brook, New York, 11794 USA
⁶Smithsonian Institution, Washington, District of Columbia, 20560 USA
⁷Maine Mineral and Gem Museum, Bethel, Maine, 04217 USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 8418 is an unusual chondrite whose properties do not exactly match those of any other known chondrite. It has similarities to the CV (Vigarano group), CK (Karoonda group), and CL (Loongana group) chondrites, but its abundance of large calcium-aluminum-rich inclusions (CAIs) and the low NiO content (<0.2 wt%) of its matrix olivine ally it most closely with the CV group. The absence of grossular, monticellite, wollastonite, and sodalite from the alteration products of the CAIs; the magnesium-rich nature of the matrix olivines (Fa38) relative to that of the CV3 chondrites (~Fa50); and the presence of secondary Na-bearing plagioclase and chlorapatite indicate a metamorphic temperature >600 °C. NWA 8418 contains kamacite, taenite, and troilite, and lacks magnetite and pentlandite. We propose that NWA 8418 be reclassified as a reduced CV4 chondrite, which makes it the first CV chondrite of petrologic type 4.

¹S.J.Ralston,¹T.S.Peretyazhko,¹B.Sutter,²D.W.Ming,²R.V.Morris,¹A.Garcia,^1,3A.Ostwald
Earth and Planetary Science Letters 603, 117987 Link to Article [https://doi.org/10.1016/j.epsl.2022.117987]
¹Jacobs, NASA Johnson Space Center, Houston, TX 77058, United States of America
²NASA Johnson Space Center, Houston, TX 77058, United States of America
³University of Nevada, Las Vegas, NV 89154, United States of America
Copyright Elsevier

Smectites are widespread on Mars, but the water-rich, neutral-to-alkaline pH conditions favorable for smectite formation would be expected to have also produced abundant carbonates on early Mars, which are not observed. Smectite formation from basaltic glass on Mars could occur in acidic environments unfavorable for carbonate formation. Acidic smectite formation has been previously demonstrated in batch experiments (closed hydrologic systems), however, the mechanisms and octahedral composition of smectite forming in acidic flow-through (open hydrologic systems) environments are still not fully understood. We conducted hydrothermal (190 °C) alteration experiments on Stapafell basaltic glass at 0.01 and 0.25 mL min⁻¹ flow rates corresponding to low and high water to rock ratio (W/R) flow conditions, and initial pH (pH₀) values of 2, 3, 4 and 6. A batch low W/R experiment was conducted at pH₀ 2 for comparison to the open system experiment. Kaolinite, montmorillonite and chlorite formed at pH₀ 2 at low W/R; no phyllosilicates formed at pH₀ 2 at high W/R; and lizardite formed at pH₀ ≥ 3 at both W/R ratios. Lizardite, kaolinite, and montmorillonite in these experiments formed by precipitation from solution and chlorite likely formed through alteration of montmorillonite and/or basalt. Saponite formed at pH₀ 2 in batch conditions by alteration of basaltic glass. Comparison of experimental data with martian observations of phyllosilicate assemblages indicated that smectite formation on Mars likely occurred under water-limited environmental conditions. Al-rich smectite could form in low W/R open system subsurface environments under a very narrow range of pH (pH < 3) while saponite could form in closed low W/R systems under acidic to alkaline conditions. The combination of open and closed hydrological regimes could be responsible for development of clay mineral stratigraphies observed on Mars. The acidic conditions required for formation of Al-rich smectite montmorillonite were unfavorable for carbonate precipitation, but carbonate precipitation could occur together with Fe/Mg-smectite saponite in closed systems at pH > 4. The lack of widespread carbonates on Mars could not therefore be explained solely by acidic conditions. Phyllosilicate formation under acidic conditions on Mars may affect biosignature stability in martian regolith as preservation capacity is lower in phyllosilicates that formed in or experienced acidic pH.

^1,2,3Kateřina Chrbolková,⁴Patricie Halodová,^1,3Tomáš Kohout,²Josef Ďurech,⁵Kenichiro Mizohata,⁶Petr Malý,⁷Václav Dědič,⁸Antti Penttilä,⁶František Trojánek,⁴Rajesh Jarugula
Astronomy & Astrophysics 665, A14 Open Access Link to Article [DOI https://doi.org/10.1051/0004-6361/202243282]
¹Department of Geosciences and Geography, PO Box 64, 00014 University of Helsinki, Helsinki, Finland
²Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
³Czech Academy of Sciences, Institute of Geology, Rozvojová 269, 16500 Prague, Czech Republic
⁴Research Centre Řež, Hlavní 130, 250 68 Husinec–Řež, Czech Republic
⁵Department of Physics, PO Box 43, 00014 University of Helsinki, Helsinki, Finland
⁶Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague, Czech Republic
⁷Institute of Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic
⁸Department of Physics, PO Box 64, 00014 University of Helsinki, Helsinki, Finland
Reproduced with permission (C)ESO

Context. Airless planetary bodies are studied mainly by remote sensing methods. Reflectance spectroscopy is often used to derive their compositions. One of the main complications for the interpretation of reflectance spectra is surface alteration by space weathering caused by irradiation by solar wind and micrometeoroid particles.

Aims. We aim to evaluate the damage to the samples from H+ and laser irradiation and relate it to the observed alteration in the spectra.

Methods. We used olivine (OL) and pyroxene (OPX) pellets irradiated by 5 keV H+ ions and individual femtosecond laser pulses and measured their visible (VIS) and near-infrared (NIR) spectra. We observed the pellets with scanning and transmission electron microscopy. We studied structural, mineralogical, and chemical modifications in the samples. Finally, we connected the material observations to changes in the reflectance spectra.

Results. In both minerals, H+ irradiation induces partially amorphous sub-surface layers containing small vesicles. In OL pellets, these vesicles are more tightly packed than in OPX ones. Any related spectral change is mainly in the VIS spectral slope. Changes due to laser irradiation are mostly dependent on the material’s melting temperature. Of all the samples, only the laser-irradiated OL contains nanophase Fe particles, which induce detectable spectral slope change throughout the measured spectral range. Our results suggest that spectral changes at VIS-NIR wavelengths are mainly dependent on the thickness of (partially) amorphous sub-surface layers. Furthermore, amorphisation smooths micro-roughness, increasing the contribution of volume scattering and absorption over surface scattering.

Conclusions. Soon after exposure to the space environment, the appearance of partially amorphous sub-surface layers results in rapid changes in the VIS spectral slope. In later stages (onset of micrometeoroid bombardment), we expect an emergence of nanoparticles to also mildly affect the NIR spectral slope. An increase in the dimensions of amorphous layers and vesicles in the more space-weathered material will only cause band-depth variation and darkening.