¹David Čapek,¹Pavel Koten,¹Jiří Borovička,¹Vlastimil Vojáček,¹Pavel Spurný,¹Rostislav Štork
Astronomy & Astrophysics 625, A106 Link to Article [https://doi.org/10.1051/0004-6361/201935203]
¹Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
Reproduced with permission (C) ESO

Context. A significant fraction of small meteors are produced by iron meteoroids. Their origin and the interaction with the atmosphere have not been well explained up to now.

Aims. The goals of the study are to observe faint, slow, low altitude meteors, to identify candidates for iron meteoroids among them, to model their ablation and light curves, and to determine their properties.

Methods. Double station video observations were used for the determination of atmospheric trajectories, heliocentric orbits, light curves, and spectra of meteors. Meteors with iron spectra or of suspected iron composition based on beginning heights and light curves were modeled. The immediate removal of liquid iron from the surface as a cloud of droplets with Nukiyama–Tanasawa size distribution and their subsequent vaporization was assumed as the main ablation process on the basis of our previous work. The numerical model has only five parameters: meteoroid initial velocity v_∞, zenith distance z, initial mass m_∞, mean drop size D_dr, and luminous efficiency τ. The theoretical light curves were compared with the observed ones.

Results. The model is able to explain the majority of the selected light curves, and meteoroid parameters that are not directly observable – m_∞, D_dr, and τ – are determined. Unlike in most meteor studies, the mass and luminous efficiency are determined independently. Luminous efficiency ranges from 0.08 to 5.8%; it weakly decreases with increasing initial meteoroid mass. No simple dependency on initial velocity was found. The mean size of iron drops depends on the meteoroid velocity. Slower meteoroids can produce drops with a wide range of mean sizes, whereas faster ones are better matched with larger drops with a smaller dispersion of sizes.

^1,7Alian Wang,²Z.C.Ling,^1,7Y.C.Yan,³Alfred S.McEwen,⁴Michael T.Mellon,⁵Michael D.Smith,^1,7Bradley L.Jolliff,⁶JamesHead
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.06.024]
¹Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
²Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai 264209, China
³Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
⁴Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14850, USA
⁵NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶Department of Earth, Environmental and Planetary Sciences, Brown University, RI 02912, USA
⁷The McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
Copyright Elsevier

We report laboratory experimental results that support a brine-related hypothesis for the recurring slope lineae (RSL) on Mars in which the subsurface Cl-salts, i.e., hydrous chlorides and oxychlorine salts (HyCOS) are the potential source materials. Our experiments revealed that within the observed RSL temperature window T_RSL (250–300 K), the deliquescence of HyCOS could occur in relative humidity ranges (RH ≥ 22%–46%) much lower than those for hydrous (Mg, Fe)-sulfates (RH ≥ 75%–96%). In addition, we demonstrated that the RH values kept by common HyCOS and hydrous sulfates in enclosures have a general trend as RH_sulfates > RH_perchlorates > RH_chlorides (with same type of cation) in wide T range. It means that the required RH range for a Cl-bearing salt to deliquescence can be satisfied by a co-existing salt of different type, e.g., in the subsurface layers of mixed salts on Mars. Furthermore, we found a strong temperature dependence of the deliquescence rates for all tested HyCOS, e.g., a duration of 1–5 sols for all HyCOS at the high end (300 K) of T_RSL, and of 20–70 sols for all tested HyCOS (except NaClO₄·H₂O) at the low end (250 K) of T_RSL, which is consistent with the observed seasonal behavior of RSL on Mars. From a mass-balance point of view, the currently observed evidences on Mars do not support a fully-brine-wetted track model, thus we suggest a brine-triggered granular-flow model for the most RSL. Considering the recurrence of RSL in consecutive martian years, our experimental results support the rehydration of remnant HyCOS layers during a martian cold season through H₂O vapor-to-salt direct interaction. We found that the evidences of HyCOS rehydration under Mars relevant P-T-RH conditions are detectable in a few minutes by in situ Raman spectroscopy. This rehydration would facilitate the recharge of H₂O back into subsurface HyCOS, which could serve as the source material to trigger RSL in a subsequent warm season. The major limiting factor for this rehydration is the H₂O supply, i.e., the H₂O vapor density carried by current Mars atmospheric circulation and the diffusion rate of H₂O vapor into the salt-rich subsurface in a cold season. In a worst-case scenario, these H₂O supplies can support a maximum increase of hydration degrees of two for totally dehydrated HyCOS, whereas the full rehydration of subsurface HyCOS layers can be easily reached during a >30° obliquity period that has H₂O vapor density 10× to 20× times the value of current obliquity. Overall, our results imply the existence of a large amount of Cl-bearing salts in the subsurface at RSL sites.

¹Li Q.,¹Dai W.,^1,2,3Liu B.S.,⁴Sarre P.J.,⁵Xie M.H.,¹Cheung A.S-C.
Molecular Astrophysics 13, 22-29 Link to Article [https://doi.org/10.1016/j.molap.2018.09.002]
¹Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China
²Department of Chemistry, Tianjin University, Tianjin 300072, China
³The National Collaborative Innovative Center of Chem. Sci. Eng. Tianjin, Tianjin 300072, China
⁴School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
⁵Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, China

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¹Hyun Na Kim,²Changkun Park,²Sun Young Park,²Hwayoung Kim,¹Min Sik Kim
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE005998]
¹Department of Earth and Environmental SciencesKongju National University, Gongju, Republic of Korea
²Division of Polar Earth‐System Sciences, Korea Polar Research Institute, Incheon, Republic of Korea
Published by arrangement with John Wiley & Sons

Determining the formation mechanism of maskelynite is essential to understanding the shocked environments of meteorites on their parent bodies. Maskelynite has been accepted as a diaplectic glass for several decades, but there have been suggestions that it is a normal glass quenched from a dense melt. Morphological characteristics have been generally investigated to identify the formation mechanism of amorphous plagioclase in meteorites, but the chemical difference between crystalline and amorphous plagioclase has not been fully understood. In this study, we investigated the morphological, atomic‐scale structural, and chemical characteristics of amorphous plagioclase in the lunar meteorite DEW 12007 to constrain its formation mechanism via chemical analysis. The morphological characteristics showed that plagioclase was partially converted into amorphous phase through partial melting. Two‐dimensional Raman mapping confirmed the structural difference between amorphous and crystalline regions. Quantitative chemical analyses revealed that the amorphous regions were more albite‐rich than the crystalline regions, likely due to the partial melting of plagioclase. Under shocked conditions, the partial melting of plagioclase induced a chemical variation between amorphous and crystalline regions. The morphological and structural changes correspond well with the chemical variations, indicating that amorphization induced such variations. The chemical differences between amorphous and crystalline plagioclase in other meteorites also could be understood to be the results of partial melting. Thus, the chemical differences between amorphous and crystalline plagioclase in partially amorphized grains could elucidate the formation mechanism of amorphous plagioclase in many meteorites.

¹S.Potin,^1,2P.Beck,¹B.Schmitt,³F.Moynier
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.06.026]
¹Université Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 414 rue de la Piscine, 38400 Saint-Martin d’Hères, France
²Institut Universitaire de France, Paris, France
³Institut de Physique du Globe de Paris (IPGP), 1 Rue Jussieu, 75005 Paris, France
Copyright Elsevier

Here were report on a laboratory study aiming to reproduce specificities of near-Earth Asteroid. We study how the elevated surface temperature, their surface roughness (rock or regolith), as well as observation geometry can affect the absorption features detected on asteroids. For that purpose, we selected a recent carbonaceous chondrite fall, the Mukundpura CM2 chondrite which fell in India in June 2017. Bidirectional reflectance spectroscopy was performed to analyze the effect of the geometrical configuration (incidence, emergence and azimuth angle) on the measurement. Our results show that reflectance spectra obtained under warm environment (NEA-like) tends to show shallower absorption bands compared to low-temperature conditions (MBA-like), but still detectable in our experiments under laboratory timescales. Irreversible alteration of the sample because of the warm environment (from room temperature to 250 °C) has been detected as an increase of the spectral slope and a decrease of the band depths (at 0.7 μm, 0.9 μm and 2.7 μm). Comparing the meteoritic chip and the powdered sample, we found that surface texture strongly affects the shape of the reflectance spectra of meteorites and thus of asteroids, where a dust-covered surface presents deeper absorption features. We found that all spectral parameters, such as the reflectance value, spectral slope and possible absorption bands are affected by the geometry of measurement. We observed the disappearance of the 0.7 μm absorption feature at phase angle larger than 120°, but the 3 μm band remains detectable on all measured spectra.

¹Timms, N.E. et al. (>10)
Contributions to Mineralogy and Petrology 174, 38 Link to Article [DOI: 10.1007/s00410-019-1565-7]
¹The Institute for Geoscience Research (TIGeR), Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

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¹Jin, Z.,¹Bose, M.
Science Advances 5, eaav8106 Link to Article [DOI: 10.1126/sciadv.aav8106]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States

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