¹Ustinova, G.K.,¹Alexeev, V.A.
Doklady Physics 64, 139-143 Link to Article [DOI: 10.1134/S1028335819030029]
¹Institute of Geochemistry and Analytical Chemistry (GEOKHY), Russian Academy of Science, Moscow, 119991, Russian Federation

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¹James McFadden,^1,2IanGarrick-Bethell,²Chae K.Sim,²Sungsoo S.Kim,³DougHemingway
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.05.033]
¹Earth and Planetary Sciences, University of California, Santa Cruz, USA
²School of Space Research, Kyung Hee University, Republic of Korea
³Carnegie Institution for Science, Washington, DC, USA
Copyright Elsevier

Previous work has established that the solar wind and micrometeoroids produce spectral changes on airless silicate bodies. However, the relative importance of these two weathering agents, the timescales over which they operate, and how their effects depend on composition have not yet been well determined. To help address these questions we make use of the fact that solar wind and micrometeoroid fluxes vary with latitude on the Moon. Previous work has shown that this latitudinally varying flux leads to systematic latitudinal variations in the spectral properties of lunar soils. Here we find that the way in which a lunar soil’s spectral properties vary with latitude is a function of its iron content, when we consider soils with 14–22 wt% FeO. In particular, a 50% reduction in flux corresponds to a significant increase in reflectance for 14 wt% FeO soils, while the same flux reduction on 21 wt% FeO soils is smaller by a factor of ~5, suggesting that this brightening effect saturates for high FeO soils. We propose that lower iron soils may not approach saturation because grains are destroyed or refreshed before sufficient nano- and micro-phase iron can accumulate on their rims. We compare our results to the spectral variations observed across the Reiner Gamma swirl, which lies on a high‑iron surface, and find it has anomalous brightness compared to our predictions. Swirls in Mare Marginis, which lie on a low iron surface, exhibit brightness differences that suggest reductions in solar wind flux between 20 and 40%. Our inferences suffer from the limited latitudinal extent of the maria and the convolution of micrometeoroid flux and solar wind flux changes with latitude. Superior constraints on how space weathering operates throughout the inner solar system would come from in situ measurements of the solar wind flux at lunar swirls.

¹M.Telus,¹C.M.O’D.Alexander,¹E.H.Hauri,¹J.Wang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.012]
¹Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA
Copyright Elsevier

To constrain the conditions of aqueous alteration in early planetesimals, we carried out in situ C and O isotope analyses of calcite and dolomite and O isotope analyses of magnetite from the highly altered CM chondrites ALH 83100, ALH 84034, and MET 01070. Petrographic and isotopic analyses of these samples support previous findings of multiple generations of carbonate growth. We observe wide ranges in the C and O isotope compositions of carbonates of up to 80‰ and 30‰, respectively, that span the full range of previously reported bulk carbonate values for CM chondrites. Variations in the Δ17O values indicate that fluid evolution varied for each chondrite. ALH 83100 dolomite-magnetite δ18O fractionation of 23‰ ± 7‰ (2SD) corresponds to dolomite formation temperature of 125°C ± 60°C. δ13C vs δ18O values fall into two groups, one consisting of primary calcite and the other consisting of dolomite and secondary calcite. The positive correlation between δ13C and δ18O for primary calcite is consistent with the precipitation of calcite in equilibrium with a gas mixture of CO (or CH4) and CO2. The isotopic composition of calcite in CM1s and CM2s overlap significantly; however, many CM1 calcite grains are more depleted in δ18O compared to CM2s. Altogether, the data indicate that the fluid composition during calcite formation was initially the same for both CM1s and CM2s. CM1s experienced more episodes of carbonate dissolution and reprecipitation where some fraction of the carbonate grains survive each episode resulting in a highly disequilibrium assemblage of carbonates on the thin-section scale.