^1,2Charity M.Phillips-Lander,¹Jamie L.Miller,¹Megan Elwood Madden
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114478]
¹School of Geology and Geophysics, University of Oklahoma, Norman, OK, United States of America
²Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, United States of America
Copyright Elsevier

Plagioclase minerals have been detected over large portions of the Mars surface. Average plagioclase mineral compositions are approximately An60 for dust free areas on Mars; however, these compositions likely reflect a mixture of more and less sodic plagioclase minerals at the surface. Plagioclase minerals have also been observed in association with chloride and sulfate salts on the Mars surface and in meteorites. Understanding plagioclase dissolution in brines provides insight into post-Noachian weathering on Mars. Batch reactor dissolution experiments were conducted at 298 K to compare albite dissolution rates in water (18 MΩ cm−1ultrapure water (UPW) adjusted to pH 2 with H2SO4; activity of water (ɑH2O) = 1.0), 2.7 mol kg−1 MgSO4 (aH2O = 0.92), 1.24 mol kg−1 MgCl2 (aH2O = 0.92), 2.9 mol kg−1 MgCl2 (aH2O = 0.75), and 5.8 mol kg−1 MgCl2 (aH2O = 0.33) brines at pH 2 to determine how changing solution chemistry and activity of water influence albite dissolution. Aqueous Si-based dissolution rates indicate albite dissolution rates decreased from −8.80 ± 0.02 to −9.49 ± 0.40 log mol m−2 s−1 as the activity of water decreased from 1 to 0.33. Na-based dissolution rates followed the same trend, decreasing from −8.58 ± 0.04 in UPW to −9.45 ± 0.15 log mol m−2 s−1 in 2.9 mol kg−1 MgCl2. Anion chemistry did not appear to effect albite dissolution in high salinity brines at acid pH. Estimated 1 mm albite grain lifetimes increase from ~60–100 to ~587 of years as activity of water decreases, suggesting that post-Noachian weathering on Mars was limited in duration and/or extent.

¹D.L.Laczniak,¹M.S.Thompson,²R.Christoffersen,³C.A.Dukes,²S.J.Clemett,⁴R.V.Morris,⁴L.P.Keller
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114479]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, United States of America
²Jacobs, NASA Johnson Space Center, Mail Code X13, Houston, TX 77058, United States of America
³Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904, United States of America
⁴ARES, Mail Code X13, NASA Johnson Space Center, Houston, TX 77058, United States of America
Copyright Elsevier

We performed H+ and He+ ion irradiation experiments on slabs of the Murchison CM2 meteorite to simulate solar wind irradiation of carbonaceous asteroids. Two separate 6 mm × 6 mm regions were irradiated with 1 keV H+ and 4 keV He+, respectively, to fluences of 8.1 × 1017 ions/cm2 for H+ and 1 × 1018 ions/cm2 for He+. Unirradiated and irradiated surfaces were analyzed using X-ray photoelectron spectroscopy (XPS), visible to near infrared spectroscopy (VNIR; 0.35–2.5 μm), and microprobe two-step laser-desorption mass spectrometry (μL2MS). We also performed analytical field-emission scanning transmission electron microscopy (FE-STEM) of focused ion beam (FIB) cross-sections extracted from olivine grains and matrix material within the H+- and He+-irradiated regions. In situ XPS analyses suggest that ion irradiation results in the removal of most surface carbon and the partial reduction of surface iron to lower oxidation states. In response to He+-irradiation, we observed reddening and brightening of reflectance spectra, which is a departure from typical lunar-style space weathering. Additionally, H+- and He+-irradiation have opposing effects on organic carbon content: H+-irradiation increases the abundance of some free organic species by breaking down macromolecular material while He+-irradiation causes a decrease in overall organic content by cleaving bonds and sputtering constituent atoms. This suggests that solar wind H+-irradiation and solar wind He+-irradiation change the organic functional group chemistry of asteroidal regolith in different ways. In contrast to some previous experimental space weathering studies, we observe an increase in H2O and OH− abundances in our sample in response to both types of ion irradiation. FE-STEM and energy dispersive X-ray spectroscopy (EDX) analyses show complete amorphization of matrix phyllosilicates in ion-affected rims, partial amorphization of olivine, and changes in Si and Mg concentrations at and/or near the surface. We discuss the implications of these results for understanding the complex nature of space weathering of primitive, carbon-rich asteroids and for analyzing future returned samples from carbonaceous asteroids Bennu and Ryugu.