^1,2Nandita Kumari,^2,3Laura B. Breitenfeld,⁴Katherine Shirley,⁵Timothy D. Glotch
Jopurnal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2024JE008814]
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA
²Planetary Science Institute, Tucson, AZ, USA
³Department of Astronomy, Mount Holyoke College, South Hadley, MA, USA
⁴Department of Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, Oxfordshire, USA
⁵Department of Geosciences, Stony Brook University, Stony Brook, NY, USA
Published by arrangement with John Wiley & Sons

The ultramafic and silicic rocks on the lunar surface have played an important role in expanding our knowledge regarding its thermal and magmatic evolution. The surface identification and quantification of these rocks on the global scale can significantly improve our understanding of their spatial extents, relationships and formation mechanisms. Christiansen feature positions using Diviner data have aided in global identification and mapping of relatively silica-rich and silica-poor lithologies on the lunar surface. We have used laboratory thermal infrared spectra of silicic and ultramafic rocks to analyze the variation in Christiansen feature in simulated lunar environment. We have characterized the absolute bulk silica content of the rocks and minerals and their Silica, Calcium, Ferrous iron, Magnesium index. We find that they are linearly correlated to the Christiansen feature despite particle size variations. Furthermore, we find that the Christiansen feature shifts toward longer wavelengths with increase in ilmenite content in the ilmenite-basalt mixtures. We have explored the effect of instrument’s spectral band position on the accuracy of the parabolic method that is currently used for the estimation of Christiansen feature position from Diviner data. We find that this method performs poorly for the estimation of the Christiansen feature for ultramafic and silicic rocks and minerals/mineral mixtures. We propose using a machine learning algorithm to estimate the Christiansen feature with higher accuracy for all kinds of silicate compositions on the Moon. This method will lead to increased accuracy in absolute quantification of bulk silicate composition of the lunar surface at varying spatial scales.

¹Kaydra Barbre,¹Andrew Elwood Madden,¹Caitlin Hodges,¹Megan Elwood Madden
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2024JE008650]
¹School of Geosciences, University of Oklahoma, Norman, OK, USA
Published by arrangement with John Wiley & Sons

Ferrihydrite has been observed within the Martian regolith; therefore, ferrihydrite transformation pathways are likely critical to iron cycling and mineral transformation on Mars and other extraterrestrial systems. Data from Mars rovers and orbiters indicate that ferrihydrite is associated with significant salt deposits. Previous studies show these salts likely formed as the planet desiccated and may rehydrate to form modern brines today that strongly influence(d) mineral alteration. We hypothesize that the salts observed on Mars’ surface may help preserve ferrihydrite for longer periods than typically observed on Earth. This study investigates the effects of brine chemistry on ferrihydrite alteration through laboratory experiments. Lab-synthesized ferrihydrite was reacted with near-saturated brines and ultra-pure water at 20°C for 30 days in a series of batch reactor experiments. X-ray diffraction and Raman spectroscopy showed that ferrihydrite was preserved without evidence of dissolution/transformation in near-saturated solutions of MgSO4, Na2SO4, and NaClO4, while additional iron-oxyhydroxide phases formed in other brines. We also compared mineral reaction products formed from freeze-dried ferrihydrite and undried ferrihydrite slurry. The freeze-dried ferrihydrite was more likely to be preserved, whereas ferrihydrite in a slurry resulted in the precipitation of goethite and lepidocrocite, indicating that particle aggregation and/or drying history affect ferrihydrite stability and alteration. Overall, ferrihydrite remained largely unaltered in the presence of concentrated sulfate and perchlorate brines. In the context of soils/regolith observed on Mars, our research demonstrates that ferrihydrite is more likely to be preserved when found in areas where these salts are dominant, and desiccated in a cold/arid environment prior to brine exposure.