^1,2,3Jinfei Yu,^1,3,4Haibin Zhao,⁵Edward A. Cloutis,^3,6Hiroyuki Kurokawa,^1,7Yunzhao Wu
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.115951]
¹Key Laboratory of Planetary Sciences, Purple Mountain Observatory, CAS, Nanjing 210023, China
²School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
³Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
⁴CAS Center for Excellence in Comparative Planetology, CAS, Hefei 230026, China
⁵Centre for Terrestrial and Planetary Exploration, University of Winnipeg, Winnipeg, Manitoba R3B 2E9, Canada
⁶Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁷State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China
Copyright Elsevier

Carbonaceous chondrites (CCs) are windows into the early Solar System and the histories of their parent bodies. Their infrared spectral signatures are powerful proxies for deciphering their composition and evolution history, but still present formidable challenges in relation to determining the degree of secondary processing such as aqueous alteration and thermal metamorphism via comprehensive data and mid-infrared feature. In our study, we delved into the infrared spectra spanning 1–25 μm of 17 CCs, with distinct petrological characteristics and varying degrees of alteration. Through this investigation, we uncovered distinct spectral patterns that shed light on the processes of alteration and metamorphism. As aqueous alteration intensifies, two key spectral features, the 3 μm-region absorption feature associated with OH-bearing minerals and water, and the 6 μm band indicative of water molecules, both grow in intensity. Simultaneously, their band centers shift towards shorter wavelengths. Moreover, as alteration progresses, a distinctive absorption feature emerges near 2.72 μm, resembling the OH absorption feature found in serpentine and saponite minerals. Comparison of aqueous alteration to laboratory-heated CCs suggests that the 3 μm region OH/H2O absorption feature differs between CC heated to less than or more than ~300 °C. Further insights are gained by examining the vibrational features of silicate minerals, notably influencing the 10 μm and 20 μm regions. The 12.4 μm /11.4 μm reflectance ratio diminishes, and the reflectance peak in the 9–14 μm range shifts towards shorter wavelengths. These changes are attributed to the transformation of anhydrous silicates into phyllosilicates. In the 15–25 μm region, the influence of thermal metamorphism becomes evident and results in the appearance of more spectral features, the single reflectance peak at 22.1 μm undergoes a transformation into two distinct peaks at 19 μm and 25 μm, which is primarily attributed to the increased presence of anhydrous silicates and olivine recrystallization. These findings offer novel insights into the volatile-rich compositions of C-complex asteroids and the thermal evolution histories of their parent bodies.

^1,2Kierra A. Wilk,¹J.F. Mustard,¹R.E. Milliken,¹C.M. Pieters
Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2024.115945]
¹Brown University, Providence, RI, United States of America
²NASA Goddard Space Flight Center, Greenbelt, MD, United States of America
Copyright Elsevier

Spectral variations due to the removal of surface adsorbed H₂O at 3 and 6 μm in reflectance spectra on lunar soils and relevant minerals (olivine, pyroxene, and plagioclase) have been assessed. This study characterizes variations in hydration features as a function of lunar relevant surface temperatures, to further understand current (i.e., M³, HRI-IR, VIMS) and future (i.e., Lunar Trailblazer) observations of diurnal changes in surface hydration. Additionally, we explore the utility of using the 6 μm H₂O feature to discern the speciation of surface hydration at 3 μm. We perform controlled temperature measurements (25–200 °C) in a Linkam THMS600 Environmental Stage fixed to a Bruker LUMOS Microscope Fourier Transform IR (μFTIR) spectrometer. We observe clear and systematic changes in the strength of the 3 μm H₂O/OH feature associated with the thermal removal of adsorbed H₂O, in addition to changes in the overall shape and band position of the feature in both the terrestrial and lunar samples. The strength of the 3 μm feature for the compositionally distinct and relatively brighter Apollo highland soil (62231) is stronger and more symmetric than the 3 μm feature observed for the darker mare soil (10084). While several silicate related absorption features are identified near 6 μm, neither a distinguishable hydration feature nor any changes in reflectance that could be attributed to the presence or a change in the amount of surface adsorbed H₂O were observed at 6 μm.