^1,2,3,4,5Yuhuan Yuan,^1,2,3,4Ke Wen,^1,2,3,4Yiping Yang,⁶Chaoqun Zhang,^1,2Xiaorong Qin,^1,2,3,4,5Jianxi Zhu,^1,2,3,4,5Hongping He,⁷Joseph W. Stucki
Journal of Geophysical Research: Planets (in Press) Link to Article [https://doi.org/10.1029/2026JE009683]
¹State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy ofSciences, Guangzhou, PR China,
²Guangdong Provincial Key Laboratory of Mineral Physics and Materials, GuangzhouInstitute of Geochemistry, Chinese Academy of Sciences, Guangzhou, PR China,
³Center for Advanced Planetary Science,Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, PR China,
⁴Guangdong Research Centerfor Strategic Metals and Green Utilization, Guangzhou, PR China,
⁵University of Chinese Academy of Sciences, Beijing,PR China,
⁶Key Laboratory of Deep Petroleum Intelligent Exploration and Development, Institute of Geology andGeophysics, Chinese Academy of Sciences, Beijing, PR China,
⁷Department of Natural Resources and EnvironmentalSciences, University of Illinois at Urbana–Champaign, Urbana, IL, USA
Published by arrangement with John Wiley & Sons

Nontronite, a Fe3+-rich smectite widely identified on Mars, serves as a key mineral indicator for reconstructing paleo-redox and paleo-aqueous environments. However, uncertainties in interpreting its spectral data hinder a precise understanding of its formation conditions and paleoenvironmental implications. To fill this gap, the present study investigated the controls of nontronite formation and its crystallographic-spectral relationships by synthesizing a series of Fe-Si-Al samples with varying Fe/Si molar ratios under hydrothermal conditions. Results demonstrated that crystalline nontronite forms exclusively within a Fe/Si molar ratio of 0.21–0.48 under the simulated alkaline conditions. Incorporation of Fe3+ into tetrahedral sites as [IV]Fe3+ reduced the tetrahedral-octahedral sheet mismatch, thereby enhancing the crystallinity of nontronite. This crystallographic evolution was systematically observed in Mid Infrared and Visible-Shortwave Infrared spectra: [IV]Fe3+ content negatively correlated with the Si-O vibration wavenumber (near 1,000 cm−1) but positively correlated with the 2Fe3+-OH band position (∼1,430 nm) and depth. Furthermore, band depths at ∼1,430 and ∼2,290 nm are robust proxies for the crystallinity of nontronite in the absence of byproducts. These findings constrain the formation of nontronite on Mars to oxidizing, alkaline subsurface hydrothermal environments during the early Noachian, which represents one of the possible pathways for nontronite formation. These results provide a refined framework for interpreting orbital and in situ spectral data, advancing the understanding of clay mineral formation and environmental evolution on Mars.

¹J. Shawcross,^2,3D. J. Adams,^3,4M. L. Wong,^1,5,6K. J. Smith,^1,7Y. L. Yung
Journal of Geopyhsical Research: Planets (in Press) Open Access Link to Article [https://doi.org/10.1029/2025JE009146]
¹Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Departmentof Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
³NHFP Sagan Fellow, NASA HubbleFellowship Program, Space Telescope Science Institute, Baltimore, MD, USA
⁴Earth and Planets Laboratory, CarnegieInstitution for Science, Washington, DC, USA
⁵Department of Water Resources Management, Environmental EngineeringProgram, Central State University, Wilberforce, OH, USA
⁶Department of Planetary Sciences, The University of Arizona,Tucson, AZ, USA, 7NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
Published by arrangement with John Wiley & Sons

Noachian and early Hesperian Mars were likely warm and wet, with an atmosphere abundant in molecular nitrogen. The recent discovery of nitrate deposits in the Yellowknife Bay mudstones at Gale Crater confirm the existence of nitrogen oxides (NO_X) on Noachian Mars. The processes responsible for the production of these nitrates would fractionate nitrogen isotopes: nitrogen oxides will have different isotopic signatures depending on how they were formed—lightning, solar energetic particles (SEPs) and galactic cosmic rays, or photolysis. We used the Caltech–JPL 1D photochemical and transport model KINETICS to simulate nitrogen isotope fractionation in the formation of nitrogen oxide species. At the surface, where deposition occurs, we predict a depletion of δ¹⁵N = −0.845‰ relative to the isotopic composition of atmospheric N₂. Near 200 km altitude, photolysis contributes to positive fractionation. From 300 km to the top of the modeled atmosphere, mass fractionation in the negative direction becomes relevant and produces a depletion of ¹⁵N above 450 km relative to source N₂. Our study predicts a depletion of ¹⁵N in atmospherically derived NO_X relative to the assumed background ratio, which is critical knowledge for constraining the formation history of nitrate on Mars, and whether nitrogen isotopic fractionation could be used as a biosignature.