Visible and near-infrared spectrogoniometry of natrojarosite, a Martian analogue from Matanomadh, India

Natrojarosite, a hydrous ferric sulfate, is a key mineralogical indicator of past acidic and aqueous environments on Mars. Terrestrial analogue studies of nearly pure natrojarosite are therefore critical for improving spectral interpretations of the Martian surface. This study presents the first comprehensive spectro-goniometric and Hapke radiative transfer modeling of naturally occurring pure natrojarosite from the Matanomadh locality, Kachchh district, Gujarat, India, a recognized Martian analogue site due to its distinctive sulfate mineralogy. Reflectance spectra were acquired in the 0.4–2.5 μm range using a precision spectro-goniometer at the Planetary Remote Sensing Laboratory, Ahmedabad, across multiple viewing geometries and grain-size fractions. Hapke modeling was applied to retrieve single-scattering albedo (SSA), phase function parameters, and extinction coefficient (imaginary component of the refractive index, k). The optical results reveal strong backscattering and significant grain-size-dependent spectral behavior, highlighting the need to account for particle properties in planetary reflectance modeling. This work delivers the first comprehensive dataset of optical constants for naturally occurring natrojarosite, derived from laboratory spectro-goniometric measurements and radiative transfer inversion. These findings advance the spectral library for Martian sulfate terrains and establish Matanomadh natrojarosite as a valuable endmember analogue for Mars remote sensing applications.