Low temperature phase transitions in the visible and near-infrared (VNIR) reflectance spectra of (NH4)2HPO4 and (NH4)HSO4 salts

The detection of ammonium bearing crystalline solids in salt-water systems on icy bodies and solar system bodies could provide information about the ascent of these salts from a deep reservoir within the hydrosphere. Due to their chemical-physical properties, NH₄⁺ compounds play a key role both in the internal dynamics of celestial bodies and in the potential habitability of ocean worlds. In this work we analysed the reflectance spectra of two synthetic NH₄⁺ salts: ammonium hydrogen phosphate (NH₄)₂HPO₄ and ammonium hydrogen sulphate (NH₄)HSO₄ in the 1–4.2 μm spectral range at low temperature, between 110 and 290 K. For (NH₄)₂HPO₄ we also examined the effect of three different grain sizes (150–125 μm; 125–80 μm; 80–32 μm). The collected reflectance spectra show absorption features related to NH₄⁺ group overtone and combination modes in the 1–2.5 μm range. In particular, the bands located at ∼1.09 μm (3ν₃), ∼1.30 μm (2ν₃ + ν₄), ∼1.58 μm (2ν₃), ∼2.02 μm (ν₂ + v₃) and ∼ 2.2 μm (v₃ + v₄) could be useful to discriminate these salts. The low temperature spectra, compared to those at ambient temperature, reveal finer structures, displaying sharper and narrower absorption bands. The selected NH₄⁺-bearing salts are subjected to reversible low temperature phase transitions, which are revealed in the spectra by a progressive growth and shift of the bands toward shorter wavelengths with a drastic change of their depth. We performed laboratory measurements of ammonium (NH₄⁺) compounds to address the limited data available expanding the existing database. The collected cryogenic spectra can be directly compared with remote sensing data from planetary missions of the upcoming decade such as NASA's Europa Clipper, and ESA's JUICE and the newly launched James Webb Space Telescope expanding the existing database of ammonium compounds at cryogenic temperature.