Reflectance spectra (1–5 μm) at low temperatures and different grain sizes of ammonium-bearing minerals relevant for icy bodies

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1Department of Physics and Geology, University of Perugia, I-06123 Perugia, Italy
2Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
3Department of Chemistry, Biology and Biotechnology, University of Perugia, Via Elce di sotto 8, 06123 Perugia, Italy
Copyright Elsevier

It has been proposed that ammonium-bearing minerals are present in a varying amount in icy planetary bodies. Their observation at the surface of large objects was related to the upwelling and cryovolcanism of ammoniated water from possible subsurface oceans forming ammonium-bearing minerals (NH4+) mixed with ice at the surface. We analyzed the temperature evolution of the near-infrared spectra of a selected number of anhydrous and hydrated ammonium-bearing minerals containing different anions and water content. Reflectance spectra were collected in the 1–4.8 μm spectral range at cryogenic temperatures ranging from 293 K to ~65 K and the effect of sample’s grain size between 32 and 150 μm was also investigated at room temperature. Reflectance spectra of anhydrous samples show well-defined absorption bands in the 1–2.5 μm range. The bands located at ~1.06, 1.3, 1.56, 2.02, and 2.2 μm could be useful to discriminate these salts and their characteristics are examined in detail in this work. On the other hand, the reflectance spectra of water-rich samples show H2O fundamental absorption bands strongly overlapping the NH4+ bands, thus dominating the spectra from 1 to 2.8 μm and fully saturating above 2.8 μm. The position of the absorption bands changes with temperature and grain size, shifting to higher frequencies as temperature decreases. The low-temperature spectra also reveal a fine structure compared to the room temperature ones and display narrower and more defined absorption bands. Granulometry mainly affects the band depth and band area parameters. Moreover, mascagnite, salammoniac, ammonium phosphate, tschermigite, and ammonium nitrate are subjected to a reversible low-temperature phase transition, which is manifested in the spectra by a progressive growth and shift of the bands towards shorter wavelengths with an abrupt change in their depth. This new set of spectra at cryogenic temperatures can be directly compared with remote sensing data to detect the presence of ammonium-bearing minerals on the surface of icy bodies. Their identification can impact our knowledge of the internal composition and dynamics of these bodies as well as their potential habitability.


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