Reflectance spectroscopy of ammonium-bearing minerals: a tool to improve the knowledge of the icy planetary bodies

Recent discoveries demonstrated that the surface of Mars, Ceres and other celestial bodies like asteroids and comets are characterized by the presence of ammonium bearing minerals (Dalle Ore et al., 2018; Berg et al., 2016, Poch et al., 2020). Data collected by New Horizon LORRI and Ralph emphasized the presence of ammonia on Charon, one of the Pluto’s satellites, that is, ammonium chloride, ammonium nitrate and ammonium carbonate have been claimed as the best candidates for its composition (Cook et al., 2018). Moreover, the analysis of the absorption features of Mars spectra at ~1.07, 1.31 and 1.57 μm can be related to ammonium bearing minerals (Sefton-Nash et al., 2012) as well as the presence of oceans underneath the Europa’s crust (Zimmer et al., 2000) suggests a hypothetical composition of water + ammonia, as anti-freezing water element (Sphon and Schubert, 2003). In this scenario, cryovolcanism activity (Jia et al., 2018) can give rise to an interaction between water ammonia and the surface.

This study focuses on, by taking into account sulfates, phosphates, aluminates and borates, understanding how different anionic groups and the different amount of water, affect the ammonium spectra features. Ammonium bearing minerals are of significant interest as hydrogen bonds can affect the NH₄⁺ absorption features and the configuration of the hydrogen bonds, N-H….X, in ammonium salts (e.g. NH₄Cl, NH₄Br), can be quite different (Harlov et al., 2001).

All this, with a careful analysis of remote data compared with the analyses of more accurate laboratory data, should allow a better remote characterization of planetary bodies.

In this work, the reflectance spectra of some ammoniated hydrous and anhydrous salts, namely sal-ammoniac NH₄Cl, larderellite NH₄B₅O₇(OH)₂·H₂O, mascagnite (NH₄)SO₄, struvite (NH₄)MgPO₄·6H₂O and tschermigite (NH₄)Al(SO₄)₂·12H₂O, were collected at room temperature and at 193K. These samples were selected to improve the NH₄-bearing mineral reflectance spectra database and to extend the investigated spectral range with respect to the literature data: e.g. Berg et al., 2016.

We analyzed natural ammonium bearing minerals using reflectance spectroscopy in the long-wave ultraviolet (UV), visible, near-infrared (NIR), and mid-infrared (MIR) regions (~1 – 16 μm) at 298 and 198 K. In addition, thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were made to evaluate the amount of water/ammonium loss and the potential phase transitions occurring in the investigated temperature range. X-ray diffraction analyses were performed on the samples before and after thermal treatments to study the evolution of their crystal structure.

Reflectance spectra of ammoniated minerals show absorption features at 1.3, 1.6, 2.06, 2.14, 3, 3.23, 5.8 and 7.27 μm, related to ammonium group. The 2ν₃ at ~1.56 μm and the ν₃+ν₄ at ~2.13 μm, are the most affected modes by crystal structure type, since their position is strictly related to hydrogen bonds. The reflectance spectra of water-rich samples (struvite (NH₄)MgPO₄·6(H₂O) and tschermigite (NH₄)Al(SO₄)₂·12(H₂O)) show only fundamental absorption features in the area from 2 to 2.8 μm and a strong water feature at 3 μm. An endothermic peak at 192° C was detected in the DSC diagram of sal-ammoniac sample, due to the phase transition from CsCl structure type to NaCl type.

Important was the application of a new proprietary tool (areal mixing model RE-Mix) created to fit remote sensing data coming from planetary bodies with a mixing of the reflectance spectra of single minerals. The RE-Mix tool is based on Hapke model, the most common scattering theory used to calculate synthetic reflectance spectra (Hapke, 1981, 2012). We can assume that the surfaces of planetary bodies contain mixtures of different minerals. In the interpretation of the remote sensing data, it is therefore necessary to assume a mixture of spectra of different minerals. The spectral modelling method used inside Re-Mix is an areal mixing model, which is the most used and the least computationally intensive process. It is based on the least-squares method and the goodness of fit (χ²) is adjusted changing the weight coefficients of the single minerals. The tool is based on Wolfram Mathematica software (Wolfram 1999). A full graphical interface was developed.

The method interprets the remote sensing data from Jupiter’s moon, Europa and Ceres asteroid. We found a number of NH₄-bearing mineral mixtures can fit the planetary spectra together with other mineral species, improving the hypothesis that ammonium species should be among the non-icy materials present on the surface of Galilean moons and mixed with carbonate mineral on Ceres surface.

These knowledges will give us more detailed information from the remote data and suggestions which areas and data should have higher priority for remote investigations in the future space missions.

References:

• Berg, Breanne L., et al. "Reflectance spectroscopy (0.35–8 μm) of ammonium-bearing minerals and qualitative comparison to Ceres-like asteroids." Icarus 265 (2016): 218-237.
• Cook, Jason C., et al. "Composition of Pluto’s small satellites: Analysis of New Horizons spectral images." Icarus 315 (2018): 30-45.
• Dalle Ore, C. Morea, et al. "Ices on Charon: Distribution of H₂O and NH₃ from New Horizons LEISA observations." Icarus 300 (2018): 21-32.
• Hapke, B. (1981). Bidirectional reflectance spectroscopy: 1. Theory. Journal of Geophysical Research: Solid Earth, 86(B4), 3039-3054.
• Hapke, B. (2012). Theory of reflectance and emittance spectroscopy. Cambridge university press.
• Harlov, D. E., M. Andrut, and B. Pöter. "Characterisation of tobelite (NH₄)Al₂ [AlSi₃O₁₀](OH₂) and ND₄-tobelite (ND₄)Al₂ [AlSi₃O₁₀](OD)₂ using IR spectroscopy and Rietveld refinement of XRD spectra." Physics and Chemistry of Minerals 28.4 (2001): 268-276.
• Jia, Xianzhe, et al. "Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures." Nature Astronomy 2.6 (2018): 459-464.
• Poch, Olivier, et al. "Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids." Science 367.6483 (2020).
• Sefton-Nash, E., et al. "Topographic, spectral and thermal inertia analysis of interior layered deposits in Iani Chaos, Mars." Icarus 221.1 (2012): 20-42.
• Spohn, Tilman, and Gerald Schubert. "Oceans in the icy Galilean satellites of Jupiter?." Icarus 161.2 (2003): 456-467.
• Wolfram, Stephen. The MATHEMATICA® book, version 4. Cambridge university press, 1999.
• Zimmer, Christophe, Krishan K. Khurana, and Margaret G. Kivelson. "Subsurface oceans on Europa and Callisto: Constraints from Galileo magnetometer observations." Icarus 147.2 (2000): 329-347.