Reflectance spectra of mascagnite and salammoniac minerals with varying viewing geometry

• Published in: Icarus (New York, N.Y. 1962) Vol. 403; p. 115633
• Main Authors: Fastelli, M., Schmitt, B., Beck, P., Poch, O., Zucchini, A., Frondini, F., Comodi, P.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01-10-2023; Elsevier
• Subjects: Icy bodies; Mascagnite; Reflectance spectra; Salammoniac; Sciences of the Universe; Viewing geometry; Salammoniac; Icy bodies; Mascagnite; Reflectance spectra; Viewing geometry; reflectance spectra; salammoniac; viewing geometry; icy bodies; mascagnite
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2023.115633

We analyse the quantitative effects of viewing geometry variations on the near-infrared reflectance spectra of mascagnite-(NH4)2SO4 and salammoniac-NH4Cl samples. Bi-directional reflectance spectra are collected in the 1–4.2 μm range considering a set of 3 incidence (i) angles (i = 0°; 30°; 60°) and 9 emergence (e) angles between −70° and 70° at room temperature and computed with respect to the normal direction. The NH4+ overtone and combinations bands located at ∼1.09, 1.32, 1.62, 2.04, 2.2 and 3.05 μm are experimentally investigated. The bidirectional reflectance spectra of these samples show significant variations with the observation geometry in terms of reflectance values, spectral slope, and absorption bands parameters. The band positions remain essentially unchanged by varying the incident and emergence angles. On the other hand, bands' area and depth show the highest variability for i ≥ 30° and e greater than ±40°(up to a factor 2.3 in relative mean variation). The area and depth parameters of these bands show a dual behaviour: (i) for the weak-medium spectral features below 2 μm the area and depth decrease as the phase angle increases. (ii) The strong spectral features above 2 μm increase their values only at phase angles above 90°, but also at low phase angles for high incidences, i ≥ 30°. This behaviour is linked both to the non-linear radiative transfer in particulate media and to the way the band depth and area are defined, relative to the local continuum. We observe important dependence (up to ∼60% relative mean variation) of band depth and area on the incidence angle, up to 60°, compared to moderate variation with emergence angles (up to ∼20% relative mean variation). Furthermore, the ∼3 μm features becomes more saturated at ±70° emergence angles. A general trend of spectral bluing with change in observation geometry is observed. The current dataset is a contribution in the framework of present and future space missions focused on understanding the nature and quantification of ammonium-bearing minerals on icy bodies. The NH4+ − bearing minerals identification could provide information on: (i) ocean/brine compositions, (ii) possible explanations of geological phenomena and (iii) implications for biological activity.
•(NH4)2SO4 and NH4Cl minerals were characterized through BRDF spectroscopy with different incidence (i) and emergence (e) angles.•Relationship between the viewing geometry variations and the spectral parameters (band area, depth, slope) of NH4+ features were observed.•High spectral variability was observed for i ≥ 30° and e ≥ 60° angles.•Results from such investigation will be useful to interpret occurrence of ammonium salts on planetary surfaces, especially small bodies.