IR Reflectance measurements and quantitative analysis of ammonium abundance in two-component mixtures

Introduction:

Recent planetary missions such as Dawn at Ceres [1], Hayabusa2 at Ryugu [2], and OSIRIS-REx at Bennu [3]  have revealed the widespread occurrence of OH-bearing, hydrated, and ammonium-bearing minerals. Ammonia-rich compounds have also been identified on distant bodies including Pluto, Charon [4,5], and Umbriel [6], suggesting a broader presence of ammoniated materials in the outer solar system. These discoveries provide critical constraints on the origin and chemical evolution of planetary bodies.

Forthcoming instruments like the MWIR Imaging Spectrometer for Target-Asteroids (MIST-A) [7] on the Emirates Mission to the Asteroid Belt  (EMA)[8] are expected to provide valuable insights into the presence and distribution of these compounds in that region.

The detection and quantification of volatiles such as OH, H₂O, and NH₃ are fundamental for reconstructing the evolutionary history of planetary surfaces and subsurfaces with implications for astrobiology [9].

Estimating the abundance of these species provides critical constraints on (i) formation and accretion processes, (ii) their surface and subsurface distribution, and (iii) potential associations with possible tenuous exospheres. While laboratory reflectance studies quantifying H₂O content exist [10], relatively few have examined ammonium-bearing compounds (mainly in the VIS-NIR) [11–16], and even fewer have used mid-IR transmittance spectroscopy to determine NH₄⁺ concentrations [17]. This work focuses on quantifying ammonium abundance in controlled two-component mixtures via IR reflectance spectroscopy over the 1.4–11.5 μm range.

Experimental setup and Samples:

Measurements were performed in the 1.4–11.5 μm range using the CAPSULA setup at IAPS CLAB [18,19]. A Bruker INVENIO FTIR spectrometer equipped with a liquid nitrogen-cooled Mercury Cadmium Telluride (MCT) detector, operating at a spectral sampling of 0.13 nm, was employed. The internal SiC lamp of the FTIR system, suitable for mid-IR measurements, provided illumination. Light was transmitted through a CaF₂ viewport into a TVC chamber, where a system of mirrors collected the reflected signal. The measurement geometry was set to i= 45° and e = 0°.

The samples comprised binary mixtures of a synthetic ammonium salt and a second endmember. The considered Ammonium salts are: Ammpnium Sulfate, Ammonium chloride, and monoammonium phosphate. Unlike previous work focused on specific planetary analogues (e.g. Ceres [12,14]), the second component was selected based on its spectral properties—specifically, its flatness in spectral regions where ammonium features occur.

Two materials were chosen as second endmember: Glencoe Rhyolite (Scotland) and Magnetite, representing bright and dark endmembers respectively, both with grain sizes <50 µm. Mixtures were prepared by weight, varying the proportions of ammonium salt and the selected endmember.

An example of studied mixtures is reported in the following table.

Mix 1	Endmember 1 (Rhyolite)	Endmember 2 (NH4)2SO4
End1	100%	0%
Mix1-A	80%	20%
Mix1-B	60%	40%
Mix1-C	40%	60%
Mix1-D	20%	80%
End2	0%	100%

Results:  As an illustrative case, Figure 1 presents the reflectance spectra of mixtures of Glencoe Rhyolite and ammonium sulfate measured with CAPSULA.

Figure 1: Reflectance spectra of Glencoe Rhyolite and Ammonium Sulfate mixtures in air, spanning 1–11 μm. Data normalized at 1.45 μm. Red and blue lines represent the pure endmembers.

It is evident that rhyolite exhibits slight hydration, as indicated by the 3 μm absorption feature, along with several absorptions at longer wavelengths, notably the fundamental SiO₂ band complex near 8–9 μm [20], where the rhyolite background is flat, and there is minimal overlap with water absorptions.

However, it remains spectrally flat between 1–2.5 μm. In the first mixture (Mix 1-A), where ammonium sulfate constitutes 20% by weight (equivalent to 5% NH₄⁺ by weight in the mixture), ammonium absorption bands become clearly discernible. The black arrow in Fig. 1 indicates the increasing of NH4⁺ amount in the spectra from top to bottom. This effect is especially pronounced in the bands at 1.6 μm and 2.1 μm (Fig.2). This facilitates the identification and analysis of NH₄⁺ features.

 

Figure 2: Continuum-removed spectra of Glencoe Rhyolite and Ammonium Sulfate mixtures in air. The upper panel highlights NH₄⁺ vibrational mode 2𝜈_3, while the lower panel shows additional NH₄⁺ features (𝜈₂ + 𝜈₃ and 𝜈₃ + 𝜈₄ vibrational modes) [12,16].

Figure 3 shows the correlation between NH₄⁺ abundance (wt%) and the corresponding spectral band area for three different mixtures of ammonium salts and rhyolite (Mix 1, Mix 2, and Mix 3). The observed positive linear trend indicates that increasing NH₄⁺ concentrations are associated with a proportional increase in the band area, suggesting a consistent and quantifiable spectral response to ammonium content within these mixtures. This relationship provides a basis for interpreting ammonium-related absorptions in planetary remote sensing datasets.

Figure 3: Correlation between NH₄⁺ abundance (wt%) and the corresponding spectral band area for three laboratory-prepared mixtures of ammonium salts with rhyolite (Mix 1, Mix 2, and Mix 3). Data points include 1σ uncertainties on both axes.

Future work: All mixtures will also be analysed in vacuum and heated to high temperature for volatile analysis by mass spectrometry. This process can be useful, particularly at 3 μm, to distinguish the contribution of water from that of ammonia. 

References: [1] De Sanctis M.C. et al., Nature Letter 528, 241-244, 2015. [2] Kitazato K., et al., Science, 364, 272-275, 2019. [3] Hamilton V.E., et al., Nature Astronomy, 2019. [4] Cook J.C., et al., Icarus 315 (2018) 30–45. [5] Cruikshank D.P. et al., 2019, Icarus, 330, 155-168. [6] Cartwright R.J. et al., The Planetary Science Journal, 4:42 (28pp), 2023. [7] Filacchione G., et al., ACM 2023 (LPI Contrib. No. 2851). [8] Al Mazmi et al., 2024, COSPAR, b1.1-0036. [9] Glavin, Daniel P., et al.  Nature Astronomy (2025): 1-12. [10] Milliken R.E.,  (2006). [11] Bishop, J. L., et al., PSS 50 (2002) 11-19. [12] Berg B. L., et al., Icarus 265 (2016) 218-237. [13] Ferrari M., et al., Icarus 321 (2019) 522-530. [14]Poch et al., Science 367, 1212 (2020). [15] De Angelis S., et al., JGR Planets, 126.5 (2021). [16] Fastelli M., et al., Icarus 382 (2022) 105055. [17] Busigny V. et al., American Mineralogist, Volume 89, pages 1625-1630 (2004). [18] De Angelis S., et al. (2022): 16, EPSC2022-540. [19] De Angelis S., et al., Memorie S.A.It, 2024. [20] Salisbury J.W., et al., Journal of Geophysical 94 (1989) 9192-9202.

Acknowledgments: This work has been funded by AMMONHIA (Abundance Mass spectrometry Measurements Of NH In Analogues materials) “Bando Ricerca Fondamentale  INAF 2023”.

The experimental setup used has been funded with the Agreement ASI-INAF n.2018-16-HH.0.