Detection of soluble and fixed NH 4+ in clay minerals by DTA and IR reflectance spectroscopy: a potential tool for planetary surface exploration

Abstract

Nitrogen is an essential element for life. It is the only element among the six major biogenic elements, C, N, S, O, P, H, whose presence in the Martian soil has not been positively and directly established. We describe here a study assessing the ability to detect NH 4 + in soils using two methods: differential thermal analysis (DTA) and infrared (IR) reflectance spectroscopy. Four standard clay minerals (kaolinite, montmorillonite, illite and attapulgite) and an altered tephra sample from Mauna Kea were treated with NH 4 + in this study, then leached in order to remove the non-chemically bound ammonium species. Aliquots of these NH 4 +-treated and then leached samples were analyzed by DTA and IR reflectance spectroscopy to quantify the detectability of soluble and sorbed/fixed NH 4 +. An exotherm at 270–280°C was clearly detected in the DTA curves of NH 4 +-treated (non-leached) samples. This feature is assigned to the thermal decomposition reaction of NH 4 +. Spectral bands observed at 1.56, 2.05, 2.12, 3.06, 3.25, 3.55, 4.2, 5.7 and 7.0 μm in the reflectance spectra of NH 4 +-treated and leached samples are assigned to the sorbed/fixed ammonium in the clays. The montmorillonite spectra have shown the most intense absorption features due to fixed ammonium among the leached samples in this study, as a result of its high cation sorption capacity. It is concluded that the presence of fixed NH 4 + in clays may be detected by IR reflectance or emission spectroscopy. Distinction between soluble and fixed NH 4 + may be achieved through the presence or absence of several spectral features assigned to the fixed NH 4 + moiety and, specifically, by use of the 4.2 μm feature assigned to solution NH 4 +. Thermal analyses furnish supporting evidence of ammonium in the clays/soil through detection of N released at temperatures of 270–330°C. Based on the results of this study, it is estimated that IR spectra measured from a rover should be able to detect ammonium if present above a few mg NH 4 + per g sample in the surface layers. Orbital IR spectra and thermal evolved-gas analyses measured on a rover may be able to detect ammonium in soils as well, but only if present at higher abundances. The spectral features at 3.06 and 7.0 μm due to bound NH 4 + in clays and altered volcanic tephra appear to be the most promising for detection by orbital spectrometers. If N species are present in a solid phase on the Mars surface, sedimentary deposits may be the best regions to look for them.