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Effect of halite coatings on thermal infrared spectra
Author(s) -
Berger Jeff A.,
King Penelope L.,
Green Andy,
Craig Michael A.,
Spilde Michael N.,
Wright Shawn P.,
Kunkel Tara S.,
Lee Rachel J.
Publication year - 2015
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2014jb011712
Subject(s) - halite , mineralogy , coating , infrared spectroscopy , substrate (aquarium) , infrared , spectroscopy , optics , materials science , geology , chemistry , composite material , physics , organic chemistry , gypsum , oceanography , quantum mechanics
Abstract Characterizing the occurrence and distribution of soluble salts on planetary surfaces allows us to model or monitor aqueous and geochemical conditions. Thermal infrared (TIR) remote sensing is useful for identifying some types of salt deposits; however, mineral abundance determinations are based on the interaction of infrared with only the top few hundred micrometers of the surface. Thus, distinguishing massive deposits from coatings presents a challenge to TIR remote sensing. To better understand the TIR properties of spectrally transmissive halite coatings, we investigated the effects of coating thickness and texture on the TIR reflectance spectra of halite‐coated glasses. We analyzed two coating textures with variable thickness: (1) continuous and (2) discontinuous particulate coatings. As halite coating thickness increases, the intensity of substrate absorption bands decreases nonlinearly. The substrate is not detected with halite coatings >150 µm thick. Therefore, when coatings are present, it is not possible to apply TIR models of mineral abundances using linear deconvolution algorithms. We show that continuous and coarse particulate halite coatings increase the reflectance minimum (emissivity maximum), making them potentially detectable on planetary surfaces by the methods of Osterloo et al. (2008). However, the reflectance minimum does not change if coatings have low areal coverage (<50%) or are composed of fine particles (<5 µm). Thus, halite that forms as efflorescent coatings or has been redistributed as fine dust is not detected using TIR spectroscopy. Our results explain why Cl salts are not detected with TIR spectroscopy at many locations on Mars despite high Cl contents.