Reflectance Spectra of Low-temperature Chloride and Perchlorate Hydrates and Their Relevance to the Martian

Introduction: Chlorides and perchlorates present on Mars can form aqueous solutions because of low temperature eutectics and related lower evaporation rates [1]. The recent discovery of chlorides and perchlorates on Mars by NASA’s Phoenix Lander [2] reinforced the potential for chlorides on Mars. Chlorides present in databases lack spectral features (absorption bands) in the near-infrared. Most spectral libraries contain only high temperature “Earthrelevant” chlorides, such as NaCl or KCl. However, at typical Martian relevant temperatures, hydrates are more stable [1]. Similarly, not much is really known about perchlorate spectral features. Experimental: The following salts and their lowtemperature hydrates were synthesized and measured: KCl, NaCl, CaCl2, MgCl2, FeCl2, FeCl3, NH4Cl, NaClO4, KClO4, and Mg(ClO4)2. We also measured eutectic mixtures of ice and salt. Reflectance spectra were taken using a Nicolet 6700 FTIR Spectrometer to allow analysis in the range 1.0-2.5 m. For comparison reasons, chlorides and perchlorates stable at ambient temperature were also measured. In the case of eutectic mixtures, we corrected for ice content using KCl + ice as a control sample since it does not form any hydrate. Results: Chlorides. After correcting the spectra for water ice, we see KCl does not exhibit any significant features, nor does anhydrous NaCl (Fig. 1). However, other anhydrous chlorides (e.g. CaCl2), as well as their hydrated forms, clearly show evidence of additional absorption bands. The most significant bands are located at 1.19, 1.44 and 1.97, the latter two due to hydration. Also, notice that the absorption band depth increases with increasing hydration state.