Recurring slope lineae and chlorides on the surface of Mars

Recurring Slope Lineae (RSL) are dark streaks that appear on steep slopes on the Martian surface. While their apparent movement suggests liquid flow, remote sensing studies have not found evidence of fresh water. Some RSL have been observed at temperatures below the freezing point of water; if liquid water is present, therefore, it is likely in the form of a brine. In contrast to studies of other dissolved species such as perchlorates, this study explores the possibility of chloride as a dissolved component in RSL. Using Thermal Emission Imaging System decorrelation stretch products and High-Resolution Imaging Science Experiment images, the presence of chloride lag deposits at RSL sites was assessed on a global scale. Though faint potential chloride signatures were detected at two RSL sites, emissivity spectra showed no significant variation from surrounding geology. No other RSL showed indications of chlorides. Using a simple model, the upper limit of chloride required for positive detection is estimated to be ~30 ± 15 g/m². The total brine required to produce a detectable lag deposit is ~80 ± 35 mL/m² at a eutectic concentration of calcium chloride. Over the last climate cycle, brine production could be up to 8 ± 3 × 10⁻⁴ mL/m²-yr without producing a detectable lag deposit. Based on these results, we conclude that chloride abundance is not the limiting factor for RSL generation. RSL could be produced by capillary wicking of chloride-rich brines to the surface, where small amounts of brine produce lag deposits below the detection limits of currently orbiting instruments.