Distinctive properties of recurring slope lineae (RSL) include slow incremental growth, formation on warm slopes in warm seasons, and annual fading and recurrence, showing a strong correlation with solar heating.
The leading hypothesis involves the flow of brines —very salty water. Salt deposits over much of Mars indicate that brine was abundant in Mars's past. Salinity lowers the freezing point of water to sustain a liquid flow. Less saline water would freeze at the observed temperatures. Thermal infrared data from the Thermal Emission Imaging System (THEMIS) onboard the 2001 Mars Odyssey orbiter, have allowed the temperature conditions under which RSL form to be constrained. While a small number of RSL are visible at temperatures above the freezing point of water, most are not, and many appear at temperatures as low as −43 °C (230 K). Some scientists think that under these cold conditions, a brine of iron sulphate (Fe2(SO4)3) or calcium chloride (CaCl
2) is the most likely mode of RSL formation. Another team of scientists, using the CRISM instrument onboard MRO, reported that the evidence for hydrated salts is most consistent with the spectral absorption features of magnesium perchlorate (Mg(ClO4)2), magnesium chloride (MgCl2(H2O)x) and sodium perchlorate (NaClO