A number of different hypotheses for RSL formation have been proposed. The seasonality,
latitude distribution, and brightness changes strongly indicate a
volatile material—such as water or liquid —is involved. One hypothesis is that RSL could form by rapid heating of nocturnal frost Another one proposes flows of carbon dioxide, but the settings in which the flows occur are too warm for
carbon-dioxide frost (), and at some sites is too cold for pure 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. 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 (). These observations are the closest scientists have come to finding evidence of liquid water on the planet's surface today.
Source of water Liquid brine flows near the surface might explain this activity, but the exact source of the water and the mechanism behind its motion are not understood. A hypothesis proposes that the needed water could originate in the seasonal oscillations of near-surface
adsorbed water provided by the
atmosphere;
perchlorates and other salts known to be present on the surface are able to attract and hold water molecules from the surrounding environment (
hygroscopic salts), Deeper
groundwater may exist and could reach the surface at springs or seeps, but this cannot explain the wide distribution of RSL, extending from the tops of ridges and peaks. The authors demonstrated that the RSLs stopped at an angle of 28° in Garni crater, in agreement with dry granular avalanche. In addition, the authors pointed out several limitations of the wet hypothesis, such as the fact that the detection of water was only indirect (salt detection but not water). This theory pushed back the dry flow theory. Research published in November 2017 concludes that the observations are best explained by dry flow processes, and remark that there is no actual
spectrographic evidence for water. but the new research article acknowledged that hydrated salts could draw some humidity from the atmosphere and seasonal changes in hydration of salt-containing grains might result in some trigger mechanism for RSL grainflows, such as expansion, contraction, or release of some water, that would change the cohesion of grains and cause them to fall or "flow" downslope. Furthermore, neutron spectrometer data by the
Mars Odyssey orbiter obtained over one decade, was published in December 2017, and shows no evidence of water (hydrogenated regolith) at the active sites, so its authors also support the hypotheses of either short-lived atmospheric water vapour deliquescence, or dry granular flows. Nevertheless, the footprint of this instrument (~100 km) is much larger than the RSLs (~100m). ==Habitability and planetary protection ==