Seasonal flows on warm Martian slopes
[4] Research indicates that in the past there was liquid water flowing on the surface of Mars,[5][6][7] creating large areas similar to Earth's oceans.
[14] The HiRISE camera onboard the MRO is at the forefront of the ongoing RSL studies as it helps chart the features with images of closely monitored sites typically taken every few weeks.
[15] The 2001 Mars Odyssey orbiter has been using spectrometers and a thermal imager for over 16 years to detect evidence of past or present water and ice.
[21] RSL appear and lengthen in the late southern spring and summer from 48°S to 32°S latitudes that favor equator-facing slopes, which are times and places with peak surface temperatures from −23 °C to 27 °C.
[24][25] The instrument team found that some false positives were caused by a filtering step when the detector switches from a high luminosity area to shadows.
[3][29] Experiments and calculations demonstrated that recurring slope lineae could be produced by the deliquescence and rehydration of hydrous chlorides and oxychlorine salts.
[32] 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.
[18][21] Deeper groundwater may exist and could reach the surface at springs or seeps,[35][36] but this cannot explain the wide distribution of RSL, extending from the tops of ridges and peaks.
[21] An analysis of near-subsurface data from the Mars Odyssey neutron spectrometer revealed that the RSL sites do not contain any more water than found at anywhere else at similar latitudes.
Research published in November 2017 concludes that the observations are best explained by dry flow processes,[38][39][40] and remark that there is no actual spectrographic evidence for water.
[39][19] Their research shows RSL exist only on slopes steeper than 27 degrees, enough for dry grains to descend the way they do on faces of active dunes.
[39] A 2016 report also cast doubt on possible sources of underground water at RSL sites,[41] 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.
[16] These features form on Sun-facing slopes at times of the year when the local temperatures reach above the melting point for ice.
[42] While the wet flows hypothesis has lost some ground since 2015,[24][25][38][39] these regions are still amongst the most favoured candidate sites to support Earth bacteria brought by contaminated landers.
