Discovered in 2011, recurring slope lineae (RSL for short) are narrow, dark lines on steep slopes. They appear and grow longer during the warmest time of year, then fade and disappear over winter. They recur in the same places the following Mars year, and appear to emerge from bedrock layers. The best current explanation for the lineae is that they are seeps of brine, water enriched in salts that lower the freezing point and slow its evaporation.
Up to now confirmed RSL have been found on Sun-facing slopes in the middle latitudes of Mars, and mostly in the southern hemisphere. However, a new paper in Nature by a team led by Alfred McEwen (University of Arizona) reports a number of new confirmed and probable sites. Among these, the scientists report the first active RSL sites to be found in the giant equatorial rift canyon, Valles Marineris. The team used images taken by the HiRISE camera on the Mars Reconnaissance Orbiter (MRO).
This result is unusual because scientists have long believed that the surface and near surface in Martian equatorial regions become too warm in summer for groundwater or ground ice to survive more than a geologically brief time. Southern latitude sites may produce more RSLs, the team says, because that hemisphere has the highest ground temperatures, with Mars being closest to the Sun near the start of southern summer.
“The lineae in Valles Marineris seem similar to those in the southern middle latitudes,” say the scientists. In addition, they note, some of the longest RSL known so far occur in Valles Marineris, up to 1.2 kilometers (1 mile) long at a site near Eos and Capri Chasma. Moreover the Valles Marineris RSL remain active throughout the year.
Where the water to make a brine comes from is still unresolved. “Flowing or seeping water or wet debris are attractive models for RSL as they can explain the seasonal darkening and fading,” the team says. The CRISM spectrometer, also on MRO, has not made strong detections of water at RSL sites, but the instrument passes overhead in mid-afternoon.
Lead author McEwen notes, “The lack of [water] absorption bands in CRISM means there is very little free water at the surface at 3 p.m.” He adds, “We need a spectrometer that can observe in the morning in daytime and which has high spatial resolution.”
“An alternative hypothesis,” the scientists write, “is that water comes from the atmosphere, trapped by hygroscopic salts, analogous to water tracks in Antarctica.” They note that Valles Marineris sees water-ice fogs that might recharge shallow subsurface ice by keeping the near-surface air more humid.
In the end, the team says, “If the abundant RSL in Valles Marineris are due to flowing water, then there must be much more water available near the surface in equatorial regions than predicted.”