Recurring slope lineae (RSL) are finger-like dark lines on steep slopes that appear and grow longer during the warmest time of year, then fade and disappear over winter. They repeat the following Mars year in the same places. While scientists have not pinpointed the fluid responsible for the flows, the betting runs heavily on brines: water full of salts that lower the freezing point.
Brines may be involved, says David Stillman (Southwest Research Institute, Boulder), but fresh water can do the job on its own. Reporting (PDF) on the work of a team of scientists, Stillman spoke at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.
The team used images from the HiRISE and Context (CTX) cameras, and derived ground temperatures using data from the Thermal Emission Spectrometer (TES), Mars Climate Sounder (MCS), and Thermal Emission Imaging System (THEMIS). The survey revealed the stages the RSL typically go through each Mars year.
“RSL start lengthening when near-maximum (roughly 2 p.m.) temperatures reach 296 Kelvin (73° F),” the scientists note, “and they stop lengthening at 289 K (61° F).” THEMIS mid-afternoon surface temperatures indicate that RSL lengthen only when the temperature is above 273 K, or the freezing point of pure water. Water-saturated soil can be expected to be above freezing to a depth of about 10 centimeters (4 inches) for several hours a day, the team says.
Mid-latitude RSL appear to be associated with surface features suggesting buried ice (for example, ice-rich latitude-dependent mantled units, pedestal craters, concentric crater fill, etc.). The team notes, “This ice is more than 400,000 years old and would likely maintain a saturated subsurface atmosphere.”
Stillman’s group says, “We suggest that the reason RSL emanate from bedrock outcrops is because bedrock has a thermal conductivity about 40 times greater than regolith (dry soil) does.” This allows the wave of annual heat to penetrate in bedrock to a depth of several meters (yards).
During winter, the subsurface temperature of outcropping bedrock will fall below the frost point of the subsurface atmosphere. Consequently, water vapor will condense into the bedrock unit throughout the winter. In spring, subsurface temperatures begin to rise, ultimately melting the ice in the bedrock, allowing it to flow.
Finally, the team concludes, “RSL lengthen when surface temperatures rise above 273 K [32° F]. This suggests high concentrations of brine are not necessary to generate the RSL. Our proposed flow mechanism explains the repeatability of RSL and allows vapor-deposited ice to recharge bedrock, even at topographic highs such as crater central peaks.”