Recurring slope lineae (RSL) are enigmatic dark streaks on steep martian slopes that appear during the martian summer and disappear during the winter. RSL have been hypothesized to be flows, but whether they are flows of liquid water, brines, or dry sandy material (or all of these in different locations) is uncertain. Hiroyuki Kurokawa from the Tokyo Institute of Technology and colleagues investigated possible evidence for the wet origin of RSL using atmospheric observations of water vapor and water ice. They hypothesized that if water triggers the formation of RSL, it will evaporate, and the vapor may be observable.

To determine whether the evaporated water vapor would be observable, the team ran computer simulations of the martian atmosphere using a global climate model. Water vapor expected to be released from six potential wet RSL (1 millimeter per hour for liquid water sites and 0.1 millimeter per hour for brine sites) was added to the lowest level of the model. Water vapor was averaged across model grid cells, which were about 67 kilometers (42 miles) wide, and model runs simulating 20 martian days showed where the water would go. Kurokawa and colleagues then compared the amount of water evaporated from RSL to the background amount of water observed by satellites. They found that the amount of water evaporated from RSL was less than a thousandth of the amount of water in the atmosphere and would therefore be undetectable by satellite observations. Furthermore, the amount of water evaporated from RSL would be dwarfed by the exchange of water between the regolith and the atmosphere by a factor of more than ten. Water could accumulate to observable levels in small craters, and searches for water vapor released from RSL should focus on small craters and other basins. READ MORE