Recurring slope lineae are narrow, dark markings that appear on Martian slopes between 25 and 40 degrees. They were first observed by NASA’s Mars Reconnaissance Orbiter in 2011, and it has since been theorized by many scientists that these lines are caused by brine water on Mars’ surface. However, the source of the water, and the mechanism by which the water creates these RSLs, are under dispute. The markings themselves lengthen and darken between late spring and early fall, and there are thousands of them present on Mars.
There are multiple theories behind the cause of these lineae. One popular theory is that of CO2 defrosting over polar dunes, which can form gullies on the planet, but the problem with this hypothesis is that RSLs appear in areas that are much too warm for CO2 frost to occur. Likewise, the temperatures at which these flow marking occur are much too cold for pure water to be causing them, which is why another theory, the brine water theory, has become popular. Salinity lowers the freezing temperature of water, so if the surface water on Mars was sufficiently salty enough, it is entirely possible for liquid brine to be able to form and move across a steep enough slope. The lineae also appear to be the most active when facing the sun, which …show more content…
A more popular theory is that the water comes from the atmosphere, becoming trapped by hygroscopic salts. (This is similar to a process that creates water tracks on the continent of Antarctica.) There are even some people that argue that it might be a combination of the two, with the atmospheric water concentrating the surface water as it moves along, which helps make the tracks more prominent. Either way, it is still very much uncertain what causes these lineae, though water seems to be the most popular