Back in 2011, scientists discovered that liquid water could still exist on the equator on Mars - if only seasonally. The discovery came in the form of dark streaks that appeared on the Martian landscape during the summer months and disappeared during the colder months. Since then, researchers have been further investigating the sites where water appeared to be flowing in search of minerals that could be associated with free-flowing water. The study was led by Lujendra Ojha and James Wray and recent publications have appeared in the journals Geophyiscal Research Letters and Icarus.

What is believed to be water is appearing in narrow trench-like geological features called recurring slope lineae (RSL). The floor and the walls of the RSL become darkened during the summer months, like the dirt and rocks have gotten wet. Though scientists could not definitively say where the water would be coming from, they believed it was the most likely scenario to explain the seasonal darkening.

Following up from the 2011 discovery, Ojha went in search for chemical traces of water or salt, which would indicate flowing water. The first step was going to be to locate the RSL for study in the first place. The High Resolution Imaging Science Experiment (HiRISE) collected images of 200 locations that were scouted based on their mid-latitude in the southern hemisphere and rocky cliff locations, though only 13 were verified to have RSL, which was much lower than they had been anticipating. 

For the 13 confirmed RSL sites, the team used the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) in order to survey the sites for water-associated minerals. Unfortunately, they did not find any traces of water or salts, but they didn’t come away completely empty-handed. Signatures indicating the presence of ferric (Fe³⁺) and ferrous (Fe²⁺) minerals were readily found at RSL sites yet were not quite as prominent in the non-RSL locations. While this does not prove the presence of water, the researchers aren’t sure how it could have happened without it.

Just as the dark streaks in RSL sites appear darker during the summer months, the mineral signals grows stronger during warmer weather. The team also determined that RSL sites cannot be predicted from one year to the next, as not all of the sites are abundant year after year. It is a cat-and-mouse game of trying to locate the RSL and observe it before the mid-afternoon heat, but researchers will be persistent in determining what is causing the RSL sites to darken each summer. Though water would be the simplest explanation, it will be difficult to prove.

This gif shows RSL sites in time lapse, beginning in the winter when the ground is driest and the color is lightest, progressing through spring and summer, as the dark streaks become apparent. Photo credit: NASA/JPL