NASA says surface features on Mars known as seasonal dark flows or "recurring slope lineae” are the “the strongest indication of possible liquid water” on the planet. The RSL were spotted by the Mars Reconnaissance Orbiter and the Mars Odyssey orbiter.

According to the space agency, dark flows, or RSL, are thin markings seen on some slopes on Mars during warmer temperatures. These seasonal lines disappear in the Martian winter and only occur in a few areas of the planet. While the researchers cannot conclusively say the RSL are the result of seasonal liquid water running down Martian slopes during the summer, all the evidence suggests the presence of liquid water.

Lead author Lujendra Ojha, a graduate student at the Georgia Institute of Technology, said in a statement, “We still don't have a smoking gun for existence of water in RSL, although we're not sure how this process would take place without water.”

Ojha’s team observed 13 RSL sites using the MRO’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument and the High Resolution Imaging Science Experiment (HiRISE) camera. The researchers discovered iron-containing minerals, with different oxidation states, in the RSL sites with levels that increased during warmer weather.

Any explanation for these seasonal changes in mineral levels requires water, notes NASA. The darkening could be due to moisture or an increase in ferric minerals, iron-containing material with a higher oxidation state than ferrous minerals. The source of these seasonal dark flows may be near-surface water that did not freeze due to the presence of salt.

Recurring SlopeLineae The CRISM data is mapped over the HiRISE image with purple/pink areas indicating a ferric mineral reading Photo: NASA/JPL-Caltech/UA/JHU-APL

RSL vary year-to-year and the next step in finding liquid water on Mars may require forecasting models that can predict wet areas. James Wray, from Georgia Tech, said in a statement, “"NASA likes to 'follow the water' in exploring the Red Planet, so we'd like to know in advance when and where it will appear. RSL have rekindled our hope of accessing modern water, but forecasting wet conditions remains a challenge.”

Alfred McEwan, from the University of Arizona in Tucson, had previously discovered RSL near the Martian equator. The new research builds off his findings and adds new RSL sites as well as a spectral analysis of these areas. Ojha’s spectral analysis of RSL using MRO’s CRISM instrument will be published in the journal Geophysical Research Letters. The observation of candidate RSL sites and the discovery of new RSL sites using the HiRISE camera will be published in the journal Icarus.