RSL: it’s what all the coolest Mars researchers are talking about these days, but it’s not an acronym for a spacecraft or some new organic compound (or even some hip social media slang ROFLMAO). Rather it’s our best evidence yet that water exists on Mars in liquid form today, right now — even if it’s not yet 100 percent conclusive.

RSL, or recurring slope lineae, are dark streaks that have been observed appearing on the steep inner walls of craters or bedrock outcrops in the southern mid-latitudes on Mars. Images from the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter have repeatedly imaged these stains, which are thought — perhaps even hoped — to be the result of liquid water flowing downhill after the slopes have received enough sunlight to melt any ice tucked just below the surface.

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Any water in RSL is expected to be very salty, which would allow it to stay liquid at considerably low temperatures, and even possibly contain some sort of iron-rich “antifreeze” component.

But despite several years of investigation, the water hasn’t been definitively found… yet.

“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,” said Lujendra Ojha, graduate student at the Georgia Institute of Technology in Atlanta and lead author of two new reports about RSL. He first discovered them in HiRISE images in 2011 while an undergraduate at the University of Arizona.

Watch an animation of seasonal RSL on a Martian slope here.

In recent observations of 13 RSL sites using another of MRO’s high-tech instruments — the Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM – Ojha and Georgia Tech assistant professor James Wray could not identify any clear indication of water, but there were signatures of iron-containing minerals left behind by the flows. And, like a process that results from a freeze-thaw cycle, there seemed to be more of this ferrous material when the seasonal temperatures were higher.

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So what could cause RSL streaking besides water? The culprit could actually be a dry process: liquid-like flows of fine, grainy material that can create markings visible from orbit but don’t actually require water. If dry, lighter-toned surface material were to slide downhill, streaks of darker soil could be left behind — an ongoing process that’s been seen in other locations on Mars.

Still, water could very well be what’s darkening these Martian slopes but it hasn’t been directly detected yet with MRO’s impressive but nearly decade-old instruments — or perhaps it just isn’t looking at the right time.

According to a NASA news release, “the orbital observations have been made only in afternoons and could miss morning moisture.” Maybe on Mars it pays to be an early riser!

The team isn’t giving up yet, though. A direct discovery of liquid water on Mars existing in the present day would be a veritable breakthrough, and would most certainly shape future exploration plans.

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“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,” Wray said. “RSL have rekindled our hope of accessing modern water, but forecasting wet conditions remains a challenge.”

“The flow of water, even briny water, anywhere on Mars today would be a major discovery, impacting our understanding of present climate change on Mars and possibly indicating potential habitats for life near the surface on modern Mars.”

(I hope this article on RSL doesn’t count as TL;DR.)