A few billion years ago, we wouldn’t have any trouble finding liquid water on Mars. There were lakes and rivers, as indicated by the twisting trails they etched into the surface. There might have even been an ocean. Today is a different story. We know that Mars has water, but it’s locked underground or in the polar ice caps. The idea of liquid water, the essence of life as we know it, on Mars has up until this point has been as wispy as the Martian atmosphere.

But today, scientists are reporting in the journal Nature Geoscience that they have detected definitive signs of flowing, liquid water on the surface of Mars.

In 2011, Dr. Alfred S. McEwen, a professor of planetary geology at the University of Arizona, and his team discovered that as summer time on Mars came and went, so did mysteriously dark streaks on the slopes of the planet’s craters and mountains. These recurring slope lineae or RSLs reappeared each summer as the temperatures on Mars warmed up to a toasty 31 degrees Fahrenheit (~0 degrees Celsius), detectable from orbit as darker brush strokes against red dust.

The recurring slope lineae observed at Hale Crater (dark streaks on the left slope).

Scientists suspected that water played a role in the streaks – the RSLs appeared on warm slopes and extended downslope like liquid would – but at the time the basic photographs from the Mars Reconnaissance Orbiter that identified the RSLs in the frist place just didn’t have the information needed.

If water were flowing on Mars, it would likely have to do with salts, in this case chemical compounds that could only be produced through an interaction with liquid water. Salts such as sulphates, chlorides and perchlorates have been detected on Mars from both orbital investigations and on the surface itself, and can lower the freezing temperature of water significantly. So, if salts could be detected in the RSLs, then it would be direct evidence that salty water was flowing down Martian slopes in summer time, and disappearing as the temperatures got too cold for the salts to help with in winter. But finding them would mean using a different kind of camera.

That’s exactly what Lujendra Ojha, a graduate student at the Georgia Institute of Technology, and his team have now done. Using the Mars Reconnnaissance Orbiter’s spectrometer – a device that analyzes chemical compounds based on the light they absorb – Ojha found evidence for hydrated salts in the RSLs, in four different locations on the Martian surface.

The dark streaks cascading down a  “warm” Garni Crater are now confirmed to be liquid water.

Although we now know that briny water is flowing on Mars, Ojha and his team aren’t sure where it is coming from. “Water could form by the surface/subsurface melting of ice,” the team writes in Nature Geoscience, “but the presence of near-surface ice at equatorial latitudes is highly unlikely.”

It’s also unlikely that either condensation – Mars’ atmosphere is so thin that holding water vapor is a problem – or underground aquifers are the cause of the RSLs.

An ocean RSLs are not, but the presence of liquid water on Mars does give us a new place to look for life. That life would likely be microbial, and would have to thrive in both low temperatures and extremely salty conditions, but the idea isn’t inconceivable.

For NASA, this discovery solidifies the special status that RSLs are already under, though our Martian rovers are currently barred from exploring them for fear of potential contamination, reports the New York Times. But for us, it means renewed hope that Mars isn’t the barren wasteland we once thought, and that we are not alone.

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STUDY: Spectral evidence for hydrated salts in recurring slope lineae on Mars

IMAGES: NASA/JPL-Caltech/University of Arizona