NASA teased us last week with the promise of a “major science finding” regarding our planetary neighbor, Mars. Today they delivered news of an exciting discovery – and no it’s not Mark Watney or even little green men. Using data collected by the Mars Reconnaissance Orbiter (MRO), scientists have detected evidence of salty water flowing on the surface of Mars.

Water is one of the key ingredients for life as we know it. We’ve known for a while that Mars has frozen water at its poles, and earlier this year the Curiosity rover detected the possibility of salty water below the surface, but this is the first evidence for it flowing on the surface.

During the warm seasons, temperatures on the Red Planet reach about 250-300 Kelvins (-23 to 27 degrees Celsius, -10 to 80 degrees Fahrenheit) and in order for liquid water to survive (even temporarily) on the surface of Mars today, it would have to contain some salt. Both remote and in-situ investigations have shown that various salts, such as perchlorates, sulfates, and chlorides are present on the Martian surface. These salts can significantly lower both the freezing point and the evaporation rate of water, and also easily absorb moisture from the atmosphere.

Recurring slope lineae (RSL) – thin, dark streak-like gullies seen creeping down the sides of craters – have been spotted by MRO in low and mid-latitudes on the Red Planet. High-resolution images from MRO’s HiRISE camera show that the RSL are typically less than 5 meters (16 feet) wide, appear on slopes during the warmer months, lengthen and fade away during the cooler months. Scientists first proposed the idea that the RSL could be a product of seasonal water flows back in 2007, but there was no direct evidence to support that until now.

Spectral data from MRO’s Compact Reconnaissance Imaging Spectrometer for Mars instrument (CRISM) observed four different locations where the RSL are most extensive. The data showed evidence of three different hydrated salts – magnesium perchlorate, magnesium chlorate and sodium perchlorate – at all four locations, indicating that salty water (also called brine) flows are responsible for the RSL activity.

Image credit: NASA/JPL/University of Arizona

Scientists now know that salty water flowing downhill is behind the RSL, but where does the water come from? There are a few possibilities, but researchers have yet to narrow it down. The water could form by the melting of surface or subsurface ice, but the likelihood of near-surface ice around the equatorial region is slim. Another possibility is the seasonal discharge of a local aquifer, but also unlikely. Since the salts have the potential to absorb moisture from the atmosphere through a process known as deliquescence, that could be another possibility.

Here on Earth, in the Atacama Desert, we know that the deliquescence of certain salts offers the only known refuge for active microbial communities. If RSL form as a result of deliquescence of perchlorate salts, they could have major astrobiological implications.

The findings have been published in Nature Geosciences and provide new insight into the current Martian hydrologic cycle.