Water flows freely on the surface of Mars. Wait, what?

In recent months, NASA’s Curiosity rover has confirmed that Mars, millions of years ago, used to be covered in free-flowing rivers. According to some new imagery from NASA’s Mars Reconnaissance Orbiter (MRO) and analysis by Alfred McEwen of the University of Arizona, however, it appears that water still flows on the surface of Mars today.

In the image above, the dark lines are postulated to be caused by the flow of salt water down hills and mountains on Mars. These streaks, officially called “recurrent slope linea,” appear to ebb and flow with the seasons, growing longer in Mars’ summer months, and then receding in the winter. We have known about these streaks for a few years, and there have been other theories about their formation, but according to McEwen the best explanation is that ice — trapped about a meter underground — melts in the summer months and streaks down the hill, causing these markings. “Water will darken most soils,” McEwen says.

In the video below and animated GIF above, a sequence of images shows the growth of these dark streaks. The frames were captured over a period of months as NASA’s MRO periodically orbited the same spot on the surface.

Adding credence to the melting-subsurface-water theory, these recurring slope lineae only seem to appear near the Martian equator, where temperatures are relatively warm. Most people don’t realize that the surface of Mars is incredibly cold: Unless it’s summer and you’re near the equator, the air temperature is nearly always below freezing (0 Celsius, 32 Fahrenheit). It’s around -153C (-243F) at the poles, with an overall average temperature for the planet being -55C (-67F). Near the equator, in the summer, at noon, surface temperatures might reach 20C (68F) — warm enough to melt those underground deposits of ice. As the summer wears on, melted water would streak down the hill, until autumn comes and daytime temperatures are no longer high enough to melt the subsurface supply.

The image below shows the location of every Mars rover and lander, and the location of every dark streak identified by MRO’s HiRISE camera. The equator runs roughly through the middle, in between Pathfinder and Opportunity.

If life still exists on Mars, these fertile regions, where ice appears to melt for a few months of the year, are where we’re most likely to find it. The problem with exploring these areas, though, is that it would be incredibly easy to contaminate these patches of damp soil with terrestrial, Earth-based life. If even just a handful of microbes are on the wheels of a future rover, we run the risk of those microbes transferring onto Mars. If that rover, or a future rover, later detects life on Mars… we don’t know if that life is native to Mars, or whether we just cheated and brought some ready-made life from Earth.

The solution to this is sterilizing probes and rovers [What a good band name! -Ed], but as you can imagine it’s very hard to sterilize something as large and unwieldy as a spaceship. Viking 1 and 2, the first probes we sent to Mars, were made safe by encasing them in a pressurized “bioshield,” and then cooking the whole thing at 111 Celsius (232F) for 40 hours. The bioshield was then jettisoned once the probe had left Earth orbit. We could do this for future landers and rovers, but it increases the cost of the mission significantly — an estimated 10% in the case of the Viking probes.

McEwen, working with John Rummel, chair of the Committee on Space Research (COSPAR) panel on planetary protection, are working with NASA to cordon off some areas of Mars as “special regions.” The idea is that landers and rovers would have to be sterilized before they can visit one of these special regions, which would include any area that appears to have surface water (the ice caps, and any area that presents these dark streaks).

There are a large number of Mars landers and rovers scheduled to launch over the next decade, but we don’t currently know where they intend to land (the final landing location is usually chosen quite close to the launch date). As far as we know, there are no plans to sterilize any of these spacecraft, so they’re unlikely to visit a dark streak in search of Martian life — but who knows: That could change!

Now read: Life on Earth originally came from Mars, new study suggests