In the image above, do you see those dark, narrow streaks flowing downhill on the steep slopes at Mars's Horowitz crater? Scientists say they provide definitive evidence of water flowing on the red planet. The findings were published today in the journal Nature Geoscience. NASA also held a press conference today to discuss the discovery.

That they might represent water flow has been suspected for several years. In 2011, a team of researchers working on the University of Arizona's HiRISE (High Resolution Imaging Science Experiment), an imaging system aboard the Mars Reconnaissance Orbiter (MRO), hypothesized that these streaks, known as recurring slope lineae, or RSLs, might be evidence of intermittent salty water flows that change with the seasons: 

The current research team (which includes planetary scientists from HiRISE, a few U.S. universities, NASA, and a French research center) combined the HiRISE documentation of RSLs—which were subsequently found at dozens of sites—with spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), an instrument also onboard the MRO.

RSLs have low reflectance compared to the surrounding terrain, and they appear to get larger during warm seasons. The team analyzed at what wavelengths these RSLs absorb light, and then compared their abilities to absorb different wavelengths to those of minerals on Earth. The closest matches were magnesium perchlorate, magnesium chlorate, and sodium perchlorate—hydrated salts, which were detected at four locations during the seasons when RSLs are most extensive.

"Our findings strongly support the hypothesis that recurring slope lineae form as a result of contemporary water activity on Mars," the researchers write. They don't know where the water originates, or how it formed; the favored theory is that it's the result of deliquescence, in which salts absorb moisture from the atmosphere to create liquid water. This water is likely much saltier than our oceans.

Here are two views of slopes where hydrated salts were detected.

Dark narrow streaks known as recurring slope lineae emanating out of the walls of Garni crater on Mars. The dark streaks here are up to few hundred meters in length. Image credit: NASA/JPL/University of Arizona

Planetary scientists have detected hydrated salts on these slopes at Hale crater. The blue color seen upslope of the dark streaks are thought not to be related to their formation, but instead are from the presence of the mineral pyroxene. This is a false-color image. Image credit: NASA/JPL/University of Arizona

What are the implications of this discovery? Potential life on Mars, of course; either native life—which if it does exist is likely microbial and subsurface—or human life, in the future, as part of a manned Mars mission. 

As for Martian life, "I think it’s likely there’s life in the crust of Mars—microbes," said University of Arizona planetary geologist and study co-author Alfred McEwen, speaking at the press conference from Nantes, France. "To me, the chances of life being in the subsurface of Mars has always been very high."

But as Mars Exploration Program lead scientist Michael Meyer noted, “We have only one example of life, and that is us. We don’t know how it started, and so one of the things we found at Mars is that it could have supported life. But we don’t know how life started here, so we don’t know if it’s possible for life to start on Mars.”

As for the possibility of human life on Mars, "these observations are giving us a much better view that Mars has resources that are useful to future travels," said John Grunsfeld, astronaut and associate administrator of NASA’s Science Mission Directorate. For one thing, he noted, there's the potential to make rocket fuel (which is commonly made from liquid hydrogen and liquid oxygen). "The exciting thing is that we'll send humans to Mars in the near future," Grunsfeld said.

Before humans ever set foot on the red planet, there are several unmanned missions to Mars on the horizon. Next year, NASA will send the InSight lander to Mars to peer into its interior for the first time. The European Space Agency is launching two ExoMars missions—one in 2016 and the other, in collaboration with the Russian Federal Space Agency, in 2018. And in 2020 NASA's Mars Exploration Program continues with the launch of another rover, which will collect samples and bring them back to Earth.

Because the slopes featuring these briny water flows are steep, they're not good landing places for rovers. Nimble-footed astronauts, on the other hand, might one day be able to make the climb for a closer look.

"We are on a journey to Mars, and science is leading the way. Each time we learn something new about Mars, Mars becomes more and more interesting," Grunsfeld said. "I think it's going to provide us with a great sense of our place in the universe and our solar system in particular."