Where there's brine, there's water.
Granted, they haven't caught the liquid in the act — and what they've detected looks less like salty water and more like watery salt. But nonetheless, the discovery disclosed in a paper published Monday by the journal Nature Geoscience helps solve a long-standing Martian mystery and sheds light on the potential for life on our nearest planetary neighbor.
Flowing water, "suggests that it would be possible for there to be life today on Mars," said NASA's science mission chief, John Grunsfeld, during a news conference.
The paper was co-written by a scientist at Johns Hopkins Applied Physics Laboratory in Laurel. Scott Murchie helped analyze the data produced by NASA's Mars Reconnaissance Orbiter.
The existence of frozen water on Mars was confirmed by scientists in 2008. But instruments aboard the $720 million orbiter have yielded new and strong evidence that salt water in liquid form flows down certain Martian inclines known as recurring slope lineae each summer, according to the scientists.
The puzzling slopes were first noticed in 2010 by Lujendra Ojha of the Georgia Institute of Technology in Atlanta, lead author of the report on the findings, while he was an undergraduate student using images from the orbiter's camera called the High Resolution Imaging Science Experiment, or HiRise.
They seemed to grow and fade with the seasons.
"We found the hydrated salts only when the seasonal features were widest, which suggests that either the dark streaks themselves or a process that forms them is the source of the hydration," Ojha said. "In either case, the detection of hydrated salts on these slopes means that water plays a vital role in the formation of these streaks."
The flowing water doesn't mean the planet is now considered that much more hospitable — the dark slopes appear in dozens of locations on the planet when temperatures are above minus-10 degrees Fahrenheit, or minus-23 Celsius, which are considered the warmer times.
It's still like a "less tropical Antarctica," said Murchie, a principal investigator at the Hopkins Applied Physics Lab and one of eight co-authors of the paper behind the announcement.
Salt keeps the water from completely freezing, like salt or ice melt on the sidewalks in wintertime, he said. In this case, the salts are perchlorates, which are very salty, and known to keep liquids from freezing even when temperatures reach minus-94 degrees Fahrenheit, or minus-70 Celsius.
NASA said that naturally produced perchlorates on Earth are generally concentrated in deserts and some types can even be used as rocket propellant.
The salinity of the Mars water is uncertain and will require more tests with more sophisticated equipment in space or on the planet's surface, Murchie said.
But he said the new evidence could mean that astronauts wouldn't have to carry water with them to the planet.
"We suspected there was water, but it was still an open debate," Murchie said. "Inside I'm going, 'Hurray!' This is find we wanted to find. This is as close to my wildest dreams of seeing liquid water on the surface."
The findings may further whet the appetite of astrobiologists looking to probe past habitable environments on Mars, researchers said.
"I think it's incredibly exciting, because when we look back at the broad scope of Mars history, it's always in the past where there's evidence for the most water," said Bethany Ehlmann, a planetary geologist at the California Institute of Technology, who was not involved in the paper.
"But if there's liquid water even today, when Mars is supposedly at its driest, I think that says that there was probably liquid water for all of the last 4.5 billion years, just like there was on Earth. Not in the same quantity, but at least ephemerally, episodically, it's there."
Still, the water is so briny that it's difficult to imagine microbes being able to survive in the harsh fluid.
Where exactly the water comes from, how it's released and how it gets back into the soil to repeat the cycle every year also remain open questions, scientists said. Such questions could be answered by a future missions around and to Mars, Murchie said.
One thing is certain about the planet: When the NASA-approved movie "The Martian" premieres Friday, Murchie plans to go see it.