Liquid water exists on the surface of Mars during the planet's warmer seasons, according to new research published in Nature Geosciences. This revelation comes from new spectral data gathered by NASA’s Mars Reconnaissance Orbiter (MRO), a spacecraft that studies the planet from orbit. The orbiter analyzed the chemistry of weird dark streaks that have been known to appear and disappear seasonally on the Martian surface. The analysis confirms that these streaks are formed by briny — or salty — water flowing downhill on Mars.

NASA has advertised these findings as the solution to a major Mars mystery: does the Red Planet truly have liquid water on its surface? Researchers have known that water exists in ice form on Mars, but it's never been confirmed if water can remain in a liquid state. The space agency is claiming that we now have that answer.

NASA has advertised these findings as the solution to a major Mars mystery

This isn't the first study to suggest liquid water is present in some form on Mars. Scientists have theorized for years that Mars was once home to a large ocean more than 4 billion years ago. And recent findings from the Mars Curiosity rover suggest that liquid water exists just underneath the Martian surface. The discovery of water on Mars has almost become a joke among planetary scientists. Alfred McEwen, a planetary geologist at Planetary Image Research Laboratory who also worked on this research, wrote in Scientific American that the studies have become extremely commonplace: "Congratulations — you've discovered water on Mars for the 1,000th time!" he joked.

Today's findings seem to offer more direct evidence of liquid water than most, though the study only confirms what NASA has long suspected — that flowing liquid water forms the strange, dark streaks that have been observed on Mars. These streaks — called recurring slope lineae — were first observed by the MRO spacecraft in 2010. The lines are blackish and narrow at less than 16 feet across. During the warmer seasons, the streaks grow thicker and longer; they then fade and shrink at times when Mars is colder.

Dark narrow streaks called recurring slope lineae emanating out of the walls of Garni crater on Mars. (NASA/JPL/University of Arizona)

This led scientists to believe years ago that perhaps water and salt were involved in the creation of these lines. "[The streaks] loved forming at temperatures that were right for liquid water to exist," study author Lujendra Ojha, a graduate student at Georgia Tech, told The Verge.

The average temperature on Mars is a frigid -80 degrees Fahrenheit, but on a summer day near the equator, the temperature can reach up to 70 degrees Fahrenheit. Ojha and his team speculated that when conditions are warm enough, liquid water filled with perchlorates — a type of salt — flow downhill on the planet's sloping geological features. Together, water and perchlorates form a brine solution, which has a much lower freezing point than water. This allows the brine to stay in a liquid state even when temperatures grow colder. Ultimately, the streaks are the leftover salt deposits from these briny flows, Ojha believed.

The new study published today offers direct evidence that liquid water is indeed involved. Using the MRO's imaging spectrometer, the researchers studied the chemical makeup of the recurring slope lineae. The visible-infrared spectrometer, which can determine the composition of minerals by observing them in different light wavelengths, showed that the dark streaks were indeed composed of hydrated salts that have molecular water in their crystal structure. "What that seems to be telling us is that water plays a key role in the formation mechanism of these features," said Ojha.

Water strengthens the possibility of finding microbial life on the Red Planet

As for where this water is coming from, Ojha noted there are three possible sources. The perchlorates may be pulling water out of the Martian atmosphere when the air grows particularly humid. The water also may be from a subsurface reservoir of ice that turns into liquid when it comes in contact with the salts. There's even the possibility of an aquifer that is generating the water needed for the briny flows.

Whatever the source, Ojha said the evidence is unambiguous proof that liquid water exists on Mars. And if so, that strengthens the possibility of finding microbial life on the Red Planet. The presence of liquid water on Earth is intimately linked with the formation of life, so the odds are better than ever that extraterrestrial organisms are nearby in our Solar System.

Except we kind of already knew that. But now, we're really, really sure.

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Correction: Alfred McEwen works at the Planetary Image Research Laboratory, not Arizona State University.