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The dark streaks – sometimes nearly 100 meters long and five meters wide – stretch down the slopes of Martian craters and hills in images captured by orbiting spacecraft.

Ever since scientists spotted the streaks four years ago, the leading theory has been that they could hold flowing water.

On Monday, NASA scientists said a closer study of those streaks reveals that the surface of Mars has water in liquid form – raising the question of whether that water can support life.

The Globe and Mail spoke to scientists and astronomers about the importance of NASA’s announcement – and what comes next.

WATCH: SCIENTISTS FIND LIQUID WATER ON MARS

What exactly is the substance in the dark streaks?

NASA scientists described a briny water substance – saltier than any of Earth’s oceans. The streaks are thin layers of wet soil that are responsive to the sun’s rays – expanding during the Martian spring and summer and then disappearing for the remainder of the year when the climate is coldest.

Nili Patera, one of the most active dune fields on the planet Mars is shown in this handout photo taken by NASA's Mars Reconnaissance Orbiter March 1, 2014 and provide by NASA May 2, 2014. By monitoring the sand dune changes, NASA can determine how winds vary seasonally and year-to-year. (Reuters)

How did scientists verify the existence of flowing water?

NASA needed to establish strong evidence that the streaks carried water. To do so, it looked to a high-powered instrument on its Mars Reconnaissance Orbiter high above the planet.

“What they did in this research is they looked at the spectra – they looked at the types of light emitted by those patches of ground and they were able to find in them salt that basically only forms in the presence of water,” said Mike Reid, a lecturer in the department of astronomy and astrophysics at the University of Toronto.

“It seems a lot more certain now that those patches – those dark splotches on the sides of the craters – were in fact water,” he added.

The planet Mars is seen in an image from NASA's Hubble Space Telescope taken August 27, 2003. (Reuters)

Where does that water-like substance come from?

“This is the next puzzle to this recent discovery,” said Michael Meyer, lead scientist for the Mars Exploration Program at NASA.

The authors offered three possible sources: melting ice, underground water aquifers and surface molecules absorbing water in the atmosphere, which is the theory the authors lean towards, said Laura Parker, astronomer and professor in the department of physics and astronomy at McMaster University.

The 2008 NASA Phoenix Mars Lander, equipped with technology provided by the Canadian Space Agency, detected a Mars snowfall – a discovery noted at Monday’s NASA press conference.

“It’s an active planet that has some kind of water cycle. So there’s water in the atmosphere, there’s water frozen into ice on the surface, and now there’s evidence for liquid water. So we know water is existing in all three of the states that we’re finding on Earth,” Dr. Parker said.

An undated handout photo of a section of the Gale Crater composed of nearly 900 images taken by the Mars Curiosity Rover. (NYT)

Could it support life on Mars?

The latest news does not offer a definitive answer about the existence of microbial life on Mars.

But where there is water, there can be life.

The discovery of water on the moons of Jupiter and Saturn were important, but distance from the sun and gas-like quality of their environments made life there unlikely, Dr. Reid said.

“Mars is by far the most Earth-like place in the whole solar system. It’s very much like Earth in many regards, and finding that there is liquid water there in some form raises the hope that there either is life there now or that there was life there in the past, which would still be very exciting,” he said.

Paul Delaney, senior lecturer in physics and astronomy at York University, says the finding of water in liquid form is promising.

“If you ask 100 scientists working on Mars, most of them would say that it did exist [in the past] and with today’s news it increases the likelihood of it existing now,” he said.

Evidence of methane and perchlorate salts – which are associated with energy that microbes could use – and liquid water show that some kind of microbial life could be supported on Mars, Dr. Delaney said.

“There are more and more signatures and indications that life could have the building blocks and the necessary environment to survive. But we haven’t answered the question of whether life exists on Mars. Absolutely not,” he said.

An artist's rendering shows what researchers believe Mars looked like around four billion years ago, when water now frozen in the planet's icecaps formed a great northern ocean. (NYT)

How important is the finding?

NASA scientists exuded the kind of enthusiasm that would accompany a major breakthrough.

But Dr. Reid said the announcement is not as “Earth-shattering” as the press releases suggest.

“Every time they make an announcement about water, the hype that it attracts is: This is the first time anyone has found water on Mars,” he said. “Which isn’t true. We have known about water on Mars for literally decades, going back to the Viking landers in the seventies,” he added.

What makes this announcement different is that instead of frozen water or water in the atmosphere, it pertains to liquid water – which is accompanied by a greater likelihood of life on the red planet, Dr. Reid said.

Illustration of the Phoenix lander near the north polar permanent ice cap of Mars. (NASA/JPL)

If some kind of microbial life does exist, where would scientists find it on Mars?

Microbial life on Mars could exist in parts of the planet where liquid water is found near the surface, but it would have to withstand the powerful UV rays of the sun, Dr. Parker said.

Dr. Parker believes it is more likely that if Mars has microbial life, it will be well below the surface.

“On Earth, we have this great atmosphere that protects us, but if you’re on Mars, it’s much safer to spend your time underground,” she said.

Members of Crew 125 EuroMoonMars B mission collect geologic samples for study at the Mars Desert Research Station (MDRS) in the Utah desert. (REUTERS)

What comes next?

The source of the water and whether it supports any life will be the focus of future missions, including a manned one NASA hopes to achieve within the next 25 years.

In the near-term, a NASA Mars mission in 2020 will focus on the planet’s geology and studying the soil for evidence of life. Scientists will equip the Mars land rover to drill into the planet’s surface, study the soil and collect samples that could be returned to Earth.

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