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As the second-closest planet to Earth (after Venus), Mars is the subject of endless fascination to us humans. This has only increased in recent decades, as spacecraft observations have revealed how similar the red planet is to our own in many ways—including its clouds, winds, seasons, days that last roughly 24 hours and familiar natural features like volcanoes and canyons. Since the late 19th century, Mars has been thought of as the most likely place for any extraterrestrial life to exist, as well as the most probable site for any future human habitation.

There’s been one big obstacle in the way: Mars’ lack of water, one of the most crucial components of sustaining life as we know it. Scientists believe that a few billion years ago, Mars used to have a great deal of water, including rivers, lakes and possibly even an ocean. The majority of the planet’s moisture dried up and disappeared over time, and what did remain was believed to have frozen solid. But in a news briefing held this morning at NASA Headquarters in Washington, scientists announced new findings confirming what they believe are definitive signs of liquid water on Mars’ surface.

Using imaging collected by the Mars Reconnaissance Orbiter, the researchers analyzed dark streaks draped along the slopes of Martian canyons, mountains and craters. Formally called recurring slope linae (or RSLs), the streaks appear seasonally, showing up at warmer temperatures and fading at cooler ones. Since 2011, when RSLs were first discovered, scientists have suspected them of being associated with liquid water, but it was difficult to prove.

The researchers’ analysis of the high-resolution images from the orbiter looked at light waves from the streaks and determined the presence of salts known as perchlorates. These chemicals absorb water from the Martian atmosphere and allow the water to remain liquid and flowing even at very low temperatures—the average temperature on Mars is about minus 70 degrees Fahrenheit—but also keep it from boiling off in Mars’ thin atmosphere.

This isn’t the first time scientists have found clues pointing to liquid water on Mars today, but it’s certainly the most convincing case. It’s still unclear where the water comes from. Possibilities include the process known as deliquescence, by which the the perchlorates act like sponges and absorb moisture from the atmosphere; melting subsurface ice; or an aquifer below the surface that freezes during the winter and melts during the summer, when water seeps to the surface. Discovering the water’s source will be the subject of the next round of investigations, according to Michael Meyer, lead scientist for NASA’s Mars Exploration Program.

It also remains to be seen how meaningful the new findings will be in the search for life—past, present or future—on Mars, though the scientists are certainly hopeful. As John Grunsfeld, associate administrator for NASA’s Science Mission Directorate, put it: “The existence of liquid water, even if it is super salty briny water, gives the possibility that if there’s life on Mars…we have a way to describe how it might survive.”

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