NASA/JPL/University of Arizona

First we learned that Mars may be hiding a lake under its southern ice cap. Now a new study points to an active groundwater system that may be lurking deep below the planet's equatorial regions, feeding a mysterious phenomenon on the Martian surface.

If the study holds water, it would be a huge paradigm shift for the dusty, barren planet. And it could open up new avenues of exploration.

Two researchers at the University of Southern California, Abotalib Z. Abotalib and Essam Heggy, posit that "recurring slope lineae" (RSL), dark streaks that periodically appear on the side of Martian craters, are being created by an active, deep reservoir of salty water.

The study, published Thursday in Nature Geoscience, used images provided by HiRISE, a high-resolution camera aboard the NASA's Mars Reconnaissance Orbiter circling the red planet, to analyze distinct surface features.

The duo examined images of Palikir crater, which has previously been shown to harbor RSL and is a prime location for present-day water flows, to come up with their new hypothesis. They discovered that RSL most commonly emanate from fractures and cracks in Palikir's surface.

"Mars is not hydrologically dead," says Heggy. "There is an active groundwater system on Mars that is causing the recurring slope lineae."

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In the past scientists, have hypothesized a number of reasons the RSL appear on the side of Martian craters, including the idea that they may just be sand flows rather than a phenomenon caused by water. Other groups have suggested they're caused by seeping salt water that originates from just below the surface. However, radar experiments imaging the Martian surface have not been able to identify any shallow pools of groundwater.

Heggy and Abotalib's background, studying aquifers and groundwater flow in Earth's desert environments, prompted them to look at the Mars RSL hypothesis from another angle -- and they began to see similarities with our planet's deserts. Their proposal suggests that the water originates from about half a mile below the surface (approximately 750 meters) and that high pressure forces the liquid up through the soil and eventually through cracks and fractures on the surface -- resulting in the RSL.

"We have seen the same mechanisms in the North African Sahara and in the Arabian Peninsula, and it helped us explore the same mechanism on Mars," says Abotalib.

An important feature of the Mars' RSL is seasonality. Typically, they appear during the Mars summer and disappear in the fall. Heggy suggests that in the fall, when it's cooler, the water freezes over the fractures, preventing the RSL from forming. In summer, warm weather allows the pressurized water to flow up and out through the cracks, seeping down the slopes.

But with all this high-resolution imaging, why haven't we spotted the liquid flows before?

"They last for a very short amount of time due to the condition of the surface and pressure on Mars," explains Heggy. "We don't have sufficient temporal coverage on Mars to witness that happen in real time."

The tantalizing prospect of water on Mars has long been discussed and researched because of the opportunities it presents to discover life or provide a means for colonization. But for Heggy, this study is less about proving Mars is somewhere humans may one day set up shop and more about our own planet.

"Water on Mars is very important, not to colonize Mars, but to understand how our own planet is evolving," he says. "The importance of groundwater is never about using it as a sustainable resource for humans on Mars."

The mystery of the Mars' RSL now has another viable theoretical candidate, but there's still work to be done. NASA Mars rover Curiosity has been exploring regions close to where the RSL form but has not studied them, and direct evidence of an active groundwater system still eludes areologists. Heggy and Abotalib will focus on rectifying that.

"We're going to be trying to see what are the best ways to find direct evidence or map these groundwater systems, potentially using future probing experiments," he says.

It's been a bumper year for Mars water discoveries. In February, researchers at the University of Utrecht suggested they'd discovered evidence that a deep groundwater system had once existed on Mars, given the features they'd observed in craters. On Wednesday, scientists from the University of Chicago suggested that rivers still raged less than 1 billion years ago.

With NASA and the European Space Agency sending rovers to Mars in 2020 -- and researchers scouring the planet for signs of groundwater -- it might not be long before we have definitive evidence that Mars isn't as dry as we once thought.

Originally published 9 a.m. PT

Updated 2:10 p.m. PT: Adds Nature Geoscience paper