Streaks are ‘strongest evidence yet’ of Mars water

False-color photo of streaks, believed to be flowing water on Mars, at Hale Crater. (Credit: NASA/JPL/University of Arizona)

Scientists have discovered what they say is the strongest evidence yet that there is intermittent flowing liquid water on modern Mars.

Using instruments on board NASA’s Mars Reconnaissance Orbiter (MRO), researchers measured spectral signatures of hydrated minerals on slopes where mysterious, possibly water-related streaks are found on the red planet.

“Salts would keep the water in a liquid form and allow it to creep down Martian slopes.”

The streaks, called recurring slope lineae (RSL), form and snake down the planet’s steep slopes during warm seasons when temperatures exceed -10 degrees Fahrenheit (-23 degrees Celsius) and disappear at colder times during the Martian year.

Lujendra Ojha, a PhD candidate at Georgia Institute of Technology, first noticed these puzzling features in High Resolution Imagine Science Experiment (HiRISE) images returned by the MRO spacecraft when he was an undergraduate at the University of Arizona in 2010.

streaks on Mars
Dark narrow streaks called recurring slope lineae flowing out of the walls of Garni Crater on Mars. The streaks are up to a few hundred meters in length. Photo is color enhanced. (Ccredit: NASA/JPL/University of Arizona)

The spectral signatures of hydrated salts were visible in many RSL if they were relatively wide in diameter. The clincher came when the researchers looked at the same locations when RSL weren’t visible and the hydration signatures had disappeared.

“Something is hydrating these salts, and it appears to be these streaks that come and go with the seasons,” Ojha says. “This means the water on Mars is briny, rather than pure. It makes sense because salts lower the freezing point of water. Even if RSL are slightly underground, where it’s even colder than the surface temperature, the salts would keep the water in a liquid form and allow it to creep down Martian slopes.”

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Up until this point, the recurrence of RSL during the warmest seasons suggested action of a volatile. The still cool ground temperatures suggested brines, but it was difficult to get the footprint of the orbital mapping spectrometers on such narrow features.

The researchers believe that the signatures are caused by hydrated minerals called perchlorates. The hydrated salts most consistent with the chemical signatures are probably a mixture of magnesium perchlorate, magnesium chlorate, and sodium perchlorate. Some perchlorates keep liquids from freezing even when conditions are as cold as -70 degrees Celsius. On Earth, naturally produced perchlorates are concentrated in deserts. Some types of perchlorates can be used as rocket propellant.

Current day water

Perchlorates have previously been discovered on Mars. The Phoenix lander and Curiosity both found them in the planet’s soil, and some scientists believe that the Viking missions in the 1970s measured signatures of these salts. However, this study of RSL detected perchlorates in entirely different areas from where the landers explored. This is also the first time perchlorates have been identified from orbit.

For the study, published in Nature Geoscience, researchers used data acquired by MRO’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) to identify the spectral signatures. The orbiter began circling the planet in 2006. Four years later, RSL were discovered in MRO/HiRISE images, leading some scientists to think that liquid water might be flowing on the present-day Martian surface.

“Almost 40 years after the Viking landers first analyzed the soils on Mars, it’s exciting that we’re able to expand the places where this important resource might be concentrated on the red planet,” says Mary Beth Wilhelm, another Georgia Tech PhD candidate and NASA researcher who is a coauthor of the paper.

The current process for observing spectral signatures is a bit limited, says James Wray, an assistant professor in the School of Earth and Atmospheric Sciences. Because of its fixed orbit, MRO sees the same locations of Mars at the same time every day.

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“We’re only able to receive data on these RSL locations in mid-afternoon, when the air is dry and the morning humidity is almost gone,” he says. “When future spacecraft arrive in the planet’s orbit, we could see larger, wetter RSL at earlier times of the day. This could just be the tip of the iceberg.”

slope lineae flowing downhill on Mars
False color image recurring slope lineae flowing downhill on Mars. (Credit: NASA/University of Arizona)

Regardless, the new findings are more proof that the mysterious lines meandering down Martian slopes are indeed current-day water, Ojha says.

“When most people talk about water on Mars, they’re usually talking about ancient water or frozen water. But there’s more. This is the first spectral detection that unambiguously supports our liquid water-formation hypotheses for RSL.”

Other researchers from NASA Ames Research Center, Johns Hopkins University Applied Physics Laboratory, University of Arizona, Southwest Research Institute, and Laboratoire de Planétologie et Géodynamique are coauthors of the study.

The National Science Foundation Graduate Research Fellowship Program funded the work.

Source: Georgia Tech