U.S. space agency NASA announced on Monday it has found "strong" evidence that there is liquid water intermittently flowing on present-day Mars.
Images taken from the Mars orbit showed dark, finger-like markings that, typically less than five meters in width, appear on slopes during warm seasons, lengthen and then fade during cooler seasons on the Red Planet.
These surface features, known as recurring slope lineae (RSL), have been hypothesized to form by the activity of brines, but direct evidence has been lacking.
Now, using an imaging spectrometer from NASA's Mars Reconnaissance Orbiter (MRO), researchers detected signatures of hydrated minerals on slopes where the mysterious streaks are seen on Mars.
"Our quest on Mars has been to 'follow the water,' in our search for life in the universe, and now we have convincing science that validates what we've long suspected," said John Grunsfeld, associate administrator of NASA's Science Mission Directorate in Washington.
"This is a significant development, as it appears to confirm that water -- albeit briny -- is flowing today on the surface of Mars."
These puzzling surface features were first noticed in 2010 by Lujendra Ojha, then a University of Arizona undergraduate student, using images from the MRO's High Resolution Imaging Science Experiment.
In the new study, Ojha, now a graduate student at Georgia Institute of Technology, and his colleagues analyzed spectra data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the MRO from four different locations where the RSL features were present.
They reported in the British journal Nature Geoscience that signatures of hydrated salts at these RSL locations existed only when the dark features were relatively wide.
"We found the hydrated salts only when the seasonal features were widest, which suggests that either the dark streaks themselves or a process that forms them is the source of the hydration," Ojha said.
"In either case, the detection of hydrated salts on these slopes means that water plays a vital role in the formation of these streaks."
The researchers interpreted the spectral signatures as caused by hydrated minerals called perchlorates, which are likely a mixture of magnesium perchlorate, magnesium chlorate and sodium perchlorate.
Some perchlorates have been shown to keep liquids from freezing even when conditions are as cold as minus 94 degrees Fahrenheit (minus 70 Celsius), NASA said. On Earth, naturally produced perchlorates are concentrated in deserts, and some types of perchlorates can be used as rocket propellant.
Perchlorates have previously been seen on Mars. For example, NASA's Phoenix lander and Curiosity rover both found them in the planet's soil, and some scientists believed that the Viking missions in the 1970s measured signatures of these salts.
"However, this study of RSL detected perchlorates, now in hydrated form, in different areas than those explored by the landers," NASA said. "This also is the first time perchlorates have been identified from orbit."
For Ojha, the new findings are more proof that the mysterious lines he first saw darkening Martian slopes five years ago are, indeed, present-day water.
"When most people talk about water on Mars, they're usually talking about ancient water or frozen water," he said.
"Now we know there's more to the story. This is the first spectral detection that unambiguously supports our liquid water-formation hypotheses for RSL."
