As usual, NASA is hedging its bets. Until some human being goes to Mars, finds Hale Crater and sticks a finger in the soil and feels the damp salty sand for himself, what we’ll have to be satisfied with is “best evidence yet.” That said, the images and other telemetry from NASA’s Mars Reconnaissance Orbiter (MRO) provide the strongest evidence yet that liquid water flows intermittently on present-day Mars.

Let’s break it down and make it simple.

Dark streaks appear to ebb and flow ever time on various slopes where the temperatures get up to -10 degrees Fahrenheit (that’s -23 Celsius), and disappear when it gets colder than that. If you go with the idea that Mars once had abundant oceans and fresh water, and that most of it was lost along with much of the atmosphere, whatever water was left behind is going to be very very salty – so salty you’d have to call it brine.

Brine freezes at a much lower temperature than fresh water does. It’s the salt in the water that makes it possible for the little water that’s left to flow under certain conditions, so when it’s “hot” during the summer, this brine can melt and flow. The NASA scientists are pretty darn sure that this is what’s causing these dark streaks. Since they’re seasonal, the NASA mission scientist can observe this change over time. The only explanation for this is some kind of fluid that makes these streaks wetter during the Martian summer, and the only fluid that would be active at these temperatures is – you guessed it – water.

“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, astronaut and 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.”

This briny water is flowing downhill and making these streaks. The spectrographic data shows that these are hydrated salts on the slopes. The scientists think it’s pretty likely that the water is a subsurface flow, with enough water wicking to the surface to darken up the streaks on the hillsides. Of course they have to make up a TLA (three letter acronym) to describe them – that’s something space scientists tend to do. In this case the term is Recurring Slope Lineae (RSL), which essentially just means lines on hills that keep showing up. When the streaks disappear, so do the hydrated salt readings. It’s kind of a no brainer to assume that what we are seeing is salt water soaking through the sand on the hillsides.

Dark narrow streaks called recurring slope lineae emanating out of the walls of Garni crater on Mars. The dark streaks here are up to few hundred meters in length. They are hypothesized to be formed by flow of briny liquid water on Mars. The image is produced by draping an orthorectified (RED) image (ESP_031059_1685) on a Digital Terrain Model (DTM) of the same site produced by High Resolution Imaging Science Experiment (University of Arizona). Vertical exaggeration is 1.5. Credits: NASA/JPL/University of Arizona

Dark narrow streaks called recurring slope lineae emanating out of the walls of Garni crater on Mars. The dark streaks here are up to few hundred meters in length. They are hypothesized to be formed by flow of briny liquid water on Mars. The image is produced by draping an orthorectified (RED) image (ESP_031059_1685) on a Digital Terrain Model (DTM) of the same site produced by High Resolution Imaging Science Experiment (University of Arizona). Vertical exaggeration is 1.5.
Credits: NASA/JPL/University of Arizona

“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. In either case, the detection of hydrated salts on these slopes means that water plays a vital role in the formation of these streaks,” said Lujendra Ojha of the Georgia Institute of Technology (Georgia Tech) in Atlanta, lead author of a report on these findings published Sept. 28 by Nature Geoscience.

Ojha first spotted these features as an undergraduate student at the University of Arizona  in 2010, using images from the MRO’s High Resolution Imaging Science Experiment (HiRISE). HiRISE observations now have documented RSL at dozens of sites on Mars. The new study pairs HiRISE observations with mineral mapping by MRO’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM).

The spectral analysis performed by Ojha and his co-authors appear to show that the streaks are actually hydrated minerals called perchlorates. While they can’t tell exactly what substance are made up by the elements they detect, the most likely combination is 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). 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. NASA’s Phoenix lander and Curiosity rover 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, now in hydrated form, in different areas than those explored by the landers. This also is the first time perchlorates have been identified from orbit.

MRO has been examining Mars since 2006 with its six science instruments, helping project scientists to piece together the history of Mars and what appears to have been a very wet world hundreds of thousands of years ago. Extremophile organisms on Earth can survive in harsher conditions than this, so the discovery of liquid water on the surface of Mars is a pretty exciting development. It means that there is still hope for finding existing life on what we had supposed was a barren, dry, lifeless rock.

“It took multiple spacecraft over several years to solve this mystery, and now we know there is liquid water on the surface of this cold, desert planet,” said Michael Meyer, lead scientist for NASA’s Mars Exploration Program at the agency’s headquarters in Washington. “It seems that the more we study Mars, the more we learn how life could be supported and where there are resources to support life in the future.”

For a time in the late 19th century, it was believed that there were canals on Mars. The Italian astronomer Giovanni Schiaparelli, who observed Mars in 1877, was the first to describe, name, and lovingly illustrate mysterious straight lines along its equatorial regions, which he called “canali”. Schiaparelli was actually describing optical artifacts caused by the crappy lenses in his telescope, but it was such an exciting, romantic notion that people would rather talk about the canals as being a real thing.

We are reasonably certain that there are no Tharks on Mars, but this discovery opens the possibility that there may be something alive there, and the human spirit almost demands that we find it if it is.

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