New hints of saltwater on Mars

Flows of saltwater may ooze from rocky outcrops on Mars. Seasonal dark streaks on some Martian slopes could come from briny water, American and Swiss researchers suggest in the Aug. 5 Science.

Still, the researchers haven’t actually detected any water, frozen or liquid. Nor can they explain how the water would be replenished in the dry, harsh Martian environment.

But given how the lines grow and fade each year with the seasons, the team’s brine conclusion is “entirely justified,” says Michael Hecht of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who was not involved with the research. If the idea pans out, the stripes will add to previous evidence suggesting salty water continues to be widespread on the Red Planet (SN: 4/11/09, p. 12). So far, no one has found undisputed evidence of liquid water currently active on the planet’s surface.

Looking at images from the Mars Reconnaissance Orbiter, the research team discovered lines that appear in late spring and grow throughout the summer, fading as the weather cools. Some look to be near small channels and all are on steep bedrock, such as crater rims. Hundreds to thousands of the skinny streaks appear at each of seven sites that cluster in the middle southern latitudes, a location akin to the subtropics on Earth. The researchers spotted other candidate sites elsewhere, including to either side of the planet’s equator.

Briny water melting and freezing just underground could explain these seasonal landscape patterns, the team argues. On Mars, liquid water would boil on the surface and freeze just below it, says study coauthor Alfred McEwen, a planetary geologist at the University of Arizona. Brine would evaporate, too, but not as fast. And brine freezes at a lower temperature than pure water does, so it could stick around long enough as a liquid to leave a mark. In the heat of the day, melting brine just below the surface could cause liquid or wet debris to flow down the steep slopes before the saltwater evaporated away.

Not everyone is convinced. Nilton Renno, who collaborated on earlier work arguing for saltwater droplets on the legs of NASA’s Phoenix Mars Lander, thinks the team is overstepping the evidence. He also doubts that in the warmer equatorial regions brine would stay liquid long enough to flow. Without chemical signals from water or onsite measurements by a future lander, Renno, of the University of Michigan in Ann Arbor, doesn’t buy the researchers’ brine conclusion.

“They were very careful,” Hecht counters. Even with unanswered questions about where the water comes from and how it is replenished each year, he says, “the evidence for liquid flow is convincing and exciting.”

“It would be more comforting to have that direct detection,” McEwen says, noting that chemical signs of liquid brine would be best. But that evidence will be hard to come by because the water would be around for only brief periods of time and in small areas.

None of the candidate saltwater streaks are anywhere near Gale Crater, where NASA plans to land its Curiosity rover next year (SN Online: 7/22/11).