Brines on Mars

Salty subsurface oceans may exist on the Red Planet and could force scientists to rethink their view of the planet

The Phoenix Mars Lander has been entombed in ice on the Red Planet since November, but the extraordinarily salty tale it has told endures. The surprisingly high concentration of perchlorate salts found in the Martian polar soil by Phoenix’s wet chemistry lab last year — as much as a few percent — could mean that shallow, extremely briny reserves of water lie just below much of the Martian surface, several researchers will report next week at the annual Lunar and Planetary Science Conference in The Woodlands, Texas.

PHOENIX MARS LANDER | This series of images shows a strut on the Phoenix Mars Lander recorded by the craft on days 8, 31, and 44 of the mission. Two spherical blobs (within circled region) appear to merge and might be the first droplets of liquid water detected on the Red Planet. But some researchers contend the blobs are only frost and that changes in lighting may have fooled scientists. Renno et al., JPL/NASA, U. of Arizona

“There seems to be at least a possibility that we have liquid water in the near subsurface over much of the planet,” comments astrobiologist Christopher Chyba of Princeton University, not a member of the Phoenix teams. “That’s a pretty startling change in the way we think about Mars.”

Perchlorates are unique among salts because they can keep water liquid at temperatures as low as –68° Celsius, close to the minimum temperature at the Phoenix landing site last May, when the craft arrived in the arctic region.

Phoenix images taken early in the mission show liquid droplets moving and merging on the legs of the craft, Nilton Renno of the University of Michigan in Ann Arbor and his colleagues will report at the conference. The droplets might have come from subsurface briny pools penetrated by the lander, Renno’s team suggests.

Other Phoenix scientists, including Michael Hecht of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., argue the blobs are likely to be frost, not liquid.

Regardless of the nature of the blobs, Hecht says he is in “violent agreement” with Renno about the possibility of briny pools. Such pools could force scientists “to go back and re-examine a lot of conclusions that we’ve reached about Mars,” Hecht says.

The existence of pools may explain the possible movement of Mars’ polar ice caps, the source of gullies found on the planet a few years ago, the regulation of water vapor in the Martian atmosphere and the prevalence of salt-encrusted soil revealed by the Martian rovers near the planet’s equator.

The high concentration of perchlorates is likely to exist across Martian soil, not just where Phoenix landed, Hecht says. But the proposed perchlorate brines would be “too cold, too salty” to up the chances for finding active life on the planet, asserts astrobiologist Chris McKay of NASA’s Ames Research Center in Mountain View, Calif.

The “activity level” of the briny water, a measure of the amount of water that’s not chemically tied up in salt ions and therefore available to organisms, would be much too low to support the gene repair necessary for life to survive, adds astrobiologist David Des Marais, also of Ames. The highly saline water could, however, act to preserve organic compounds and vestiges of life, he says.

It’s possible that some kind of organism could extract energy by munching on the perchlorate as a kind of power bar, but the bacteria would have to have access to enough water to avoid getting desiccated, Hecht says.

When researchers first announced in 2000 that a camera on the Mars Global Surveyor craft had found evidence of gullies, a few people suggested that salty brines coming out of the Martian hillsides might explain the features. But most researchers rejected that suggestion out of hand because temperatures on the Martian surface were too cold to keep any known brine from freezing. With the discovery of a high concentration of perchlorates, which could maintain an extremely low-temperature brine, the suggestion may need to be taken more seriously, Hecht says.

“That’s where we find ourselves now,” rethinking many of the assumptions made about Mars over the past decade, Hecht says.

He and his colleagues, including David Fisher of the Geological Survey of Canada in Ottawa, suggest that Mars’ polar caps may float on a bed of perchlorate brine or sludge, allowing the caps to migrate. Hecht and his colleagues also suggest that the water soaked up by perchlorate in the soil could regulate the amount of water vapor in the atmosphere, which in turn could have a profound influence on the Red Planet’s global climate. The existence of perchlorate-rich brines may also explain why a gamma-ray spectrometer on the Mars Odyssey mission found a high concentration of hydrogen in the equatorial region of the planet, says Des Marais. The detection of hydrogen at low latitudes on Mars indicates the presence of either liquid water or the hydroxyl ion (OH), which is strongly bound to a mineral or a solute, such as perchlorate, in a highly concentrated brine, he says.

Then there’s this thought: Should any of the proposed near-surface brines harbor organisms, says Chyba, “we’re in a realm where we’ve got to be cleaning our spacecraft much much better,” to avoid contaminating the creatures, he says.

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