More than a pinch

Scientists suggest liquid water on Mars would have been too salty for life

May 29, 2008 at 1:05 pm

Men aren’t from Mars, and nor are any other types of Martians, a new study suggests.

EL CAPITAN This image, taken by the Mars rover Opportunity, shows the small cavities located on the region of Meridiani Planum dubbed “El Capitan.” Millions to billions of years ago, infiltrating groundwater removed the minerals that once occupied these cavities. The water once there must have been very salty to preserve the rest of the outcrop minerals that remain in this region today. NASA, JPL, Cornell, US Geological Survey

PAYSON LEDGE Opportunity captured this image of Meridiani Planum. It shows the stack of fine layers exposed at a ledge called Payson on the western edge of Erebus Crater on the plane. The rock textures likely evolved during millions of years of interplay between wind and water erosion processes. NASA, JPL, Cornell

The report comes just days after the successful touch-down of Mars Phoenix Lander, which has buoyed scientists excitement of finding signs of past or present life on the Red Planet. But that excitement may be tempered by the argument that life never really could have gotten a foothold on Mars in the first place. The liquid water believed to have flowed on Mars several billion years ago was far too salty to foster life, researchers report in the May 30 Science. The scientist base their work on an analysis of the Red Planet’s rocks, which previously supplied the first hints that the planet may at one time have had liquid water on it surface.

“Water is the first indication of life,” says HarvardUniversity biogeochemist Andrew Knoll, coauthor of the new study. “But detecting it is not the only step to finding life. It’s just the first.”

That is why Knoll and colleagues Nicholas Tosca of Harvard and Scott McLennan of StonyBrookUniversity in New York decided to reanalyze the rocks that once held Martian water.

“Today Mars is dry,” Knoll says, “but by understanding the characteristics, like the saltiness, of the water that was once there, we can constrain how or if life could have evolved.”

To test the now nonexistent waters, the researchers calculated what they call “water activity.” This number describes how much substance is dissolved in a liquid. The value is important because water molecules that adhere to dissolved particles are not available to assist in biological processes.

Pure water measures 1.0, meaning all of its molecules are free to support the growth of life. In comparison, seawater activity is 0.98, and research shows that few organisms grow when a liquid’s value dips below 0.85.

This new study finds that the water activity on Mars could have been as low as 0.5.

The analysis of the rock is based on the chemical composition of regions across the planet’s surface. The scientists used data from the Mars Express OMEGA and Mars Reconnaissance Orbiter CRISM spectrometers. They also used information that the Mars Opportunity rover collected at Meridiani Planum, a plain located just south of the Martian equator, to calculate the planet’s water activity.

The salt and mineral deposits found on Mars condensed out of the Red Planet’s ancient waters as the liquid evaporated. By understanding the relationship between the evaporative process and the rock composition, the scientists calculated that the water activity, at most, was between 0.78 and 0.86. They conclude that terrestrial life, life as we know it, could not have survived in such water.

The authors have done a reasonable job making their calculations and arguing that it would be a challenge for life to arise, says Jack Farmer, an ArizonaStateUniversity astrobiologist.

But the authors concede in the paper that on Earth, life-forms can adapt to extremely salty environments, even within the range predicted for Meridiani. So, he says, the study does not really exclude terrestrial life on the Red planet.

“Bottom line,” Farmer says, “I think the door is still wide open for Martian life, and we need to keep searching.”

Knoll agrees that despite this study’s findings, scientists should keep looking for life.

The study is an “early second step” in understanding the history of Mars, he says, but researchers need to move beyond Meridiani.

“We have to get up close and personal with the planet,” Knoll says, “but you have to remember that there are not a lot of alternatives to exploration. We will have to wait a few years, at least for the MSL Lander 2009, to really do what we need to do.”

In late 2009, the Mars Science Laboratory, MSL, is scheduled to blast off toward the Red Planet to investigate more deeply the possibility of life, the role of water and the geochemical properties of currently unexplored areas of Mars.