BOSTON - If there was life in surface water on Mars early on, it might have enjoyed a very buoyant, salty ride, if it could thrive in such a hostile environment at all, new research suggests.

Minerals in sedimentary rocks found at Mars' Meridiani Planum by NASA's Opportunity rover suggest they formed in extremely salty water, even saltier than the oceans on Earth, said Andrew Knoll of Harvard University, who is part of a team of scientists taking a closer look at the geological data collected by the Mars Exploration Rovers mission.

Water was definitely present at least for short periods of time at Meridiani Planum, he said, but no one has known how habitable it might have been nor whether it was around long enough for life to take hold and endure.

So Knoll and his colleagues looked at data that reveal the chemistry of the salts in the rocks there as a gauge of the salinity of the ancient water. That allowed the scientists to estimate how salty the brines were at the time the minerals were deposited.

"The punchline is it was really salty," Knoll told a gathering of reporters here at the annual meeting of the American Association for the Advancement of Science, "salty enough that only a handful of terrestrial organisms would have a ghost of chance of surviving there when conditions were at their best."

Constrains the possibilities

There are a couple dozen organisms on Earth that can tolerate the salinity levels determined by Knoll and his team to have been present when the Meridiani Planum rocks were deposited, he said.

The research, to be presented today in detail to scientists as well, is currently under review at a top research journal, Knoll said.

Knoll and his colleagues also have found several different classes of minerals that reveal how little the rocks at Meridiani Planum were altered since they were exposed to the elements, he said. Water at this location was "rare and transient," according to the research, detailed in an online paper in the Journal of Geophysical Research ? Planets.

The best era during which to look for evidence of Martian life would be in Mars' earliest history, Knoll said, that is, during the first 500 million to 600 million years, before the Meridiani Planum rocks were deposited.

The new findings bring scientists no closer to determining whether there was ever microbial life on Mars, but they do "constrain our thinking about life on Mars," Knoll said, painting a picture of an environment that is very "forbidding" for life.

Double whammy

Previously, Mars mission data have revealed that liquid water at Meridiani Planum was very acidic.

"So it is doubly bad if it's acidic and salty," Knoll said.

"I'm not sure the effects are necessarily additive, but certainly there are limits to the physiological ways that microorganisms can adapt to tolerate acidity," he said. "There are also limits to ways that they can adapt to tolerate highly saline environments and those are completely different physiological systems. In a sense, it makes it harder to adapt that there are two biochemical systems that would need adapting."

Scientists are more optimistic that life might have had a chance on Mars in its early history compared to later on. In earlier times, Mars' environment was probably wetter, less oxidizing (a condition that is challenging for most life) and less acidic, but "that doesn't mean it was terrific. It just wasn't as bad as it got later," Knoll said.

Meanwhile, Mars scientists and mission engineers gathered at the AAAS meeting today also looked forward to the May 25 arrival of NASA's Phoenix Lander at Mars' north pole and discussed work on the Mars Science Laboratory (MSL) mission, set to launch in September 2009.

MSL will carry a new rover to Mars that is far more complex and sophisticated, and five times heavier, than each of the MER rovers on the red planet now. The MER rovers are learning more about the habitability of the planet in the distant past, said Richard Cook, MER project manager.

Instruments on MSL will focus on detecting organic material at the planet and collecting more precise data on the minerals at Mars.

As for the MER rovers, Spirit is hunkered down for winter, parked on a north-facing slope and serving as a weather station to monitor a nearby dune field. And Opportunity is descending down the wall of Victoria Crater, investigating its finely layered rock stacks, made mostly of sulphate salts, said Steve Squyres of Cornell University, head of the science team for the MER mission.

The rovers nearly died last summer during intense dust storms on Mars that nearly stole all their ability to gather power, but they again outperformed expectations and endured. In fact, it looks good for them to be operational when Phoenix arrives in late spring.

"The rovers have lasted so long that I am never willing to make predictions about how long these things will last," Squyres said. "? It's been four years. You do the math. I think it's pretty likely."

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