In 2004, NASA's Opportunity rover found evidence in Martian soils that water had once flowed across the surface there, buoying¹ hopes that the red planet may once have supported primitive² life.

But a new study throws some cold water, and a big pinch of salt, on those hopes.

"Liquid water is required by all species on Earth and we've assumed that water is the very least that would be necessary for life on Mars," said study team member Nicholas J. Tosca, a Harvard University postdoctoral researcher. "However, to really assess Mars' habitability we need to consider the properties of its water. Not all of Earth's waters are able to support life, and the limits of terrestrial life are sharply defined by water's temperature, acidity³ and salinity⁴."

Tosca and his team analyzed⁵ salt deposits in the 4-billion-year-old Martian rock investigated by Opportunity. The new analysis shows that the water that would have flowed across these ancient Martian rocks may have been exceedingly briny⁶.

"Our sense has been that while Mars is a lousy environment for supporting life today, long ago it might have more closely resembled Earth," said Andrew H. Knoll⁷, also of Harvard and on the study team. "But this result suggests quite strongly that even as long as four billion years ago, the surface of Mars would have been challenging for life. No matter how far back we peer into Mars' history, we may never see a point at which the planet really looked like Earth."

The research was presented in February at the annual meeting of the American Association for the Advancement⁸ of Science in Boston.

Halophiles, or organisms that can tolerate high-salinity waters, are known to exist in places on Earth, but they likely evolved from organisms that lived in purer waters, scientists think, making it unlikely that life would actually arise initially⁹ in extremely briny waters.

The high salinity, however, "doesn't rule out life forms of a type we've never encountered," Knoll added, "but life that could originate and persist in such a salty setting would require biochemistry distinct from any known among even the most robust¹⁰ halophiles on Earth."

Knoll and Tosca also say the finding doesn't rule out the possibility that less salty waters once flowed on the planet.