NASA researchers, including one Harvard professor, have found that the surface of Mars has been too acidic to sustain life for the past four billion years, suggesting that Martians are more the realm of science fiction than astronomical research.

Clues found in the rocks by two NASA space rovers—Opportunity and Spirit—have led researches to conclude that the concentrations of minerals in Mars’s water makes the planet inhospitable for even the heartiest of organisms, according to Andrew Knoll, a professor of natural history in the Earth and planetary sciences department and a member of NASA’s Mars Exploration Rover science team.

“One particular iron sulfate mineral called jarosite forms only under strongly acidic conditions,” Knoll said. “Its identification in the Martian rocks provides a smoking gun for strongly acidic conditions.”

Knoll said that differences in size and distance from the sun caused Earth and Mars to become distinct early in their lives.

“About 3.8 billion years ago—the age of the sedimentary rocks collected by Opportunity—Earth had oceans, life, and an atmosphere capable of sustaining greenhouse warmth but without oxygen,” said Knoll, who added that the calculations on ancient water activity on Mars were performed by Harvard postdoctoral fellow Nicholas J. Tosca. “Mars was dry, acidic, and at least mildly oxidizing.”

Despite the evidence collected by the rovers, there still may be some opportunity to find evidence of life on Mars.

“Most of Mars remains unvisited by rovers, although orbiting satellites are providing good maps of surface conditions across the planet,” Knoll said.

Two rovers, the Phoenix lander and the Mars Science Laboratory, will be landing on Mars in the near future for evidence of the planet’s suitability for life.

The Phoenix lander, which is set to land on Mars on May 25, will be looking at a region farther to the north where there is permafrost.

“We’re going to places where there is kind of evidence that could support life and checking them out in greater detail,” said Diana Blaney, a member on the science team for the Phoenix lander. “Phoenix starts that process.”

Meanwhile, the Mars Science Laboratory is scheduled to leave Earth in the fall of 2009 and land on Mars in the summer of 2010. According to Joy A. Crisp, a Mars Science Laboratory scientist, it has capabilities that Spirit and Opportunity did not have, like the ability to analyze drilled powders from rocks for organic compounds.

“We will be looking at the oldest part of Mars, four to 4.5 billion years old Mars,” Crisp said. “This might be a different kind of environment that might be more suitable for life.”

—Staff Writer Kevin C. Leu can be reached at kleu@fas.harvard.edu.