Both Spirit and Opportunity uncovered geologic evidence of a wet past, a sign that ancient Mars may have been hospitable to life. But new findings reveal the Red Planet was also once such a hostile place that the environment may have prevented life from developing.
"For much of its history, it was a very forbidding place," said mission principal investigator Steven Squyres of Cornell University.
Scientists stressed that the rovers were investigating a snapshot in geologic time and that it's possible that other regions of Mars that have yet to be explored could have had a different environment.
The new analyses were presented Monday at an American Geophysical Union meeting in San Francisco.
Since landing on opposite sides of Mars in January 2004, the six-wheel rovers found conclusive evidence that the planet once had water based on examination of water-altered bedrock at their respective sites.
But the sedimentary rocks in the Martian plains where Opportunity landed also painted a picture of a past environment some 3 billion to 4 billion years ago that fluctuated between being very acidic and arid — conditions that were probably unfavorable to life.
The Gusev Crater region where Spirit touched down revealed an even more violent history. Three outcrops examined by the rover displayed deposits of water-altered debris from explosive events. Hot ash rained from the sky and space objects bombarded the surface about 4 billion years ago. During that time, water was present, but not a large amount.
Scientists acknowledged that such harsh environments probably would have posed challenges for life to start, but they did not rule out the possibility that limited life forms could have thrived under these extreme conditions.
Dr. Andrew Knoll of Harvard University, Cambridge, Mass., a paper co-author, said, "Life that had evolved in other places or earlier times on Mars, if any did, might adapt to Meridiani conditions, but the kind of chemical reactions we think were important to giving rise to life on Earth simply could not have happened at Meridiani."
Scientists analyzed data about stacked sedimentary rock layers 23 feet thick, exposed inside "Endurance Crater." They identified three divisions within the stack. The lowest, oldest portion had the signature of dry sand dunes; the middle portion, windblown sheets of sand with all the particles produced in part by previous evaporation of liquid water. The upper portion corresponded to layers Opportunity found earlier inside a smaller crater near its landing site.
Materials in all three divisions were wet both before and after the layers were deposited by either wind or water. Researchers described chemical evidence that the sand grains deposited in the layers had been altered by water before the layers formed. Scientists analyzed how acidic water moving through the layers after they were in place caused changes such as the formation of hematite-rich spherules within the rocks.
Experimental and theoretical testing reinforces the interpretation of changes caused by acidic water interacting with the rock layers. "We made simulated Mars rocks in our laboratory then infused acidic fluids through them," said researcher Nicholas Tosca from the State University of New York. "Our theoretical model shows the minerals predicted to form when those fluids evaporate bear a remarkable similarity to the minerals identified in the Meridiani outcrop."
The stack of layers in Endurance Crater resulted from a changeable environment perhaps 3.5 to 4 billion years ago. The area may have looked like salt flats occasionally holding water, surrounded by dunes. The White Sands region in New Mexico bears a similar physical resemblance. "For the chemistry and mineralogy of the environment, an acidic river basin named Rio Tinto, in Spain, provides useful similarities," said Dr. David Fernandez-Remolar of Spain's Centro de Astrobiologia.
Many types of microbes live in the Rio Tinto environment, one of the reasons for concluding that ancient Meridiani could have been habitable. However, the organisms at Rio Tinto are descended from populations that live in less acidic and stressful habitats. If Meridiani had any life, it might have had to originate in a different habitat.