"We now have proof and scientific consensus that Mars was soaking wet at this place and at one window of time," said Jeff Kargel, a geologist with the U.S. Geological Survey in Flagstaff, Arizona, and author of a Science commentary.
On the opposite side of Mars, Spirit has been examining layers of a rocky cliff face that also shows signs of alteration by water. Its findings were reported in a previous Mars special issue in Science.
Water is considered a precursor to life. On Earth there is no example of life without liquid water.
"Water certainly is a great enabler of life," Kargel said. "Though I'm not one who goes as far as to say, Where there is water, there is life. It could be all kinds of nasty water that is just not suitable for any living thing."
Researchers don't know how widespread Martian water was, or whether it formed oceans or just shallow ponds. Also, if the water did not persist for very long, it could have restrained the development of microorganisms and other living things.
"Whether life could exist at the surface or in the subsurface, and whether it could exist over a limited geographical extent or over a regional or global scale, is unclear," said Bruce Jakosky, a planetary geologist at the University of Colorado in Boulder. Jakosky was not involved in the studies described in Science.
From the mineral deposits, the researchers concluded the Martian surface was not only wet, but also extremely salty and acidic. Such conditions are found only rarely on Earth, in places such as the Rio Tinto Basin in Spain.
That raises particular questions about the origin of potential life on Mars. While highly acidic environments on Earth, such as the Rio Tinto Basin, teem with life, life did not originate in these places. Instead, it evolved to inhabit the acidic environments.
"Could life begin in an environment with that kind of acidity?" Squyres said. "Frankly, I don't know. It would be a challenge."
Scientists are planning new missions to the red planet that would return physical samples of rock and soil to Earth for analysis. Rocks on Mars may have chemical and physical fossils on them.
Elaborate quarantine measures are being developed to isolate any soil and rock samples returned from Mars, even though such a mission is still at least a decade away.
For their Mars rover missions, scientists made sure that their two spacecraft were extraordinarily clean in order to avoid bringing any Earthly microbes onto Mars.
Kargel, the U.S.G.S. geologist, cautioned in his Science commentary that until scientists learn otherwise, they must assume that life exists on Mars to avoid any biological cross-contamination.
"Once you suppose that possibly Mars is a living world, there's reasonable ground for wondering whether there could be compatibility between life in the physical and biological environments [of Earth and Mars]," he said.
"There may be the ability of organisms from one planet to take fuller advantage of physical environments on the other planet, and thereby reproducing at the expense of the endemic species," Kargel said.
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