More recent layers above these minerals contain clays high in aluminum.
These may have been formed as water dissolved the iron and magnesium out of the soil or by changes to the chemistry of ancient Martian groundwater, the researchers say.
Other chemical changes in the clays, such as differences in the iron oxidation state in some layers, point to a major event that somehow heated up Martian water billions of years ago, Bishop said.
"Possibly a volcano erupted, or maybe some kind of impact made the water hot," she said.
"There are a lot of things that could have taken place, and the hard thing is that it happened four billion years ago."
Mawrth Vallis is one of the possible landing sites for the Mars Science Laboratory, which is slated to launch in late 2009. The roving lab is expected to better piece together the nature of Mars's soil chemistry.
Nicholas Tosca, an expert on Martian soils at Harvard University, called the new finding " a really interesting result."
But "it probably raises more questions than answers, I think," said Tosca, who was unaffiliated with the study.
"Unfortunately, we don't know much about the prevailing conditions at that time. I think this paper scratches the tip of the iceberg in getting more detailed information about what the clays are trying to tell us."
"Rocket Fuel" Salt in Martian Soil?
Meanwhile, NASA's Phoenix mission based near the planet's north pole recently found that soil samples taken in June and July appear to contain perchlorate salt.
Salts are minerals that, like clays, could be evidence of past water on Mars. Perchlorate, a common rocket-fuel component, occurs naturally in harsh Earth environments such as the Atacama Desert in Chile.
Some "extreme" microorganisms there have adapted to use the chemical as an energy source, but it is not considered a particularly auspicious molecule in the search for life on Mars.
The finding could be a setback from research released in June that Martian soil resembles the dirt in Earthly vegetable gardens.
Still, Phoenix scientists haven't ruled out the possibility that the Mars perchlorate is a contaminant that hitched a ride aboard the recently landed spacecraft.
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