Scientists identified the mineral goethite in bedrock studied in the Columbia Hills.
Spirit and its twin, Opportunity, were sent to Mars to look for geologic evidence of a watery past. Both rovers, especially Opportunity, have found such evidence in their nearly year-long treks over the martian surface, but the goethite find is particularly important, a mission scientist said in a JPL statement.
"Goethite, like the jarosite that Opportunity found on the other side of Mars, is strong evidence for water activity," said Goestar Klingelhoefer of the University of Mainz, Germany, lead scientist for the spectrometers on each rover. The instruments are designed to analyze iron minerals.
Goethite forms only in the presence of water, although it may be in liquid, ice or gaseous form.
The Columbia Hills bedrock was previously found to have hematite, a mineral that usually, but not always, forms in the presence of water.
Klingelhoefer's findings were being presented this week at a meeting of the American Geophysical Union in San Francisco.
Spirit will continue to study the hills to determine whether water there only remained underground or ever pooled on the surface.
On the other side of the planet, Opportunity has recently seen frost and clouds marking seasonal changes, according to science team member Michael Wolff of the Brookfield, Wis., branch of the Boulder, Colo.-based Space Science Institute.
"We're seeing some spectacular clouds," Wolff said. "They are a dramatic reminder that you have weather on Mars."
Frost has also been seen on the rover during some mornings, according to Wolff.
But the search for signs that water was once widely abundant on the planet continues.
Opportunity is concluding its survey of deep Endurance Crater (search). Close looks at lower rock layers in part of the crater wall show strong indications that the material was last transported by wind rather than water, as happened with some layers higher up, said Steve Squyres of Cornell University, the principal investigator for the rover's instruments.
"The combination suggests that this was not a deep-water environment but more of a salt flat, alternately wet and dry," Squyres said.