Robot Mars rover Spirit has so far been eclipsed in its mission by its "twin" Opportunity, which found rich evidence of a wet history at its landing site.
Spirit found the mineral in a rock called Clovis
But now Spirit has found compelling evidence that liquid water also flowed at Gusev Crater, the rocky basin it is exploring on the Red Planet.
Spirit has discovered a mineral called goethite in the bedrock at Gusev which forms only in the presence of water.
Details were outlined at the American Geophysical Union in San Francisco, US.
"Goethite, like the jarosite that Opportunity found on the other side of Mars, is strong evidence for water activity," said Dr Goestar Klingelhoefer, lead scientist for the Mössbauer spectrometer instrument on the rovers, which analyses iron-bearing minerals.
Spirit had previously identified the mineral haematite in the bedrock at Columbia Hills, where it is exploring. This mineral usually forms in wet conditions, but it can occasionally form without the presence of water.
The discovery of goethite, however, makes the evidence for a watery past at Gusev more compelling.
Frost (above) covers the calibration target on Opportunity which is usually black (below)
The new results come from a rock called Clovis.
Spirit has now driven past a feature called the West Spur and has started to ascend Husband Hill, a section of Columbia Hills.
On the other side of Mars, Opportunity has spotted clouds and frost on the flat plain of Meridiani Planum, a sign of the changing seasons on the planet.
They are caused by water vapour moving from the planet's North Pole toward the South Pole during the current northern-summer and southern-winter period.
A thin frost was observed by Opportunity on the calibration target for its panoramic camera.