The project, by researchers from the Planetary Science Institute in Arizona and the Georgia Institute of Technology, identified the clay minerals using a spectroscopic analysis from the Mars Reconnaissance Orbiter.
Clays are present in some of the rocks studied by NASA's Opportunity Mars rover when it landed in 2004 but the rover was unable to detect them then, the researchers said.
"It's not a surprise that Opportunity didn't find clays while exploring," James Wary of Georgia Tech's School of Earth and Atmospheric Sciences said. "We didn't know they existed on Mars until after the rover arrived. Opportunity doesn't have the same tools that have proven so effective for detecting clays from orbit."
Opportunity has reached an area believed to contain rich clay deposits, but since the rover was supposed to survive for only three months -- it's still exploring nine years later -- its two mineralogical instruments are no longer working.
Opportunity will have to take pictures of rocks with its panoramic camera and analyze targets with a spectrometer to try and determine the composition of rock layers, the researchers said.
"So far, we've only been able to identify areas of clay deposits from orbit," Wray said. "If Opportunity can find a sample and give us a closer look, we should be able to determine how the rock was formed, such as in a deep lake, shallow pond or volcanic system."