Clay minerals - rocks that usually form when water is present for long periods of time - cover much more of Mars than previously thought, say Georgia Institute of Technology scientists.
The minerals were found using a spectroscopic analysis from the Mars Reconnaissance Orbiter. They also exist in the Meridiani plains, and now appear to have been in some of the rocks studied by Opportunity when it landed at Eagle crater in 2004. The rover itself only detected acidic sulfates.
"It's not a surprise that Opportunity didn't find clays while exploring," says assistant professor James Wray. "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."
The clay signatures near Eagle crater are very weak, especially compared to those along the rim and inside Endeavour crater. Wray believes clays could have been more plentiful in the past, but Mars' volcanic, acidic history has probably eliminated some of them.
"It was also surprising to find clays in geologically younger terrain than the sulfates," says Eldar Noe Dobrea of the Planetary Science Institute. "This forces us to rethink our current hypotheses of the history of water on Mars."
Opportunity has reached an area believed to contain rich clay deposits - but after operating for nearly nine years longer than expected, its two mineralogical instruments don't work anymore. Instead, the rover must take pictures of rocks with its panoramic camera and analyze targets with a spectrometer to try and determine the composition of rock layers.
"So far, we've only been able to identify areas of clay deposits from orbit," says Wray. "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."