This image from the front hazard-avoidance camera (Hazcam) on the NASA Mars Exploration Rover Opportunity shows the rover's arm extended for examination of a target called "Onaping" at the base of an outcrop called "Copper Cliff" in the Matijevic Hill area of the west rim of Endeavour Crater. (Photo : NASA/JPL-Caltech)

Clay minerals are found to be more abundant on the Martian surface than previously thought, according to a new study co-authored by the Georgia Institute of Technology.

Clay minerals are rocks that are formed when water is present for long periods of time. The presence of clay minerals on Mars was first discovered in 2005, indicating that the planet once hosted liquid water on the surface.

Now, a team of researchers has found that the planet is hosting more clay minerals than expected. The research team has detected the presence of clay in some of the rocks studied by Opportunity Rover when it landed at Eagle crater in 2004. But the rover detected only acidic sulfates in the rocks and since then moved to Endeavour Crater, a place NASA's Mars Reconnaissance Orbiter pinpointed for clay minerals.

Using spectroscopic analysis from the Mars Reconnaissance Orbiter, the research team was able to identify the clay minerals at Eagle crater. They also found that clays also exist in the Meridiani plains, through which the rover trekked towards its current position.

"It's not a surprise that Opportunity didn't find clays while exploring," James Wray, a faculty member at the Georgia Institute of Technology and a member of Curiosity's science team, said in a statement.

"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."

According to Wray and his colleagues, the clay signatures at the Eagle crater are very weak compared to those clay minerals located inside the Endeavour crater. Wray suggested that clays could have been found in abundance in the past at the Eagle crater, but the Mars' volcanic and acidic history might have eliminated them.

The team was surprised to find clays in younger terrain than sulfates. It was thought that the clays were formed when the planet's waters were alkaline (lower concentration of hydrogen ions). As volcanism caused the water to acidify, the dominant alteration mineralogy became sulfates. "This forces us to rethink our current hypotheses of the history of water on Mars," said Eldar Noe Dobrea, of the Planetary Science Institute, who led the project.

Although Opportunity rover has reached an area rich in clay deposits, their mineralogical instruments are not working to examine the clay minerals.

The rover, which was launched in 2003 and landed on the Martian surface in 2004, was expected to work for mere three months. After nine years since it was launched, the rover is still sending information about the Martian surface.

Researchers noted that Opportunity must take pictures of rocks with its panoramic camera and use a spectrometer to analyze targets in order to determine the composition of rock layers. This would help experts to determine how rocks were formed.

The rover is currently working at "Matijevic Hill" at the inboard edge of "Cape York" on the rim of Endeavour Crater. It has been carrying out science experiments at a location called "Copper Cliff."

Besides Opportunity rover, one more rover is working on the other side of the Martian surface - Curiosity. The car-sized Mars rover landed on the Gale crater on Aug. 5.

Ever since its arrival on the red planet, Curiosity is performing various experiments to assess if the crater ever offered a habitable environment to support microbial life.

This week Curiosity will be driving within a shallow depression called "Yellowknife Bay," giving data to researchers in order to help them choose a rock to drill.

The findings of the study are published online in the current edition of Geophysical Research Letters.