Researchers say they have identified rocks that could hold four-billion-year-old evidence of life on Mars.

According to a scientific article published Friday, scrutiny of clay-carbonate rocks in the Nili Fossae region of Mars points to possible evidence of organisms that once lived on the Red Planet.

In 2008, NASA's Mars Reconnaissance Orbiter determined that the Nili Fossae, a region of valleys that have cut into the Martian bedrock, contains carbonate.

The mineral, which forms in the presence of water, had previously been detected in trace amounts in Martian dust and soil.

Because carbonate is typically formed when the remains of dead organisms are buried and preserved, the finding generated considerable excitement in the scientific community.

Following this latest study, lead author Dr. Adrian J. Brown of the Search for Extraterrestrial Intelligence (SETI) Institute in California said the indicators remain.

"We suggest that the associated hydrothermal activity would have provided sufficient energy for biological activity on early Mars at Nili Fossae," Dr. Adrian Brown said in a statement.

Using infrared light beamed from NASA's Mars Orbiter, Dr. Brown's team closely examined the composition of rocks in the Nili Fossae area. Then, they applied the same technique to rocks in Pilbara, Australia.

The Pilbara rocks, having remained on the surface of the Earth for 3.5 billion years, afford scientists the chance to examine evidence of the planet's early geological history.

Of particular note are the 'stromatolites' formed in the rocks by ancient microbes and preserved there for billions of years.

Now the team believes that the same 'hydrothermal' processes that preserved these markers of life on Earth could have taken place in the four-billion-year-old Nili Fossae.

Comparing the composition of rocks on the two planets, researchers found they each contained similar properties.

"They indicate that biomarkers or evidence of living organisms, if produced at Nili, could have been preserved, as they have been in the North Pole Dome region of the Pilbara craton," Dr. Brown said in a statement.

Nili Fossae was once touted as a possible landing site for the Mars Science Laboratory NASA plans to launch in 2011. The area has since been deemed too treacherous, due to its rocky, uneven terrain.

The paper "Hydrothermal formation of Clay-Carbonate alteration assemblages in the Nili Fossae region of Mars" is published in the journal Earth and Planetary Science Letters.