Although Mars appears to be an arid, lifeless place now, about four billion years ago its surface was covered with bodies of water – which may have supported microscopic aquatic life. A new study has determined what type of rocks may be most likely to contain the fossilized remains of such microbes.

The study was conducted by scientists from Scotland's University of Edinburgh, along with American colleagues from NASA's Jet Propulsion Laboratory, Brown University, California Institute of Technology, Massachusetts Institute of Technology (MIT) and Yale University.

They determined that sedimentary rocks, which formed out of compacted mud or clay, "are the best place to seek fossil evidence of life." This is due partially to the fact that the rocks formed in ancient lake beds, during the Noachian and Hesperian periods of Martian history, between three and four billion years ago. Additionally, such rocks are rich in iron and silica, both of which are known to help preserve fossils.

If there are fossil-containing sedimentary rocks on Mars, they are more likely to be intact than their same-aged counterparts on Earth. This is because unlike our planet, Mars hasn't been subject to the shifting of tectonic plates, which can grind up ancient rocks over long periods of time.

It is now hoped that the findings could help determine the landing and search sites for future rover missions, including NASA's Mars 2020 Mission, which will be specifically looking for evidence of past Martian life.