London, May 26 (IANS): Iron-rich rocks -- which formed in lake beds -- are the best place to seek fossil evidence of life on Mars from billions of years ago, researchers say.

It is believed that Mars supported primitive life forms around four billion years ago and the new study could aid the search for traces of tiny creatures -- known as microbes - on the Red Planet.

"There are many interesting rock and mineral outcrops on Mars where we would like to search for fossils, but since we can't send rovers to all of them we have tried to prioritise the most promising deposits based on the best available information," said Sean McMahon from University of Edinburgh in Britain.

The study, published in the Journal of Geophysical Research, said that sedimentary rocks made of compacted mud or clay are the most likely to contain fossils.

These rocks are rich in iron and a mineral called silica, which helps preserve fossils.

They formed during the Noachian and Hesperian Periods of Martian history between three and four billion years ago. At that time, the planet's surface was abundant in water, which could have supported life.

The rocks are much better preserved than those of the same age on Earth, the researchers said.

This is because Mars is not subject to plate tectonics -- the movement of huge rocky slabs that form the crust of some planets -- which over time can destroy rocks and fossils inside them.

The team reviewed studies of fossils on Earth and assessed the results of lab experiments replicating Martian conditions to identify the most promising sites on the planet to explore for traces of ancient life.

Their findings could help inform NASA's next rover mission to the Red Planet, which will focus on searching for evidence of past life.

The US space agency's Mars 2020 rover will collect rock samples to be returned to Earth for analysis by a future mission.

A similar mission led by the European Space Agency is also planned in coming years.

The latest study of Mars rocks could aid in the selection of landing sites for both missions. The researchers hope that their study could also help to identify the best places to gather rock samples.