Much of our fascination with Mars is due to the belief that the planet may have once been teeming with primitive forms of life. During the earliest Martian era, known as the Noachian epoch, water was abundant on the planet's surface, meaning Mars had at least some of the conditions required to support life.
Named after Noachis Terra, the oldest region on Mars and which dates back to 4 billion years ago, this period in the planet's history also benefited from warmer temperatures — another indication that the Red Planet might have hosted life.
According to a new study, ancient microbial life could still be preserved in certain types of rocks on Mars. The research, led by Dr. Sean McMahon, from the University of Edinburgh in the U.K. and Yale University in Connecticut, offers a field guide for the upcoming missions to the Red Planet, pinpointing exactly where to look for microbe fossils.
"The Martian surface is cold, dry, exposed to biologically harmful radiation and apparently barren today. Nevertheless, there is clear geological evidence for warmer, wetter intervals in the past that could have supported life at or near the surface," reads the paper, published this month in the Journal of Geophysical Research: Planets.
McMahon's team analyzed a number of studies conducted on Earth fossils and looked into the lab results of previous experiments replicating the conditions of the Martian environment.
Their research uncovered that the best place to look for fossils are sedimentary rocks formed on the floor of ancient Martian lakes, just like the one that filled the Gale Crater 3.5 billion years ago and where the Mars Curiosity Rover is currently drilling for rock samples.
These types of Martian rocks, formed between 3 and 4 billion years ago during the Noachian and Hesperian eras, are chock full of iron and silica, which helps preserve fossils, notes the University of Edinburgh.
For this reason, the researchers believe that microbial life on Mars could be contained inside rocks rich in iron and silica, found on the dried-up lakebeds where Martian life may have once flourished.
"We conclude that mudstones rich in silica and iron‐bearing clays currently offer the best hope of finding fossils on Mars and should be prioritized," the authors write in their paper.
These ancient sedimentary rocks are also far better preserved on Mars, due to the planet's lack of plate tectonics, which typically destroys similar-age rocks here on Earth, together with the fossils inside them. One such example is the Jezero Crater delta imaged above, a well-preserved ancient river delta on Mars that is also rich in clays and carbonates, according to NASA.
McMahon hopes that his study will help NASA choose a landing site for its impending Mars 2020 mission, tasked with finding signs of past life 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 prioritize the most promising deposits based on the best available information."The Mars 2020 rover will be collecting rock and soil samples, which will be retrieved and brought back for study on Earth by the Mars Sample Return mission.