Much of our fascination with Mars is due to the belief that the planet may have once been primitive forms of life. During the early Martian era, known as Noah's time, water was plentiful on the surface of the planet, which means that Mars had at least some of the conditions necessary to sustain life.
Named after Noachis Terra, the oldest region of Mars and whose dates 4 billion years ago, this period in the planet's history also benefited from warmer temperatures, another indication that the Red Planet could have harbored lifetime.
According to a new study, ancient microbial life could still be conserved in certain types of rocks on Mars. The research, led by Dr. Sean McMahon, of the University of Edinburgh in the United Kingdom and Yale University in Connecticut, offers a field guide for upcoming missions to the Red Planet, indicating exactly where to look for the fossils of microbes.
The Martian surface is cold, dry, exposed to biologically noxious radiation and apparently sterile at present. However, there is clear geological evidence of warmer and wetter ranges in the past that could have sustained life on or near the surface, "reads the article published in Journal of Geophysical Research: Planets  McMahon's team badyzed a series of studies on fossils from Earth and examined the results of previous experiments that replicate the conditions of the Martian environment.
His research found that the best place to search for fossils are the sedimentary rocks formed on the floor of ancient Martian lakes, such as the one that filled Gale crater 3,500 million years ago and where the Mars Curiosity Rover is currently drilling rock samples.
These types of Martian rocks, formed between three and four billion years during the Noe and Hesperia eras are full of iron and silica, which helps to preserve the fossils, points out the University of Edinburgh.
For this reason, the researcher believes that the microbial life on Mars could be contained within iron-rich rocks and silica, found in the beds of lakes where Martian life may have once flourished.
"We conclude that mudstones rich in silica and clays containing iron currently offer the best hope of finding fossils on Mars and should be prioritized," the authors write in their article.
These ancient sedimentary rocks are also much better preserved on Mars, due to the planet's lack of plate tectonics, which typically destroys rocks of the same age here on Earth, along with the fossils they contain. One such example is the delta of the Jezero crater, a well-preserved former 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 spot for its imminent mission of Mars 2020, with the task of finding signs of past life on the Red Planet.
"There are many interesting rocky outcrops and minerals on Mars where we would like to look for fossils, but since we can not ship rovers to all of them, we have tried to prioritize the most promising deposits based on the best information available."
The Mars 2020 rover will collect samples of rock and soil, which will be recovered and brought for study on Earth through the Mars Return mission sample.