During the Noachian period (about 4.5 to 3.5 billion years ago), the Martian atmosphere was substantially thicker than it is currently, and the planet had a magnetosphere with a surface field strength about as strong as Earth’s today. Both factors would have dimished the amount of cosmic and solar radiation reaching the surface, and thus aided the possible emergence of life on Mars.
A group of researchers at Christian Albrechts University Kiel and the German Aerospace Center have calculated the surface radiation environment assuming densities 25, 50, and 100 times greater than today’s atmosphere. Writing in the Journal of Geophysical Research (October 20, 2011), they found that radiation rates at the surface (for neutrons, protons, muons, antimuons, and alpha particles) were comparable to present-day Earth. They note, “The Noachian radiation environment should have looked different from present-day Martian conditions. In particular, it should have been less hazardous for an emergence of life.”