The Curiosity rover is preparing to drill a little hole in a slab of Martian sedimentary rock to extract material for testing in the ongoing search for life on the red planet. What can we expect to find? What was the environment like for the origin and evolution of life forms on Mars?
We know far more about the present physical and chemical conditions on the surface of Mars than we know about the distant, presumably warmer and wetter, past. Since Mars has a thin, very dry atmosphere of 97% pure carbon dioxide, ultraviolet (UV) light from the Sun readily penetrates to the surface. In the absence of more than a tiny trace of oxygen, ozone cannot be made in quantity, and cannot provide an ozone layer similar to Earth’s to protect the surface from killing UV radiation. In fact, UV light can dissociate carbon dioxide into carbon monoxide (CO) and atomic oxygen (O) even at the surface of the planet. Even a tiny trace of atomic oxygen is very bad news for organic matter: O is a very powerful oxidizing agent. Any organic matter exposed at the surface of Mars, whether exposed by weathering of ancient organic-bearing sediments or dropped onto Mars by impacts of carbonaceous meteorites, would quickly ”burn” into carbon dioxide and water vapor. It is only in the interiors of ancient sedimentary rocks, where O cannot penetrate, that organic matter might survive.
The CO2 content of the atmosphere of Mars is sufficient to provide an average pressure of about 0.006 atmospheres at the surface, although this number is very variable from place to place because of the wide range of elevations spanning a deep basin (Hellas) and several towering volcanoes. The CO2 famous for maintaining Earth’s surface temperature above the freezing point (via the greenhouse effect) has a surface pressure of less than 0.0004 atmospheres. So why is Mars so cold? Several reasons: the greenhouse effect on Earth is dominated by water vapor, which is very rare on Mars; Mars experiences about half the intensity of sunlight that Earth receives. So an earlier, warmer Mars requires that it was also a wetter Mars. You need water vapor to make Mars warm enough to have water vapor! Given favorable early conditions on Mars, with liquid water present and a strong greenhouse effect at work, life may indeed have originated there. But what evidence of that former life would we be able to find today? There are two obvious possibilities: well-protected organic matter deep inside ancient sedimentary rocks, or fossils of simple life forms. But evidence of ancient life would not necessarily prove an independent origin for life off Earth: large impact events can launch surface rocks from both Earth and Mars into orbits around the Sun, from which they can collide with and land on either planet. Martian life, if any, may be expatriate Earth life--- and vice versa.