During the next two decades, NASA will conduct several missions to address whether life ever arose on Mars. The search begins with determining whether the Martian environment was ever suitable for life.
Conditions Needed for Life to Thrive
On Earth, all forms of life need water to survive. It is likely, though not certain, that if life ever evolved on Mars, it did so in the presence of a long-standing supply of water. On Mars, we will therefore search for evidence of life in areas where liquid water was once stable, and below the surface where it still might exist today. Perhaps there might also be some current "hot spots" on Mars where hydrothermal pools (like those at Yellowstone) provide places for life. Recent data from Mars Global Surveyor suggest that liquid water may exist just below the surface in rare places on the planet, and the 2001 Mars Odyssey will be mapping subsurface water reservoirs on a global scale. We know that water ice is present at the Martian poles, and these areas will be good places to search for evidence of life as well.
In addition to liquid water, life also needs energy. Therefore, future missions will also be on the lookout for energy sources other than sunlight, since life on the surface of mars is unlikely given the presence of "superoxides" that break down organic (carbon-based) molecules on which life is based. Here on Earth, we find life in many places where sunlight never reaches--at dark ocean depths, inside rocks, and deep below the surface. Chemical and geothermal energy, for example, are also energy sources used by life forms on Earth. Perhaps tiny, subsurface microbes on Mars could use such energy sources too.
Looking for Life Signs
NASA will also look for life on Mars by searching for telltale markers, or biosignatures, of current and past life. The element carbon, for instance, is a fundamental building block of life. Knowing where carbon is present and in what form would tell us a lot about where life might have developed.
We know that most of the current Martian atmosphere consists of carbon dioxide. If carbonate minerals were formed on the Martian surface by chemical reactions between water and the atmosphere, the presence of these minerals would be a clue that water had been present for a long time--perhaps long enough for life to have developed.
On Earth, fossils in sedimentary rock leave a record of past life. Based on studies of the fossil record on Earth, we know that only certain environments and types of deposits provide good places for fossil preservation. On Mars, searches are already underway to locate lakes or streams that may have left behind similar deposits.
So far, however, the kinds of biosignatures we know how to identify are those found on Earth. It's possible that life on another planet might be very different. The challenge is to be able to differentiate life from nonlife no matter where one finds it, no matter what its varying chemistry, structure, and other characteristics might be. Life detection technologies under development will help us define life in non-Earth-centric terms so that we are able to detect in all the forms it might take.
