The early environment and its evolution on Mars - Implications for life

There is considerable evidence that the early climate of Mars was very different from the inhospitable conditions there today. This early climate was characterized by liquid water on the surface and a dense atmosphere composed predominantly of CO2. The duration of these warm initial conditions on the surface of Mars is uncertain, but theoretical models suggest that they could have persisted for hundreds of millions up to a billion years. From studies of the earth's earliest biosphere, it is known that, by 3.5 Gyr ago, life had originated on earth and reached a fair degree of biological sophistication. If Mars did maintain a clement environment for longer than it took for life to originate on earth, then the question of the origin of life on Mars follows naturally. Since over two thirds of the Martian surface is more than 3.5 Gyr old, the possibility exists that Mars may hold the best record of the events that led to the origin of life, even though there may be no life there today.