Mars is the most logical place to look for life elsewhere in the solar system because it is the most Earth-like of all the other planetary bodies in terms of its geological environment and the availability of liquid water at or near the surface throughout time. Moreover, Mars is the most accessible planetary body other than the lifeless Moon. The finding of evidence for past or present life beyond Earth would have profound philosophical and scientific ramifications, and a finding either that life was present or that it was not would have dramatic implications for the prospects for life elsewhere in the universe.
The search for life on Mars requires a detailed understanding of the nature of life on Earth and how it functions in different environments. The search also requires a very broad understanding of Mars as an integrated planetary system. Such an integrated understanding requires investigation of the following:
The geological and geophysical evolution of Mars;
The history of Mars’s volatiles and climate;
The nature of the surface and the subsurface environments;
The temporal and geographical distribution of liquid water;
The availability of other resources (e.g., energy) that are necessary to support life; and
An understanding of the processes that controlled each of the factors listed above.
Although it is not the only possible emphasis for the Mars program, astrobiology provides a scientifically engaging and broad approach that brings together multiple disciplines to address an important set of scientific questions that are also of tremendous interest to the public.
Finding. The search for evidence of past or present life, as well as determination of the planetary context that creates habitable environments, is a compelling primary focus for NASA’s Mars Exploration Program.
The astrobiology science goals for the exploration of Mars extend beyond the search for present and past life to encompass an understanding of the geological and environmental context that determines planetary habitability; habitability is defined as a general term referring to the potential of an environment (past or present) to support microbial life of any kind. Such an undertaking entails understanding the geological and geochemical evolution of the planet, its internal structure, and the nature of its interaction with the space environment. Such a broad approach will likely be required to enable astrobiologists to determine which characteristics of martian materials result from nonbiological processes and which result from biological processes and so could be used as biosignatures.
Any comprehensive program focusing on the astrobiological exploration of Mars must be undertaken with the full understanding that the outcome is uncertain. It is entirely possible that surface water did not survive on Mars for a period of time sufficient for an origin of life, or that Mars never had life. Astrobiologists seek to explore Mars to better understand the nature of the planet, to assess its biological potential and habitability, and to determine how far chemical evolution proceeded and whether life was present. A finding of “no life” would be just as important scientifically as a finding of life, in terms of constraining our views of how life originates and spreads and of how widespread life might be in the universe.
NASA’s 1995 report An Exobiological Strategy for Mars Exploration took the approach of starting from the global perspective and focusing increasingly on the local perspective. This approach involved a series of steps:
Global reconnaissance that focused on the history of water and the identification of sites for detailed in situ analysis;
In situ analysis at sites that hold promise for understanding the history of water;
Deployment of experiments that address astrobiology science questions, including the nature of martian organic molecules and the presence of features indicative of present-day or prior life;
Return of martian samples to Earth for detailed study; and
Human missions that would provide the detailed geological context for astrobiology measurements and the detection of modern-day “oases” for life.