Mars is an intriguing planet, for what it can tell us about the origin and history of planets and of life. Visible to the ancients, and distinctly reddish in the night sky, the next planet has always been an attractive subject for imaginative science fiction. As the capability for space exploration grew in the 1960's, it became clear that Mars is not, like Earth, a planet teeming with life, and is now a hostile environment for humans. The images of Mariner 4 showed a Moon-like terrain, dominated by large impact craters. This terrain now is believed to an represent ancient crust, similar to the Moon's, formed in an initial period of planetary differentiation. Mariner 9 showed for the first time that Mars was not totally Moon-like, but exhibits later volcanic and tectonic features. Large volcanos of relatively recent activity and large crustal rifts due to tensional forces demonstrate the working of internal forces. The absolute time scale is not accurately calibrated, however, by analogy with the Moon, the initial crustal formation may have occurred between 4 billion and 4.5 billion years ago, and the apparent freshness of the large martian volcanos suggests their formation within the last billion years.
Mars' atmosphere consists largely of carbon dioxide, with a typical surface pressure of about .01 Earth atmospheres, and surface temperatures that may reach 25 C on the equator in mid-summer, but are generally much colder. At these pressures and temperatures, water can not exist in liquid form on the surface. However, Mariner 9 and the subsequent Viking missions observed features which indicate that liquid water has been present on Mars' surface in past epochs. Evidence of both running water and standing water has been noted. The interpretation is that the atmosphere of Mars was thicker and warmer in former times, and perhaps much like the Earth's early atmosphere before the appearance of oxygen. Three questions arise: (a) What was the reason for the change of atmospheric conditions on Mars? (b) What are the implications of such changes for environmental changes on Earth?; and (c) Is it possible that life arose in the early Earth-like history of Mars (and, if it arose, can it still be found somewhere on Mars)?
These three scientific questions are at the core of the Mars scientific exploration defined by the Reference Mission. They can all be addressed principally by understanding the geological characteristics of the planet - the types of rocks present, their absolute and relative ages, the distribution of subsurface water, the history of volcanic activity, the distribution of life-forming elements and compounds, and others. These attributes all have to be understood in the context of what we know about the Earth, Moon and other bodies of our solar system.