Introduction

Biochemically, life on Earth is essentially a single phenomenon. All life forms on this planet share the same basic biochemical and genetic pattern and appear to have descended from a common ancestor. We don't know where, when, or how this form of life first originated, nor do we know what features of its biochemistry are general and would be found in any type of life and what features are the result of the particular history of Earth life. One way that may provide a better understanding and generalization of life would be the discovery of a second type of life. Three research programs are currently directed toward discovering a second type of life: (1) laboratory synthesis of life, (2) the search for extraterrestrial intelligence, and (3) the search for life on other planets. This chapter deals with the search for life on Mars, since this is the planet most likely to have, or have had, life.

Table 18.1, compiled from Kieffer et al. (1992), compares the surface environments on Earth and Mars. The atmosphere of Mars contains the basic elements necessary to support life – including H2O. However, it lacks sufficient atmospheric pressure and temperature for that water to exist as a liquid. The absence of water in the liquid form at any location or season on Mars is the primary reason that the surface is believed to be lifeless at the present time. However, there is direct evidence that early in its history Mars did have stable liquid water on its surface.