The schema of the biological exploration of a planet is to conduct a series of complementary experiments proceeding from general to specific. The general experiments will examine gross characteristics of the planet’s environment and surface for determining the probability of an active biota (life). Data from the general experiments will be significant in— (1) Defining the nature of specific experiments in which life detection is the major objective; and (2) Providing a high degree of confidence in undertaking specific experiments, since indications from the gross characterization of the planet in question will influence the choice and design of the specific experiments. The biological exploration of planets is then to be defined as the search for those parameters relevant to the origin, development, sustenance, and degradation of life in a planetary environment. This definition will give rise to a critical question for each progressively specific and complex experiment to determine— (1) The existence of life on the planet (2) The degree of similarity or dissimilarity (structure and function) with respect to terrestrial life (3) The origin of this planetary life The immediate objective of the biological explorations of the planet is to define the state of the planetary surface, which may exhibit the following properties: (1) A prebiota (defined as the absence of life) (2) An active biota (defined as the presence of life) (3) An extinct biota (defined as evidence of former life) The identification and the detailed characterization of each of the above stages of planetary development constitute the subject matter of the biological exploration of the planets and, specifically, Mars. THE EXPERIMENTAL INVESTIGATION OF CHEMICAL EVOLUTION Attempts have been made to simulate and approximate models of primitive Earth conditions for abiogenic synthesis, and successful synthesis of essential biochemical constituents necessary for maintaining life has been partly accomplished. Urey ([ref.11]) has clearly pointed out the possible role of a reducing atmosphere in the synthesis of prebiological organic molecules. Miller ([ref.12]) synthesized a variety of amino acids in a reducing atmosphere by means of an electrical discharge. A variety of organic compounds have been synthesized by the action of various energy sources upon reducing atmospheres, and several investigators have extended the Urey-Miller-type reactions to synthesize nucleic acid components ([ref.13]), adenosine triphosphate ([ref.14]), and a host of biologically essential organic compounds.