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.

It is likely that in the synthesis of organic moieties, simple and specific molecules were first produced when the planets had a reducing atmosphere. Further complexity or degradation of the organic compounds produced varied, depending on the geochemical changes of the planet's surface, the atmospheric constituents, the degree of interaction between surface and atmosphere, and the rate of the organic synthesis. Oparin ([ref.15]) presented the most detailed mechanisms for the spontaneous generation of the first living organism arising in a sea of organic compounds synthesized in a reducing atmosphere on Earth.

It is generally accepted that, under favorable conditions, life can arise by spontaneous generation. A primary requirement for this initiation is that there be abundant organic compounds concentrated in one or more specific zones. These simple organic molecules would undergo modification to develop a greater structural complexity and specificity, finally giving rise to a "living" organism. Therefore, because of the ease with which organic compounds can be synthesized under reducing conditions, planetary surfaces may contain an abundant source of similar organic matter. However, difficulties arise in postulating steps for further organization or modification of the above synthesized organic matter into a living state. Most of the original organic matter produced in the primary reducing atmospheres of the various planets may have been quite similar. However, major variations between planets, in chemical evolution beyond the prebiotic stage, must have been the rule rather than the exception.

The primary interest in this area of research has been the realization of the possible existence of organic molecules on planetary surfaces and, particularly, Mars. Pertinent synthesis may be either biological or abiological. Research conducted in the simulation of cosmochemical synthesis has used most of the available solar spectrum. Simulation experiments devised to study the effects of these energies on the assumed early atmosphere of the Earth have yielded products that play a dominant role in molecular and biochemical organization of the cell.