Life Sciences in the Space Program 



to fill the gap in Earth's geological record and providing a means of reconstructing 

 earlv environmental conditions. 



In the absence of a geological record, the development of phvsical models for the 

 formation and early evolution of the terrestrial planets — Earth, Mars, and Venus — 

 is essential to placing bounds on the range of physical and chemical conditions 

 that may have existed during their first billion years of historv. The value of these 

 models will lie in their ability to elucidate the properties and processes that 

 endow planets with their inventories of biogenic elements and that govern the 

 composition of atmospheres over time, the history of liquid water, and the nature 

 and distribution of environments conducive to the origin of life. 



A highly chemically reduced atmosphere dominated by methane and nitrogen was 

 postulated in early models of the primitive Earth and is exemplified by the current 

 atmosphere of Saturn's satellite, Titan. Many laboratory' experiments have shown 

 that most of the biochemical building blocks of proteins, nucleic acids, and 

 membranes can be synthesized under so-called prebiotic conditions starting with 

 these atmospheric constituents and water. Although the Voyager flyby missions 

 revealed traces of many organic compounds in Titan's atmosphere, the degree of 

 molecular complexity attained in the atmosphere and the physical processes 

 responsible for their syntheses are unclear. The deployment of chemical probes 

 into Titan's atmosphere and surface, as envisioned for the Titan/Cassini Mission, 

 will shed much more light on these uncertainties. Direct comparisons between 

 laboratory experiments and a planetarv atmosphere would provide a unique 

 opportunity to test models for abiotic organic synthesis. 



Recent models of Earth that take into account early core formation and subsequent 

 outgassing of an atmosphere during late stages of planetary accretion argue for a 

 thick primordial atmosphere dominated by carbon dioxide overlying a warm 

 ocean. In the few laboratory experiments and computer simulations that have 

 been carried out, abiotic synthesis of organic compounds appears to be difficult to 

 achieve in such an atmosphere. Plausible mechanisms tor synthesizing the organic 

 building blocks necessary to construct the first cellular metabolic and genetic 

 systems starting from carbon dioxide, water, and nitrogen in the atmosphere and 

 with the components of seawater have not been extensively studied and remain to 

 be demonstrated for these models. 



In this context, the discovery of abundant organic matter in Comet Hallev, as well 

 as in carbonaceous meteorites, has underscored the possibility that comets and 

 asteroids may have supplied some of the precursor molecules for the synthesis of 

 biochemical compounds during Earth's early history, in addition to supplying 

 much of the planet's endowment of water and biogenic elements. A quantitative 

 assessment of this question will require knowledge of the inventories of organic 

 (.(impounds in comets and asteroids, determination ol the size distribution of 

 comets and asteroids that struck the Earth in its first billion-war history, and a 

 better understanding of the phvsical and chemical effects of impacts involving 

 medium-sized (tens of kilometers in diameter) to large (hundreds of kilometers in 





