question encompasses sources for the observed molecules and aerosols, produc- 

 tion of useful biological precursors, effects of various types of energy, processes 

 that lead to the conversion of an atmosphere from oxygen-free to oxygen- 

 containing, and any other conditions that led to the various planetary 

 atmospheres. 



5. What drives atmospheric dynamics and how may these drivers affect con- 

 ditions favorable for the origin of life? 



6. Are the other atmospheres found in the solar system useful as models for 

 the atmosphere of the early Earth? 



It is probable that the information we can obtain from Earth orbital observa- 

 tions of the planets and other solar system bodies will allow us to formulate and 

 test models for the formation and subsequent evolution of the solar system. 



Lander and orbital spacecraft studies of Mars and Venus have yielded detailed 

 information on the composition of their atmospheres and some indications of 

 surface composition. Further investigations concerning the possibility of fossil 

 remains of microbial life, the search for subsurface water, and the determination 

 of the length of time that liquid water was present on the surface of both planets 

 are very relevant to exobiology. Detailed data are needed for these studies and 

 will require in situ experiments or sample returns. Several such studies are in 

 the conceptual and planning stages and one, Mars Observer, which will study the 

 atmosphere and surface from local orbit, is being developed for launch. Further 

 observations from the ground or Earth orbit will be useful in the selection of 

 sites for in situ studies or collection of samples. 



The outer planets, however, present an area where extremely useful remote 

 observations could be made because less is known about their atmospheres. 

 Moreover, in some ways they are models of the early Earth, particularly of the 

 period when there was no free oxygen in the atmosphere. One interesting study 

 has demonstrated that it is energetically feasible for organisms to live in a liquid 

 ocean under the crustal ice on Europa. Even if life itself has not developed on 

 the outer planets, or their satellites, study of their atmospheres will most cer- 

 tainly be useful for placing the origin of life in the context of the origin of the 

 solar system. From an exobiological perspective, three basic areas of study are 

 appropriate for the outer planets: origin of the atmospheres, evolution of the 

 atmospheres, and searches for other examples of satellites possessing atmo- 

 spheres. Each of these will be discussed in the following paragraphs. 



There are currently two basic models, with some variations, to explain the 

 origin of the outer planets, and they make different predictions about the nature 

 of the planets' atmospheres. In the first model, a gravitational instability in the 

 primordial nebula leads to the formation of giant gaseous protoplanets that col- 

 lapse and later segregate out a core. The second model begins with the nuclea- 

 tion of grains and their subsequent aggregation to form the cores of the giant 

 planets followed by a runaway accretion to collect the atmospheres. If the first 

 model was operative the chemical elements should have been retained in solar 



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