59 



about these two planets, this possibility became increasingly remote. 

 Our considerations of the origin of life on Earth must therefore 

 include some discussion of the unique properties of Earth itself. 

 Why is our planet so different from its nearest neighbors? 



We can quickly identify two basic characteristics that will deter- 

 mine most of a planet's development — size and distance from the 

 Sun. If a planet is too large, hydrogen will not be able to escape from 

 its gravitational field, and the result will be an object like Jupiter or 

 Saturn, with a huge, dense atmosphere and no solid surface. At the 

 other extreme, a body that is too small will not be able to retain any 

 kind of atmosphere over geological periods of time. Examples of this 

 end of the spectrum are Mercury and the Moon. Distance from the 

 Sun becomes a part of this constraint, however, since a small body at 

 a large distance could be cold enough to have an atmosphere. What 

 this means is that the thermal velocities of gas molecules will be 

 smaller than the velocity needed to escape from the body's gravita- 

 tional field (given that the molecular weight of the gas is high enough 

 and that the gas does not condense at the low temperature corre- 

 sponding to this distance from the Sun). In our solar system, the best 

 example of this situation is Titan, a satellite of Saturn, that has a 

 dense atmosphere of nitrogen with a small amount of methane and 

 traces of other compounds. 



The inner planets — Mercury, Venus, Earth, Mars — have clearly 

 been drastically affected by their proximity to the Sun. They are all 

 grossly deficient in the volatile elements, having formed in an envi- 

 ronment that was evidently at too high a temperature to permit the 

 common compounds of these elements to condense. In contrast, the 

 outer planets — Jupiter, Saturn, Uranus, Neptune — have retained 

 large amounts of hydrogen and helium. Jupiter may even represent a 

 cosmic mixture of the elements; i.e., the composition of this planet 

 may be identical to the composition of the Sun and other young 

 stars. 



ANCIENT ATMOSPHERES 



We can now distinguish between two types of atmospheres - 

 those that are primitive, representing material captured from the 

 solar nebula with only minor fractionations, and those that are 



