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secondary — produced by the degassing of the material that accreted 

 to form the planet. The first type of atmosphere is found in the 

 outer solar system; the inner planets exhibit the second type. To give 

 a quantitative illustration of the extent to which the light gases are 

 deficient on the inner planets, one can imagine adding hydrogen and 

 helium to the Earth until the abundance ratios of these elements to 

 silicon were equal to the ratios found in the Sun. The resulting planet 

 would have a mass approximately equal to the mass of Saturn. 



Since we have good evidence that the composition of our 

 planet's atmosphere has changed with time, it is natural to ask 

 whether at some earlier stage it could have been strongly reducing, 

 like the atmospheres we now see in the outer solar system. In other 

 words, did the Earth and the other inner planets capture atmospheres 

 from the solar nebula as they formed? And if they did, were the 

 compositions of these atmospheres similar to those we now find in 

 the outer solar system? With time, these planets would inevitably 

 have moved from a highly reduced to an oxidized condition, since 

 hydrogen will escape from their small gravitational fields and their 

 warm surface temperatures ensure that water will be available to pro- 

 vide a source of oxygen. They must have begun with no free oxygen 

 in their atmospheres, but were these atmospheres ever as reducing 

 (or hydrogen-rich) as Jupiter's? 



For many years this seemed the most likely scenario, but the 

 evidence that was cited in its favor has become less compelling in the 

 wake of new discoveries. An early argument involved the distribution 

 of abundances of the noble gases in our present atmosphere. Com- 

 pared with the cosmic abundance pattern, the noble gases are clearly 

 deficient on Earth, and this deficiency seems to be mass-dependent, 

 such that helium is the most depleted gas, then neon, then argon, 

 etc. This pattern was taken as evidence that the Earth once had an 

 atmosphere with its full complement of volatiles, but a "catastrophic 

 event" perhaps a strong solar wind during an early unstable and 

 flaring stage of the Sun's history (T-Tauri phase) — swept away the 

 gases, removing the lighter ones most efficiently. It seemed that the 

 noble gases left us a record of this event, since they are chemically 

 inert, and (except for helium) are too heavy to escape from the 

 Earth's gravitational field. 



But now we know that this same abundance pattern is found in 

 meteorites and in the atmosphere of Mars, although the atmosphere 



