evolution of the atmosphere Sequence of events in the development of the atmosphere

If the planet grew large (and had, therefore, a substantial gravitational field) before all gases were dispersed from its orbit, it ought to have captured an atmosphere of nebular gases. The size and composition of such an atmosphere would depend on temperature as well as planetary mass. If the solid planet had reached full size and if temperatures were greater than 2,000 K, the minimum molecular weight that could be retained might have been high enough that the very abundant gases with molecular weights between 10 and 20 (methane, ammonia, water, and neon) would have been collected inefficiently, if at all. A thinner primordial atmosphere consisting of nebular gases with higher molecular weights (such as argon and krypton—see the table), however, ought still to have been captured.

In spite of this, characteristics of the present atmosphere show clearly that a primordial atmosphere either never existed or was completely lost. Explanations offered for both of these possibilities are linked to the development of the Sun itself. Astronomical observations of developing stars (that is, bodies similar to the early Sun) have shown that their early histories are marked by phases during which the gas in their surrounding nebulas is literally blown away by the pressure of light and particles ejected from the stars as they “turn on.” (After this initial intense activity, young stars begin life with an energy output significantly below their mid-life maximum.) If the removal of gases occurred in the solar system after nonvolatile solids had condensed but before the inner planets (Mercury, Venus, Earth, and Mars) accreted, it would have been impossible for Earth to capture a primordial atmosphere. Alternatively, if planetary accretion preceded ejection of gases and Earth had accumulated a primordial atmosphere, perhaps the early solar radiation, particularly the solar wind, was so intense that it was able to strip all gases from the inner planets, meeting the second condition described above—namely, complete loss.