Overview

As we emphasized back in Chapter 1, atmospheres are not static. The mass and composition of an atmosphere evolves over time, as a result of a great variety of chemical, physical and biological processes. Now it is time to survey those processes in greater detail, and to put numbers on them to the extent possible in the limited space available in this chapter.

Throughout the following we will need to refer to some constituents of a planet as volatiles. These are "not rocks" - things that can become gases to a significant extent. The concept of a volatile is relative to the temperature of a planet. On Earth water is a volatile but on Titan it is basically a rock, as is CO2, though N2 and CH4 remain as volatiles even at the low temperatures of Titan. On Earth, sand (SiO2) is a rock, but on a roaster - a hot extrasolar Jupiter in a close orbit - it could be a volatile.

For planets in which some atmospheric volatiles exchange with a condensed reservoir, as in the case of Earth's ocean and glaciers, the whole atmosphere-ocean-cryosphere system is best treated as a unit for many purposes, and we will refer to this as the volatile envelope. In other cases, the portion of the volatile envelope which resides in the atmosphere plays a distinguished role. Only the atmospheric portion provides a greenhouse effect, and volatiles must first enter the atmosphere before they can escape to space as gases.

The main factors that govern atmospheric evolution of rocky terrestrial type planets or icy bodies with a thick solid crust are as follows. First, there is the matter of what, if anything, outgasses from the planetary interior, and at what rate. The composition of the outgassing depends on the chemistry and physics of the planet's interior; for example the early segregation of the Earth's core took iron out of the rest of the planet which allows oxygen to react with other elements. This favors the outgassing of oxidized gases such as CO2, SO2 and water vapor, though limited amounts of H2 and CH4 do exit the interior. On Titan it is speculated that ice-volcanism can release NH3 and possibly CH4 to help maintain the atmosphere, and on Venus it is speculated that the sulfuric acid clouds are maintained by outgassing of SO2 (though no active volcanism has yet been observed there.)

Next, whatever enters the volatile envelope is subject to a number of further alterations. Atmospheric constituents are lost to space, either by gradual mechanisms or in catastrophic events such as giant impacts. Different constituents are generally lost at different rates, leading to evolu tion of the composition. Then, too, chemical reactions between the volatile envelope and the solid crust can bind up constituents in mineral form, selectively removing material from the volatile envelope. If the planet has no way to engulf bits of the crust and close the cycle by cooking out the volatiles, then the volatile envelope will eventually come into equilibrium with the static upper part of the crust, and this means of evolution will mostly cease, though changes in climate could still lead to changes in partitioning between the volatile envelope and the crust. This is the situation on Mars at present. If the planet is tectonically active, as is the case for Earth, then crustal material is mixed down into the interior, and there is instead a dynamic geochemical equilibrium involving a much greater proportion of the mass of the planet. The question of whether crustal recycling occurs on Venus and Titan is one of the current Big Questions of planetary science. The discovery of extrasolar planets of the Super-Earth class raises the Big Question of whether plate-tectonics or some other form of resurfacing becomes more or less likely on rocky planets larger than the Earth or Venus.

The main energy sources that sustain plate tectonics or other forms of resurfacing are fossil heat left over from the formation of the planet and heat release by radioactive decay in the interior. These heat sources are small, but they can have a big effect on interior temperature because the diffusivity of heat through solids is so low. The importance of radioactive decay introduces another dependence of climate evolution on planetary composition, since planets can be formed with a greater or lesser endowment of radioactive materials than the Earth. Further, for planets in close orbits about their stars, or satellites in close orbits about giant planets, interior heating by tidal stresses could be important. Such processes drive spectacular volcanism on Jupiter's satellite Io, and could well play a role on Super-Earths in the habitable zone of M-dwarfs, which is quite near in to such stars.

Finally, chemical reactions within the atmosphere determine how the elemental composition is arranged into molecules. These are usually fast processes on geological time scales, requiring seconds to a few million years to operate. They nevertheless affect the long term evolution by affecting what can escape to space, what can react with the crust, and whether the elements are arranged as greenhouse gases (e.g. oxygen in the form of CO2) or not (oxygen in the form of O2). Some examples include the breakup of water vapor by ultraviolet light, the oxidation of methane or hydrogen which limits their atmospheric concentration, and the breakup of methane on Titan followed by resynthesis into ethane.

Life profoundly alters virtually every aspect of atmospheric evolution. Through the use of complex enzymes, life can break stable bonds and synthesize componds in low-temperature low-energy environments where inorganic processes do very little. Nitrogen fixation and oxygenic photosynthesis are two prime examples. We'll see in Section 8.7 that the oxygen generated in the latter process actually raises the temperature of Earth's outermost atmosphere from about 250K to over 1000K, affecting the escape of gases to space. Life can synthesize methane at rates far greater than the methane flux produced by volcanic outgassing. Life also alters the chemical environment at a planet's surface, altering the rate of reaction of atmospheric components with the crust.

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