EVOLUTION OF EARTH'S ATMOSPHERE

Although the planets Mercury, Venus, Earth, and Mars have masses within a single order of magnitude range, they possess atmospheres with extremely different properties (Table 1). These bodies may have initially possessed primordial atmospheres of solar composition whose dominant light gases (hydrogen and helium) were lost to space and replaced by outgassed water, carbon dioxide, and nitrogen (and small amounts of other gases) during the final phase of accretion 4.5 billion years (Ga) ago. The divergence in atmospheric composition seen today may in part reflect differences in initial volatile abundance, but much of the diversity can be attributed to the individual evolutionary paths of these atmospheres over the age of the Solar System. Rates of planetary atmospheric evolution have differed markedly: whereas the other planets have suffered catastrophic atmospheric evolution (Mercury has experienced complete loss, Venus a runaway greenhouse and devolatilization of surface rocks, and Mars has lost most of the atmosphere to space or the crust), the evolution of Earth’s atmosphere has been comparatively mild. Both external processes, such as radiation and the corpuscular wind from the Sun and impacts, and internal processes, such as volcanism and recycling of a planet’s crust (e.g., plate tectonics) control this evolution. While someprocesses drive exchange of compounds between the atmosphere and reservoirs in the surface, oceans, or interiors of planets, or the interconversion of different chemical species, others result in the secular, irreversible evolution of the atmosphere. Examples of the former include atmospheric photochemistry, volcanism, and plate tectonics. The latter include the accretion of new material (impacts of comets or meteorites), escape of hydrogen to space, and the sequestration of certain elements (siderophiles) into the metallic core. Earth’s atmosphere has been profoundly affected by another process: life. The modern atmosphere, containing abundant oxygen in gross chemical disequilibrium with surface organic carbon and gases such as methane, is testament to life’s ability to efficiently convert light energy into chemical energy, some of which is stored in the chemical disequilibriumbetween the atmosphere and surface. Significant disequilibrium is not present on the sterile worlds of Venus and Mars, and it has been suggested that the simultaneous presence of pairs of gases like O2 and CH4 in an atmosphere may serve as a planetary ‘biosignature’ that reveals the presence of abundant life even at a distance. Some gases such asCO2, the principal source of biological reduced carbon, are maintained at mixing ratios much lower than the level predicted in the absence of life. The current terrestrial atmosphere is far from the end state reached by Venus, where all of the surface volatiles are in the atmosphere (Table 2). Also in contrast to neighboring planets, the terrestrial atmosphere maintains conditions suitable for life (providing a modest greenhouse effect and a shield against biologically harmful radiation), and has apparently done so for 3.5 billion years, despite a 40% increase in solar luminosity, giant impacts, and the