ATMOSPHERE EVOLUTION 197 ATMOSPHERE EVOLUTION S J Mojzsis, University of Colorado, Boulder, CO, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Earth is unique among the planets of the Solar System in having sustained conditions of temperature and pressure that permit stable liquid water at the surface for more than Ga The long-term stability of liquid water is fundamental to the origin and propagation of life, and Earth is exceptional among the known planets in having a biosphere Abundant free oxygen accumulated in the atmosphere as a metabolic waste product of oxygenic photosynthesis; the high freeoxygen content contrasts markedly with the atmospheres of the neighbouring planets of the inner Solar System (Table 1) This globally oxidative condition was at first deleterious to microbial life, but led to the emergence of aerobic metabolisms, sexual reproduction, and multicellularity Appreciating how the current physical and chemical state of the atmosphere came to be requires knowledge of the initial conditions The elemental ingredients that go into making a habitable world are ultimately derived from cosmologic nucleosynthesis – the formation of primarily hydrogen (1H) and helium (4He), with minor amounts of 2D, 3He, and 6Li, at the Big Bang approximately 13 Ỉ Ga ago All the other naturally Table Atmospheric constituents of the inner planets N2à O2 H2O Ar CO2 Ne He CH4 Kr H2 N2O CO Xe O3 HCl SO2 H2S Earth Venus Mars 78.084% 20.946% 0.04 ppm nd nd nd à Abundance values are given for dry air Abbreviations: tr., trace; ppm, 10 g g 1; ppb, 10 g g 1; ppt, 10 12 g g 1; nd, not detected occurring elements in the periodic table were created by stellar nucleosynthetic and supernova reactions These events contributed gas and dust to the interstellar medium, and a cloud of such supernova ashes collapsed to form the solar nebula and, ultimately, the Sun and planets about 4.56 Ga ago (see Solar System: The Sun) Origin of a Habitable World Solar System and Planetary Formation: Relevance to Atmospheric Evolution Following Big Bang nucleosynthesis, local concentrations of gas collapsed to form the first galaxies and the nebulae and stars that comprise them Gravity binds these immense structures together, and it is the gravitational collapse when stars form that creates the pressures needed to initiate thermonuclear fusion reactions, which convert four 1H nuclei to one 4He nucleus and release energy This energy provides an outward force that counters the inward gravitational collapse Although seemingly violent and catastrophic in our Earth-bound (solid–liquid–gas) view of matter in the universe, such systems are remarkably stable and are sustained over many millions of years throughout the lifetime of a star on the main sequence In smaller stars, such as the Sun, luminosity changes over the stellar lifetime, beginning with a short (less than 100 Ma) early phase of variable brightness, which lasts until the so-called main sequence of hydrogen fusion commences (Figure 1) Time in the main sequence is long for a star such as the Sun, of the order of Ga, but it eventually draws to a close when much of the hydrogen fuel is exhausted At that stage, the star expands and enters a Figure Secular changes in solar luminosity with time Solar luminosity is expressed as fraction of current solar luminosity