"Life and the Evolution of Earth's Atmosphere." Earth

LIFE AND THE EVOLUTION OF EARTH’S ATMOSPHEREhelp us fine-tune theoretical models that attemptto shed light on conditions at the Earth’ssurface when life emerged. Studyingneighboring worlds is also relevant because itallows us to compare the very different pathstaken by these planets in the absence of life. Itis also necessary to keep in mind theimmense expanse of time involved. Geologistsapproach the geologic record much like adetective tackles problems in a detective story;the plot thickens as new evidence isuncovered, but it often yields more questionsthan answers.In science, theories evolve as evidenceaccumulates. But theories must be grounded inthe solid principles of physics, mathematics,and chemistry that describe the behavior ofmatter in the universe. We are now on thethreshold of being able to evaluate the placeof Earth in the universe; we may discoverthat we are not all that unique in having awater-rich planet with abundant life and oxygen.The search for other worlds that contain, orcontained, life stands as the grand quest forthe next millennium.Partial Pressure Pressure of O 2 of in Othe 2 in Atmospheres the OvertimeTimeO 2partial pressure ( bar )10 010 -210 -310 -410 -510 -610 -710 -8LUNAR FORMATION?10 -910 -10?10 -1110 -1210 -13?? ?paleosolsfossil evidencefor Eukaryotes( Grypania )BIFland plantsPAL10 -1 4500 4000 3500 3000 2500 2000 1500 1000 500 0Time before present ( Ma )Figure 2: This plot (after Kasting, 1993) shows estimates ofchanging concentrations of free oxygen in the atmosphereover geologic time. The area (in light blue) represents therange in possible concentrations of oxygen based on modelcalculations, and the study of ancient soils (paleosols),fossil organisms, and marine sediments that only form inthe absence of oxygen. These sediments are preserved asBanded Iron Formations (BIF) and only appear in the geologicrecord up to about 1.8 billion years ago. Although it is notknown when photosynthesis began, it is clear thatphotosynthesis only became an important producer ofoxygen in the atmosphere late in Earth’s history. PAL =present atmospheric level of O2.