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

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Stromatolites in Western Australia, today. Did ancientEarth look like this?billion years ago. So, the origin of life isprobably the origin of evolution. Life isresourceful and entrepreneurial. It takesadvantage and it changes the chemistry of itssurroundings. Life is a fantastically complexsystem, the emergence of which remains thegreatest mystery in science.Long-term changes in the composition of theatmosphere and oceans are intimately linked toboth the geophysical changes in the solid Earthitself and with the ongoing evolution of life. Theatmosphere and oceans first appeared about4.5 billion years ago, soon after the Earth andMoon completed their formational phase. Thiswas the time when gases escaping throughvolcanoes made an envelope of atmospherearound the young Earth, and a primitive crustsolidified and cooled to the point where liquidwater could condense. Water began poolinginto the first lakes, seas, and oceans. Theinteraction of water, heat, and rock set thestage for the origin of life.From the beginning, a number of factors haveaffected the makeup of the atmosphere tochange it from its initial state to what we havetoday. Several of these factors, such as platetectonics, weathering (which recycle rocks,water, and gases), and chemical changesinduced by the byproducts of life itself, areinternal to the planet. However, external factorssuch as the slowly but ever-increasingluminosity of the Sun over billions of years,gradual changes in the Earth’s orbit over manytens of thousands of years, and the rare butcatastrophic impacts of giant meteorites andcomets, have also played an important role.The atmosphere of our planet did not originallycontain all the free, breathable oxygen it doesnow. The first permanent atmosphere arosewhen gases that had been dissolved in themolten planet during its assembly from smallerbodies, called “planetesimals,” were released tothe surface by volcanism. That first, primitiveatmosphere was probably several times denserthan what we have now, and was dominatednot by oxygen, but by carbon dioxide—amajor greenhouse gas. Other gases, such asmolecular nitrogen, water vapor, and smallamounts of carbon monoxide, sulfur gases, andtrace quantities of methane, and hydrogen werealso present.Astrophysical computer models based on thestudy of young stars and of star-forming regionsin the galaxy strongly suggest that the Sun wasmuch dimmer when the first life emerged onEarth, over 4 billion years ago. A dimmer Sunwould have supplied less solar radiation towarm the early Earth. To keep the Earth fromstarting out as a frozen wasteland, with no hopefor beginning life to take hold, an atmospheric“greenhouse” must have kept the surface zonewarm enough to maintain water in liquid form.Liquid water is the prerequisite for life.Greenhouse gases in much higher abundancethan today, primarily water vapor, carbondioxide, and methane, would have formed athermal blanket over the surface of the earlyEarth that strongly absorbed outgoing thermalradiation. This leads to a significantly enhanced,warming “greenhouse effect” that offset thedimmer Sun. Without this very different,greenhouse gas-rich and oxygen-poor earlyatmosphere to begin with, life would havegotten a frozen start. (See the essay by CharlesF. Keller in Section Five, which discussesgreenhouse gases and global warming.)The composition of this early atmosphere, sodifferent from the one we have now, would bedeadly for most life that is not a primitivebacterium. Soon after the emergence of thefirst life more than 4 billion years ago, theactivities of organisms began to influence thecomposition of the atmosphere. As Earth’sbiosphere and atmosphere co-evolved over
illions of years, the product of photosynthesis,mainly, free oxygen, has come to dominate thechemistry of the atmosphere. Without thebyproducts of crucial biological processes suchas photosynthesis, there would be little or nofree oxygen for animals to breathe, nor enoughto form a protective ozone layer in the upperatmosphere. Ozone (O3) is formed by therecombination of oxygen (O2) by solar radiationin the upper atmosphere into the new moleculewith three oxygen atoms. This molecule screensharmful ultraviolet sunlight and prevents most ofit from reaching the surface. Without aneffective ozone layer in the upper atmosphere,more advanced life would not have been able toemerge from the seas to colonize the land. Wecan safely say that the vast amount of ozoneneeded to permit the colonization of landbecame possible only after significant freeoxygen accumulated from photosynthesisbeginning early in Earth’s history.Since little evidence from the Earth’s formationaltime has survived for scientists to study, it ispossible to only make a few basic conclusionsabout the nature of the first atmosphere and theconditions in which life originated. Most of whatis known about early planetary history comesfrom three sources: the sparse geologic recordremaining from more than 3.5 billion years ago,computer models of atmospheres changing withtime, and comparing Earth to its planetaryneighbors, Mars and Venus, which evolvedalong very different paths. Scientists have alsorecently started to study the sophisticatedbiochemical machinery of primitivephotosynthetic bacteria to learn more abouttheir early evolution. These primitive bacteriaare descended from the pioneeringmicroorganisms that forever changed thechemistry of the atmosphere from a primitiveunbreathable mix to oxygen-rich air.How Earth’s Atmosphere OriginatedThe elements and compounds that make up theatmosphere and oceans evaporate readily undernormal surface conditions; that’s the definitionof “volatile.” Elsewhere in the universe, thesevolatiles would assume very different physicalstates. In the space between planets and stars,where it is very cold, they would be presentmostly as solid ices; however, near starsor in regions where new stars are beingformed, where it is hot, they would be presentas plasma.Plasma is a fourth state of matter after solid,liquid, and gas. It exists only at extremely hightemperatures when electrons are stripped offatoms to form charged particles. Plasma is infact the most common state of matter in theuniverse, yet this state is useless for life; onlysolid bodies like planets can support matter inthe usable solid-liquid-gas states. Life as weknow it can occur only where a volatile such aswater is stable in the liquid state, and whereenergy resources like sunlight or chemicalenergy are available for exploitation for makingor supplying food. Such special conditions forlife seem to be satisfied only on planetaryTable 1: Comparison of atmospheric compositions of Venus, Mars, Earthpressure (bars)* CO 2 (%) N 2 (%) Argon-36 (%) H 2 0 (%) 0 2 (%)Venus 92 96.5 3.5 0.00007
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"Life and the Evolution of Earth's Atmosphere." Earth

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