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    which of the following was one of the first life forms to add oxygen to the atmosphere?


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    The Origin of Oxygen in Earth's Atmosphere

    Scientific American is the essential guide to the most awe-inspiring advances in science and technology, explaining how they change our understanding of the world and shape our lives.

    The Origin of Oxygen in Earth's Atmosphere

    It's hard to keep oxygen molecules around, despite the fact that it's the third-most abundant element in the universe, forged in the superhot, superdense core of stars. That's because oxygen wants to react; it can form compounds with nearly every other element on the periodic table. So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff?

    The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (known as chloroplasts) to do their photosynthesis for them down to this day.

    For some untold eons prior to the evolution of these cyanobacteria, during the Archean eon, more primitive microbes lived the real old-fashioned way: anaerobically. These ancient organisms—and their "extremophile" descendants today—thrived in the absence of oxygen, relying on sulfate for their energy needs.

    But roughly 2.45 billion years ago, the isotopic ratio of sulfur transformed, indicating that for the first time oxygen was becoming a significant component of Earth's atmosphere, according to a 2000 paper in Science. At roughly the same time (and for eons thereafter), oxidized iron began to appear in ancient soils and bands of iron were deposited on the seafloor, a product of reactions with oxygen in the seawater.

    "What it looks like is that oxygen was first produced somewhere around 2.7 billion to 2.8 billon years ago. It took up residence in atmosphere around 2.45 billion years ago," says geochemist Dick Holland, a visiting scholar at the University of Pennsylvania. "It looks as if there's a significant time interval between the appearance of oxygen-producing organisms and the actual oxygenation of the atmosphere."

    So a date and a culprit can be fixed for what scientists refer to as the Great Oxidation Event, but mysteries remain. What occurred 2.45 billion years ago that enabled cyanobacteria to take over? What were oxygen levels at that time? Why did it take another one billion years—dubbed the "boring billion" by scientists—for oxygen levels to rise high enough to enable the evolution of animals?

    Most important, how did the amount of atmospheric oxygen reach its present level? "It's not that easy why it should balance at 21 percent rather than 10 or 40 percent," notes geoscientist James Kasting of Pennsylvania State University. "We don't understand the modern oxygen control system that well."

    Climate, volcanism, plate tectonics all played a key role in regulating the oxygen level during various time periods. Yet no one has come up with a rock-solid test to determine the precise oxygen content of the atmosphere at any given time from the geologic record. But one thing is clear—the origins of oxygen in Earth's atmosphere derive from one thing: life.



    David Biello is a contributing editor at Scientific AmericanFollow David Biello on Twitter

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    The Origin of Oxygen in Earth's Atmosphere

    Source : www.scientificamerican.com

    evolution of the atmosphere

    Evolution of the atmosphere, process by which Earth’s modern atmosphere arose from earlier conditions. Evidence of these changes, though indirect, is abundant. Sediments and rocks record changes in atmospheric composition from chemical reactions with Earth’s crust and biochemical processes associated with life.

    Ultimate sources

    The material from which the solar system formed is often described as a gas cloud or, at a later stage, a solar nebula. The cloud was rich in volatiles (termed primordial gases) and must have been the ultimate source of the atoms in the present atmosphere. What is of primary concern, however, is the sequence of events and processes by which the volatiles present in the initial gas cloud were transferred to Earth’s inventory and the efficiency with which this was accomplished.

    Orion Nebula (M42)

    The formation of the solar system began when one portion of the gas cloud became dense enough due to compression by some external force—a shock wave from the explosion of a nearby supernova, perhaps—to gravitationally attract the material around it. This material “fell” into the region of higher density, making it even denser and attracting other material from still further away. As gravitational collapse continued, the centre of the cloud became very dense and hot, because the kinetic energy of the incoming material was released as heat. Thermonuclear reactions began at the core of the central object, the Sun.

    Capture and retention of primordial gases

    Far from the central point, the material in the gas cloud tended to settle to an extensive equatorial plane around the Sun. As the material in this disk cooled, chunks of rock grew and accreted to form the planets. The planets are much less massive than the Sun, but if they grew large enough and if the gases around them were cool enough, they could accumulate an atmosphere from the volatile components of the gas cloud. This direct capture is the first of three source mechanisms that can be described.

    A planetary atmosphere accumulated in this way would consist of primordial gases, but the relative abundances of the individual components would differ from those in the gas cloud if the gravitational field of the new planet were strong enough to hold some, but not all, of the gases around it. It is convenient to express the strength of a gravitational field in terms of escape velocity, the speed at which any particle (a molecule or spacecraft) must be traveling in order to overcome the force of gravity. For Earth, this velocity is 11.3 km (7.0 miles) per second, and it follows that, once the solid material had accumulated, gas molecules passing Earth at lower speeds would have been captured and accumulated to form an atmosphere.

    The speed at which a gas molecule moves is proportional to (T/M)1/2, where T is absolute temperature in kelvins (K) and M is molecular mass. The uppermost layers of the present atmosphere are still very hot and might have been much hotter early in Earth’s history. At temperatures below 2,000 K, however, molecules of any compound with a molecular weight greater than about 10 will have an average velocity of less than 11.3 km per second (7.0 miles per second). On this basis, it has long been thought that Earth’s earliest atmosphere must have been a mixture of the primordial gases with molecular weights greater than 10. Hydrogen and helium, with molecular weights of 2 and 4, should have been able to escape. Because hydrogen is the most abundant element in the solar system, it is thought that the most abundant forms of the other volatile elements were their compounds with hydrogen. If so, methane, ammonia, and water vapour, together with the noble gas neon, would have been the most abundant volatiles with molecular weights greater than 10 and, thus, the major constituents of Earth’s primordial atmosphere. The atmospheres of the four giant outer planets (Jupiter, Saturn, Uranus, and Neptune) are rich in such components, as well as in molecular hydrogen and, presumably, helium, which those more massive and colder bodies were apparently able to retain.

    evolution of the atmosphere

    Source : www.britannica.com

    How did Earth's atmosphere form?

    Earth is on its third atmosphere! We wouldn't have liked the first two at all!


    No one knows of any other planet where you can do this simple thing.

    Other planets and moons in our solar system have atmospheres, but none of them could support life as we know it. They are either too dense (as on Venus) or not dense enough (as on Mars), and none of them have much oxygen, the precious gas that we Earth animals need every minute.

    So how did our atmosphere get to be so special?

    Some scientists describe three stages in the evolution of Earth’s atmosphere as it is today.

    Drawing of part of Earth's surface, with helium and hydrogen atoms in atmosphere, heading upward toward space.

    Just formed Earth: Like Earth, the hydrogen (H2) and helium (He) were very warm. These molecules of gas moved so fast they escaped Earth's gravity and eventually all drifted off into space.

    Earth’s original atmosphere was probably just hydrogen and helium, because these were the main gases in the dusty, gassy disk around the Sun from which the planets formed. The Earth and its atmosphere were very hot. Molecules of hydrogen and helium move really fast, especially when warm. Actually, they moved so fast they eventually all escaped Earth's gravity and drifted off into space.

    Drawing of part of Earth's surface, with water, carbon dioxide, and ammonia molecules in atmosphere.

    Young Earth: Volcanoes released gases H2O (water) as steam, carbon dixoide (CO2), and ammonia (NH3). Carbon dioxide dissolved in seawater. Simple bacteria thrived on sunlight and CO2. By-product is oxygen (O2).

    Earth’s “second atmosphere” came from Earth itself. There were lots of volcanoes, many more than today, because Earth’s crust was still forming. The volcanoes released

    Drawing of part of Earth's surface, with water, oxygen, ozone and nitrogen in the atmosphere.

    Current Earth: Plants and animals thrive in balance. Plants take in carbon dioxide (CO2) and give off oxygen (O2). Animals take in oxygen (O2) and give off CO2. Burning stuff also gives off CO2.

    Much of the CO2 dissolved into the oceans. Eventually, a simple form of bacteria developed that could live on energy from the Sun and carbon dioxide in the water, producing oxygen as a waste product. Thus, oxygen began to build up in the atmosphere, while the carbon dioxide levels continued to drop. Meanwhile, the ammonia molecules in the atmosphere were broken apart by sunlight, leaving nitrogen and hydrogen. The hydrogen, being the lightest element, rose to the top of the atmosphere and much of it eventually drifted off into space.

    Now we have Earth’s “third atmosphere,” the one we all know and love—an atmosphere containing enough oxygen for animals, including ourselves, to evolve.

    So plants and some bacteria use carbon dioxide and give off oxygen, and animals use oxygen and give off carbon-dioxide—how convenient! The atmosphere upon which life depends was created by life itself.

    How did Earth's atmosphere form?

    Source : scijinks.gov

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