The Atmosphere of a Habitable PlanetAnimal life as we know it requires an atmosphere with sufficient oxygen. In order for plants to carry out photosynthesis, there must be a sufficient amount of carbon in the atmosphere in the form of carbon dioxide. For sustaining a stable habitable temperature, the mixture of greenhouse gases is critical. Loss of a suitable balance can lead to drastic heating or cooling of the planet. |
Index Habitable planet concepts Ward & Brownlee Ch 5 | ||
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An Oxygen Atmosphere for a Habitable PlanetEarth's oxygen content is 45% and 85% by volume according to Ward & Brownlee. This compares with about 0.1% in the Sun, the so-called "cosmic abundance". But having a lot of oxygen on a planet doesn't imply that there is an oxygen atmosphere. Oxygen tends to be bound in oxides so it would be quite possible to have almost no oxygen in the atmosphere. The models that have been constructed for the oxygenation of the Earth's atmosphere make a very long story. Ward and Brownlee's version of this story involves what they call the "oxygen revolution". In their model, the formation of continents at about 2.5 billion years ago provided increased habitat for vast quantities of stromatolites that released oxygen into the sea. Up until about 1.8 billion years ago, that oxygen was bound to the dissolved iron in the oceans, as evidenced by the deposits of banded iron. But around 1.8 billion years ago, the free iron was bound and oxygen began to be released into the atmosphere. Prokaryotic bacteria (cyanobacteria), using only water, carbon dioxide and energy from sunlight continued to release quantities of oxygen into the atmosphere. This revolution provided the basis for the oxygen metabolism of the eukaryotic cell machinery and became the basis for building higher animal life. |
Index Habitable planet concepts Ward & Brownlee Ch 5 | ||
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The Amount of Carbon Needed for LifeThe unique chemistry of carbon is essential for life as we know it. If the Earth did not have it, there would be no advanced life here. Yet too much carbon is also disastrous to life, so we should be appreciative that the Earth seems to have just about the right amount. Venus has about 100 times as much carbon and suffers from the runaway greenhouse effect that has produced a surface temperature of about 750 K compared to about 300K for the Earth. That is hot enough to melt lead! Mars hasn't been able to hold its atmosphere and has 1/100 the carbon of the Earth. Carbonaceous meteorites have up to 4% carbon, but the Earth is only 0.05% carbon. With the large variations in carbon content in our neighbordhood of the solar system, the Earth seems to be in the narrow window that provides enough carbon for life in general and and photosynthesis in particular. Yet it does not have so much as to trigger a runaway greenhouse effect.
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Index Habitable planet concepts Ward & Brownlee | ||
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Balancing Greenhouse GasesThe unique chemistry of carbon is essential for life as we know it. If the Earth did not have it, there would be no advanced life here. Yet too much carbon is also disastrous to life, so we should be appreciative that the Earth seems to have just about the right amount. Venus has about 100 times as much carbon and suffers from the runaway greenhouse effect that has produces a surface temperature of about 750 K compared to about 300K for the Earth. That is hot enough to melt lead! Mars hasn't been able to hold its atmosphere and has 1/100 the carbon of the Earth. Carbonaceous meteorites have up to 4% carbon, but the Earth is only 0.05% carbon. With the large variations in carbon content in our neighbordhood of the solar system, the Earth seems to be in the narrow window that provides enough carbon for life in general and and photosynthesis in particular. Yet it does not have so much as to trigger a runaway greenhouse effect. |
Index Habitable planet concepts Ward & Brownlee | ||
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