Lecture19 - Atmospheres An atmosphere acts as a blanket...

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Unformatted text preview: Atmospheres An atmosphere acts as a blanket which insulates the surface below and can alter it via wind and water circulation. Primary atmospheres are made of gases accreted at the time of planet formation Accreted from hydrogen-rich solar nebula The lightest molecules (H, He and H 2 ) would float to the top and escape Heavy, inert gases (like Ne) are good tracers of the primary atmosphere. From the amount of Ne in Earths atmosphere, we see that only 0.9% of its mass is primary. Earths primitive atmosphere would have been very oxygen-poor Secondary atmospheres are formed at a later date. Volcanic outgassing is the major contributor on Earth Main producer of the CO 2 and H 2 O that are the dominant volatile elements on Earth today. Oxygen-rich atmosphere evolved slowly due to dissociation of H 2 O in upper atmosphere, and photosynthesis Physical Structure atmospheres have a structure, primarily vertical, which is determined by density, pressure, chemical composition, and temperature. heated mainly from above, by radiation from the Sun. There is usually some internal heating near the surface, but it is generally less important. Outer boundary is not sharply defined and at the highest altitudes conditions are right for molecules to escape thus atmospheres can lose mass over time, some more significantly than others. Determined by the balance between pressure and gravity (hydrostatic equilibrium) dz g dP = Here we use z as the vertical distance g=MG/R 2 is the gravitational acceleration The pressure scale height H is defined as g m kT H u , so that (see example) H z e P P / = Atmospheres are heated primarily from the top, due to absorbed sunlight. It is also through the upper atmosphere that most heat loss occurs. The balance between heating and cooling determines the structure of the upper atmosphere. Thermal Structure Troposphere from the surface, temperature initially decreases with altitude because of clouds which form from condensable gases. Condensation releases heat and also clouds keep heat in at the surface. Because the condensation process is very temperature dependent, cloud layers are usually confined to a narrow altitude range. Stratosphere : temperature now increases with altitude clouds are below and the Sun (heating) is above....
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Lecture19 - Atmospheres An atmosphere acts as a blanket...

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