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| * ]s - regions where particles from the Sun become concentrated. | * ]s - regions where particles from the Sun become concentrated. | ||
| == The evolution of |
== The evolution of Terra's atmosphere == | ||
| The modern atmosphere is sometimes referred to as its third atmosphere; in order to distinguish the current chemical compostion from two notably different compositions. | |||
| Originally the Earth's surface was molten, so that any atmosphere was very unstable. About 3.5 billion years ago the surface had cooled enough to form a crust, still heavily populated with ]es shooting steam, carbon dioxide and ] in to the air. At this time the atmosphere was mostly carbon dioxide with virtually no oxygen. | Originally the Earth's surface was molten, so that any atmosphere was very unstable. About 3.5 billion years ago the surface had cooled enough to form a crust, still heavily populated with ]es shooting steam, carbon dioxide and ] in to the air. At this time the atmosphere was mostly carbon dioxide with virtually no oxygen. | ||
Revision as of 02:07, 27 June 2003
On Earth, the atmosphere consists of nitrogen (78.1%) and oxygen (20.9%), with small amounts of argon (0.9%), carbon dioxide (variable, but around 0.035%), water vapour and other gases. The atmosphere protects life on the planet by absorbing ultraviolet radiation of the sun, and reducing the temperature difference between day and night.
The temperature of the Earth's atmosphere varies with height above sea level:
- troposphere - 0 - 7/17 km, temperature decreasing with height.
- stratosphere - 7/17 - 50 km, temperature increasing with height.
- mesosphere - 50 - 80/85 km, temperature decreasing with height.
- thermosphere - 80/85 - 640+ km, temperature increasing with height.
The boundaries between these regions are named the tropopause, stratopause and mesopause.
Atmospheric regions are also named in other ways:
- ionosphere - the region containing ions: approximately the mesosphere and thermosphere up to 550 km.
- exosphere - above the ionosphere, where the atmosphere thins out into space.
- ozone layer - or ozonosphere, approximately 10 - 50 km, where ozone is found.
- magnetosphere - the region where the Earth's magnetic field interacts with the solar wind from the Sun. It extends for tens of thousands of kilometers, with a long tail away from the Sun.
- Van Allen radiation belts - regions where particles from the Sun become concentrated.
The evolution of Terra's atmosphere
The modern atmosphere is sometimes referred to as its third atmosphere; in order to distinguish the current chemical compostion from two notably different compositions.
Originally the Earth's surface was molten, so that any atmosphere was very unstable. About 3.5 billion years ago the surface had cooled enough to form a crust, still heavily populated with volcanoes shooting steam, carbon dioxide and ammonia in to the air. At this time the atmosphere was mostly carbon dioxide with virtually no oxygen.
During the next one or two billion years the water vapour condensed to form oceans, which started to dissolve the carbon dioxide. Green photosynthesizing plants evolved over the surface of the earth converting carbon dioxide in to oxygen and locking the carbon in to fossil fuels. Carbon was also locked in sedimentary rocks. As more oxygen was released in to the atmosphere it reacted with the ammonia releasing nitrogen gas. Ammonia was also converted in to nitrogen gas by nitrifying bacteria.
As more photosynthesizing plants appeared the levels of oxygen increased dramatically - leading to mass extinction of species adapted to the reducing atmosphere, and encouraging the evolution of more complex organisms. The appearance of the ozone layer further favoured these new plants by protecting them from harmful ultraviolet radiation. At this time the carbon dioxide levels became very low - as it remains today.
In modern times, the burning of fossil fuels has caused an increase in carbon dioxide in the atmosphere. This increases the amount of infrared radiation emitted from the atmosphere towards the earth's surface, thereby contributing to global warming. The IPCC concluded in their Climate Change 2001 report that "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations".