COE - Earth's Modern Atmosphere Lesson
Earth's Modern Atmosphere
As previously mentioned, Earth's original atmosphere is thought to have been very similar to the atmospheres of its two neighbors - Mars and Venus. However, Earth's atmosphere is very different today, because Earth is unique among the planets in the solar system in that it has living organisms on it. These organisms are responsible for chemically altering the composition of the atmosphere.
The oldest fossil evidence of life on Earth is of colonies of photosynthetic bacteria, called stromatolites, estimated to be around 3.5 billion years old. These stromatolites are very similar to colonies that can be found today on the coast of the Northwest Territory in Australia, living in shallow waters where the sunlight is plentiful. Like all other photosynthetic organisms, they utilize carbon dioxide from the atmosphere to create sugars and release oxygen into the atmosphere as a waste product.
As the first photosynthetic organisms consumed carbon dioxide and churned out oxygen, they slowly but steadily effected several major changes on Earth. The oxygen that they produced didn't end up in the atmosphere; rather, it was consumed in chemical reactions with iron dissolved in seawater, forming layers of rust on the ocean floor. On Earth, this rusting led to the formation of banded iron formations in rocks as the oceans receded.
Around 2 billion years ago - 1.5 billion years after photosynthetic organisms first appeared on Earth - oxygen began to leave the oceans and move into the atmosphere. Much of this oxygen was immediately consumed in chemical reactions with the elements in the Earth's crust (Mars underwent a similar rusting event in its past, which explains its red surface color), and some of it reacted higher up in the atmosphere to form ozone.
Ultimately, the buildup of atmospheric oxygen enabled life to leave the oceans and move onto land (a topic that will be revisited later in this course).
Layers of the Atmosphere
Earth's atmosphere can be classified according to changes in density and temperature. The lowest layer, the troposphere, extends from sea level up to about 10 km (6 mi), and contains around 75% of all of the air molecules that make up the atmosphere. All life exists within the troposphere. The temperature of the troposphere decreases as altitude increases, but this changes once one enters the next layer.
The stratosphere begins approximately 10 km (6 mi) above sea level. The bulk of atmospheric ozone is located in the stratosphere, so it is accurate to say that the ozone layer is found in the stratosphere. It is much less dense than the troposphere, and atmospheric temperatures begin to increase as altitude increases, primarily due to absorption of UV radiation by ozone. Because of the low density of air molecules, there is little turbulence in the stratosphere, which makes it desirable for aircraft travel.
The mesosphere (meso- meaning "middle) is the third highest layer, beginning around 50 km (30 miles) above sea level. Beginning in the mesosphere, temperatures begin to drop again, and reach a low of -100 degrees Celsius at the upper portion. Although less dense than the stratosphere, there are still enough air molecules here to cause friction with incoming objects. Thus, meteors approaching Earth heat up as they enter the mesosphere, forming "shooting stars" that can often be seen from the surface of the Earth.
Above the mesosphere is the next layer, known as the thermosphere. It begins around 80 km (48 miles) and extends up to about 600 km (360 miles) above sea level. The atmosphere is extremely thin here so, as a whole, the thermosphere is very cold. However, each of the few molecules is bombarded with a tremendous amount of energy from the sun, so the individual atmospheric molecules are extremely hot - as much as 1000 degrees Celsius! Aurora form in the thermosphere and many satellites orbit the Earth while still in this layer.
Beyond the thermosphere reside the last remnants of the Earth's atmosphere, known as the exosphere. It extends outwards from the Earth to a distance of 10,000 km (6,200 miles). There is actually very little atmosphere in this region, and it is constantly being lost to space.
IMAGES CREATED BY GAVS OR OPENSOURCE