ATMOSPHERE: AN INTRODUCTION
- Our planet earth is enveloped by a deep blanket of gases extending several thousands of kilometres above its surface. This gaseous cover of the earth is known as the atmosphere.
- Like land (lithosphere) and water (hydrosphere), the atmosphere is also an integral part of the earth and it is held in place by the gravitational influence of earth.
EVOLUTION OF EARTH'S ATMOSPHERE
The first atmosphere consisted of gases in the solar nebula, primarily hydrogen.
A) The Primordial atmosphere
- Volcanic outgassing created the primordial atmosphere which was supplemented by gases produced during the late heavy bombardment of Earth. During the Late Heavy Bombardment (4 billion years ago), a disproportionately large number of asteroids have collided with the early terrestrial planets including earth.
- Over time, the Earth’s surface solidified leaving behind hot volatile gases resulting in a heavy CO2 atmosphere with hydrogen, nitrogen, inert gases and water vapour.
- Cooling of earth caused rains for millions of years leading to the formation of oceans. The CO2 got dissolved in oceans and its percentage in total gases declined.
- Some CO2 reacted with metals to form carbonates that were deposited as sediments.
- The early atmosphere contained almost no oxygen.
- Most of the lighter gases like the hydrogen and helium escaped into space and are continually escaping even to the present day due to atmospheric escape (outer layers stripped by solar wind).
B) The Present atmosphere
The amount of oxygen reached a peak of about 30% around 280 million years ago.
- Two main processes govern changes in the oxygen levels in the atmosphere:
- Plants use carbon dioxide from the atmosphere, releasing oxygen.
- Breakdown of pyrite (iron sulphide) and volcanic eruptions release sulphur into the atmosphere, which oxidises and hence reduces the amount of oxygen in the atmosphere. However, volcanic eruptions also release carbon dioxide, which plants used and converted to oxygen.
- Periods with high oxygen in the atmosphere are associated with rapid development of animals.
- Today’s atmosphere contains 21% oxygen, which is great enough for this rapid development of animals.
COMPOSITION OF ATMOSPHERE
- The composition of Earth’s atmosphere is largely governed by the by-products of the life that it sustains.
- Dry air from Earth’s atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen, helium, and other noble gases.
- The remaining gases are often referred to as trace gases, among which are the greenhouse gases, principally carbon dioxide, methane, nitrous oxide, and ozone.
Permanent Gases of the Atmosphere
- Heavier gases like nitrogen and oxygen tend to stick at the bottom of the atmosphere.
- The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km.
- Carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.
STRUCTURE OF ATMOSPHERE
- The atmosphere can be studied as a layered entity – each layer having its peculiar characteristics. These layers are systematically discussed
- Troposphere: 0 to 12 km
- Stratosphere: 12 to 50 km
- Mesosphere: 50 to 80 km
- Thermosphere: 80 to 700 km
- Exosphere: 700 to 10,000 km
- Its altitude is 8 km at the poles and 18 km at the equator.
- The thickness is greater at the equator because of the heated air that rises to greater heights.
- The troposphere ends with the Tropopause.
- The temperature in this layer, as one goes upwards, falls (positive lapse rate) at the rate of 6.5 °C per kilometre.
- It is -45 °C at the poles and -80 °C over the equator at Tropopause (greater fall in temperature above equator is because of the greater thickness of troposphere – 18 km).
- The troposphere is marked by temperature inversion, turbulence and eddies.
- It is also meteorologically the most significant zone in the entire atmosphere (all weather phenomena like cyclones, rainfall, fog and hailstorm etc. are confined to this layer).
- It is also called the convective region since all convection stops at Tropopause.
- Topmost layer of troposphere.
- It acts as a boundary between troposphere and stratosphere.
- This layer is marked by constant temperatures.
- It lies beyond tropopause, up to an altitude of 50 km from the earth’s surface.
- The temperature in this layer remains constant for some distance but then rises (negative lapse rate) to reach a level of 0 °C at 50 km altitude.
- This rise is due to the presence of ozone (harmful ultraviolet radiation is absorbed by ozone).
- This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes.
- So, the aeroplanes fly in lower stratosphere, sometimes in upper troposphere where weather is calm.
- Sometimes, cirrus clouds are present at lower levels in this layer.
- It lies at an altitude between 20 km and 55 km from the earth’s surface and spans the stratosphere and lower mesosphere. But the highest concentration occurs between 20 km and 30 km.
- Because of the presence of ozone molecules, this layer absorbs and reflects the harmful ultraviolet radiation.
- The temperature rises (negative lapse rate) at a rate of 5° C per kilometre through the ozonosphere.
- Ultraviolet light splits O2 into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with O2 to create ozone, O3.
- The ozone molecule is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an individual atom of oxygen (ozone-oxygen cycle).
- Stratospheric ozone depletion is caused by chlorofluorocarbons, bromofluorocarbons and other ozone-depleting substances that increase the concentrations of chlorine and bromine radicals.
- Each of these radicals initiate and catalyse a chain reaction capable of breaking down over 100,000 ozone molecules.
- Most of the meteors burn up in this layer on entering from the space.
- Temperatures drop with increasing altitude to the mesopause.
- Mesopause is the coldest place on Earth and has an average temperature around −85 °C.
- In thermosphere temperature rises (negative lapse rate) very rapidly with increasing height because of radiation from the sun.
- Ionosphere is a part of this layer. It extends between 80-400 km.
- Though temperature is high, the atmosphere is extremely rarefied – gas molecules are spaced hundreds of kilometres apart.
- The International Space Station and satellites orbit in this layer.
- Aurora’sare observed in lower parts of this layer.
- The Kármán line, located within the thermosphere at an altitude of 100 km, is commonly used to define the boundary between Earth’s atmosphere and outer space, here human travellers are considered astronauts.
- speed of sound is directly proportional to temperature as we move away from earth.
- Because in an ideal gas of constant composition the speed of sound depends only on temperature and not on the gas pressure or density.
- This is the uppermost layer of the atmosphere extending beyond the ionosphere above a height of about 400 km.
- The air is extremely rarefied, and the temperature gradually increases through the layer.
- Light gases like helium and hydrogen float into the space from here.
- Certain light gases like hydrogen are constantly lost into space from exosphere due to atmospheric escape.
- Atmospheric escape of gases (atmospheric stripping) happens when gas molecules achieve escape velocity due to low gravity or due to energy received from the sun (heat, solar wind).
- Jovian planets retain gases with low molecular masses because of low temperatures and higher gravity.
- Titan, a moon of Saturn, and Triton, a moon of Neptune, possess significant nitrogen-rich
- Earth’s magnetic field reduces atmospheric escape by protecting the atmosphere from solar wind that would otherwise greatly enhance the escape of hydrogen.
IMPORTANCE OF EARTH'S ATMOSPHERE
A) Life-giving gases
- Plants require carbon dioxide to survive while animals and many other organisms need oxygen for their survival.
- Nitrogen is fixed by bacteria and lightning to produce ammonia used in the construction of nucleotides and amino acids.
B) Regulates the entry of solar radiation
- All life forms need a particular range of temperature and a specific range of frequencies of solar radiation to carry out their biophysical processes.
- The atmosphere absorbs certain frequencies and lets through some other frequencies of solar radiation. In other words, the atmosphere regulates the entry of solar radiation.
C) Temperature balance
- The atmosphere also keeps the temperature over the earth’s surface within certain limits.
- In the absence of the atmosphere extremes of temperature would exist between day and night.
D) Blocks harmful radiation
- The atmosphere helps to protect living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays.
E) Shields the earth from impact objects
- The atmosphere also takes care of extra-terrestrial objects like meteors which get burnt up while passing through the atmosphere (mesosphere to be precise) due to friction.
F) Weather and climate
- Weather is another important phenomenon which dictates the direction of many natural and human-made processes like plant growth, agriculture, soil-formation (weathering and erosion), human settlements, etc. Various climatic factors join together to create weather.
G ) Water on earth exists in liquid state due to Atmosphere
- Since liquids cannot exist without pressure, an atmosphere allows liquid to be present at the surface, resulting in lakes, rivers and oceans.
- Earth and Titan are known to have liquids at their surface and terrain on the planet suggests that Mars had liquid on its surface in the past.
H) Scattering of light
- When light passes through Earth’s atmosphere, photons interact with it through scattering.
- On an overcast, there is no direct radiation as it has all been scattered by the clouds.
- Due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light.
- This is also why sunsets are red. Because the Sun is close to the horizon, the Sun’s rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.