This article throws light upon the four main spheres of earth. The spheres are: 1. Atmosphere 2. Hydrosphere 3. Lithosphere 4. Biosphere.
Sphere # 1. Atmosphere:
The earth’s atmosphere, a complex fluid system of gases and suspended particles, did not have its origin in the beginning of the planet.
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The atmosphere as of today has been derived from the Earth itself by chemical and biochemical reactions.
Although the fluid system forms a gaseous envelope around the Earth, its boundaries are not easily defined. They can be arbitrarily defined as the Earth’s atmosphere interface and space interface.
The gases like Nitrogen, Oxygen, Argon, Carbon dioxide and Water vapor etc. together make up the total volume of atmosphere. Together with suspended particulates, viz. dust and soot, they constitute the gaseous turbidity particularly in troposphere. However, the composition of atmosphere and so also its structure is variable in time and space.
The vertical structure of atmosphere is very much related to radiant energy absorption and thus can be described in terms of variable of temperature. Below 60 km. there are two main zones of absorption at the Earth’s surface and in the Ozone layer. The absorbed energy is redistributed by radiation, conduction and convection.
There are therefore, two temperature maxima:
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At the Earth’s surface and at an elevation of 50 km. Above each of these maxima there is mainly convectional mixing. Temperatures in these mixing layers decrease with height above the heat source. The lower of these two zones is referred to as troposphere and the upper the mesosphere.
These are separated by a layer of little mixing in which the atmosphere tends towards a layered structure referred to as the stratosphere. Between the troposphere and the stratosphere is the tropopause which marks the approximate upper limit of mixing in the lower atmosphere.
The average height of this is usually given as 11 km., but this varies over the earth. In tropical latitude its average height is 16 km. and in polar latitude it is only 10 km.
There is one further zone of heating, above the mesosphere and more than 90 km. from the Earth’s surface, where shortwave ultraviolet radiation is absorbed by any oxygen molecules present at this height. This is referred to as thermosphere.
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Within this layer, ionization occurs which produces charged ions and free electrons. Beyond the thermosphere, at a height of approximately 700 km., lies the exosphere where the atmosphere has an extremely low density. At this level there are increasing numbers of ionization particles which are concentrated into bands referred to as the Van Allen radiation belts.
However, this simple model of vertical structure can be simplified to provide a model of the atmosphere as two concentric shells, the boundaries of which are defined by the stratopause, at approximately 50 km. above the Earth’s surface and a hypothetical outer limit of the atmosphere, at approximately 80,000 km.
Below the stratopause, in the stratosphere and troposphere, there is 99% of the total mass of the atmosphere and it is at this level that atmospheric circulatory systems operate.
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Beyond the stratopause a layer of nearly 80,000 km thick contains only 1% of total atmospheric mass and experiences ionization by high-energy, short wavelength solar radiation. The temperature profile of atmospheric layer is given in Fig. 2.2.
Sphere # 2. Hydrosphere:
It includes the surface water and its surrounding interface. It is vital for life molecule to survive. Water possesses a number of physical and chemical properties that help the molecule to act as best suited medium for life activities. The movement of water from earth surface to atmosphere through hydrological cycle appears to be a close system. Water is the most abundant substance on the Earth’s surface.
The oceans cover approximately 71% of the planet, glaciers and ice caps cover additional areas; and water is also found in lakes and streams, in soils and underground reservoirs, in the atmosphere, and in the bodies of all living organisms. Thus, water in all its forms—ice, liquid, water and water vapour—is very familiar to us
Humans use water in the home, in industry, in agriculture, and for recreation. These applications differ widely in the quantities and quality of the water that they require. In one way or another, we use all available sources—inland waters, ground water, and even ocean water. We pollute it, re-purify it, and reuse it, over and over again.
The demand for global water resources increased day-by-day though pure fresh water availability decreased severely. Thus, there needs precious use of available pure fresh water and their fruitful storage and conservation. A simplified outline of hydrological cycle is given in Fig. 2.3.
Sphere # 3. Lithosphere:
It is the outer boundary layer of solid earth and the discontinuity within the mantle. The outer boundary forms a complex interface with the atmosphere and hydrosphere and is also the environment in which life has evolved. The inner boundary is adjacent to rock, which is near its melting point and is capable of motion relative to the lithosphere above.
Basically lithosphere is nothing but a crustal system composed of various layers:
Core, mantle and outer crust. Various elements constitute such crustal layer in mixture of different proportion.
In general the earth crust is composed of three major classes of rocks as classified on the basis of their mode of origin:
Igneous rocks, sedimentary rocks and metamorphosed rocks.
There are two types of crusts—continental crust which is composed of granitic rocks in silicon-aluminium and with a mean density of 2.8; the other oceanic crust which is basaltic in composition, consisting of more basic minerals and has a mean density of 3.0. Overall, the average density of the earth is 5.5 gm./c.c.
Interaction between the crustal system of the lithosphere and the atmosphere and biosphere takes place where continental crust is exposed above sea-level. At the land/air interface crustal material becomes exposed to inputs of solar radiant energy, precipitation and atmospheric gases.
These inputs are often modified by or operate through the effects of the living systems of the biosphere. Under the influence of these inputs, crustal rocks are broken down by weathering processes and are transferred to fine pours crustal layer called soil.
An outline of earth layers and composition of crustal materials is given in Fig. 2.4:
Sphere # 4. Biosphere:
The biosphere encompasses all the zones on the Earth in which life is present, i.e., entire bio-resources of the earth. It develops on earth since 4.5 billions year through evolutionary processes. At the top of the lithosphere, throughout the hydrosphere and into the lower atmosphere life of diverse type exists.
These bio-resources and their surrounding constitute the “Biosphere“, where mankind acting as the most evolved creature.
The steps involved in the origin of life on earth are very complex and requires several centuries. Considerable uncertainty surrounds the details of atmospheric composition, the processes involved and even the sequence of some events leading to formation of living cells (Fig. 2.5).
The conventional view has been that the earliest organism on the plant were heterotrophic prokaryotic bacteria. Subsequently autotrophic prokaryotes & eukaryotes start appearing as stepwise evolutionary changes.
The major steps of origin of life in primitive earth are depicted in Fig. 2.5:
Life on Earth requires water, a source of energy (sunlight) and various nutrients found in the soil, water and air. Suitable combinations of these essentials cannot be found high in the upper atmosphere or deep underground. They exist only in a narrow layer near the surface of the Earth. This biosphere layer extends over most of the surface of the Earth.
It includes the upper layers of the Earth’s crust and the thin layer of soil that supports plant life. This zone of life also extends about 8 km. up into the atmosphere (air borne biota) and as much as 8 km. down into the depths of the sea.
Living organisms are not distributed uniformly on globe:
Few organisms live on polar ice caps and glaciers, whereas many live in tropical rainforests (Fig. 2.6).
Within the biosphere, there are several major regions containing specific types of ecosystems.
These major regions are called biomes. Biomes are then recognized by the types of dominant ecosystems— tropical rainforests, temperate forests, prairies, deserts, and Arctic Tundra. The ecosystems again composed of populations and population composed of individuals. The detailed subdivisions of biospheres are depicted in Fig. 2.7.
The global estimate of species of both prokaryotes and eukaryotic life forms are given in Tables 2.1 and 2.2.
These figures imply the fact that how diverse is our biological world on earth:
