🌍THE ORIGIN AND EVOLUTION OF  THE EARTH 

Early Theories - Origin Of The Earth 

Nebular hypothesis  : Immanuel Kant, a German philosopher, gave this theory. In 1796, a mathematician Pierre-Simon Laplace reexamined it. Nebular Hypothesis considered that the planets were formed out of a cloud of material association with a youthful sun, which was slowly rotating.  Later in 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped extension of material was separated from the solar surface. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets. At a later date, the arguments considered of a companion to the sun to have been coexisting. These arguments are called binary theories.     

Revised Nebular Hypothesis:   In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany somewhat revised the ‘nebular hypothesis’ though differing in details. They considered that the sun was surrounded by a solar nebula containing mostly the hydrogen and helium along with what may be termed as dust. The friction and collision of particles led to formation of a disk-shaped cloud and the planets we formed through the process of accretion.  

Modern Theories - Origin Of The Universe 

Big Bang Theory : The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also called expanding universe hypothesis. Edwin Hubble, in 1920, provided evidence that the universe is expanding. 

The Big Bang Theory considers the following stages in the development of the universe:   

i).  In the beginning, all matter forming the universe existed in one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density.  

ii). At the Big Bang the “tiny ball” exploded violently. This led to a huge expansion. It is now generally accepted that the event of the big bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within the first three minutes from the Big Bang event, the first atom began to form.  

iii) Within 300,000 years from the Big Bang, temperature dropped to 4,500K and gave rise to atomic matter. The universe became transparent. The expansion of universe means increase in space between the galaxies 

Formation Of Stars  

1. The distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for development of galaxies.  

2. A galaxy contains a large number of stars.  

3. Galaxies spread over vast distances that are measured in thousands of light-years.  

4. A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula. Eventually, growing nebula develops localized clumps of gas. These clumps continue to grow into even denser gaseous bodies, giving rise to formation of stars is believed to have taken place some five-six billion years ago.  

5. Light Year: A light year is equal to the number of kilometers traveled by light per second. It is a measure of distance and not of time. Light travels at a speed of 300,000 km/second. Therefore, the distances the light will travel in one year is taken to be as one light year. 

Formation of Planets  

The following three are considered to be the stages in the development of planets:  

1. The stars are localized lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas core.  

2. The gas cloud starts getting condensed and the matter around the core develops into small rounded objects. These small-rounded objects by the process of cohesion develop into what is called planetesimals. Larger bodies start forming by collision, and gravitational attraction causes the material to stick together. Planetesimals are a large number of smaller bodies.  

3. In the final stage, these large number of small planetesimals accrete to form a fewer large bodies in the form of planets. 

Our Solar System  

1. Our Solar system consists of eight planets. The nebula from which our Solar system is supposed to have been formed, started its collapse and core formation some time 5-5.6 billion years ago and the planets were formed about 4.6 billion years ago.   

2. Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grains and gases.   

3. Out of the nine planets, Mercury, Venus, Earth and Mars are called the inner planets as they lie between the sun and the belt of asteroids the other five planets are called the outer planets.   

4. Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high densities. The other four are called Jovian or Gas Giant planets which means Jupiter-like.

The difference between terrestrial and Jovian planets  

Inner Planets  

i) Mercury, Venus, Earth & Mars Are Called Inner Planets.   

ii) They Are Found Between Asteroid belt and The Sun.    

iii) They Are Also Called Terrestrial Planets.   

iv) Smaller in size.   

v) High density.   

vi) Solid rocky state.   

vii) They are warm.   

Outer Planets 

i) Jupiter Saturn Uranus Neptune & Pluto Are Called Outer Planets.  

ii) They Are Found Aer the Asteroid belt. 

iii) They Are Called Jovian Planets.  

iv) Larger in size.  

v) Low density.  

vi) Gaseous state.  

vii) They are cold.  

The Moon :

The moon is the only natural satellite of the earth. It is now generally believed that the formation of the moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”. A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space. This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.

Evolution Of The Earth

The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. The earth has a layered structure. From the outermost end of the atmosphere to the center of the earth, the material that exists is uniform. From the surface to deeper depths, the earth’s interior has different zones and each of these contains materials of different characteristics.  

Evolution of Lithosphere  

i). The earth was mostly in a volatile state during its primordial stage.   

ii). Due to gradual increase in density the temperature inside has increased. As a result the material inside started getting separated depending on their densities. This allowed heavier materials (like iron) to sink towards the center of the earth and the lighter ones to move towards the surface.  

iii). During the formation of the moon, due to the giant impact, the earth was further heated up. It is through the process of differentiation that the earth forming material got separated into different layers.   iv). Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core. From the crust to the core, the density of the material increases. 

Evolution of Atmosphere and Hydrosphere 

i). The present composition of earth’s atmosphere is chiefly contributed by nitrogen and oxygen.  

ii). There are three stages in the evolution of the present atmosphere. They are:  

a). The first stage is marked by the loss of primordial atmosphere.  

b).  In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere  

c).  Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis.  

iii). The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds.  

iv).  During the cooling of the earth, gases and water vapour were released from the interior solid earth which started the evolution of the present atmosphere. Continuous volcanic eruptions contributed water vapour and gases. As the earth cooled, the water vapour released started getting condensed, the carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains. The rainwater falling onto the surface got collected in the depressions to give rise to oceans.  

v). Sometime around 3,800 million years ago, life began to evolve. However, around 2,500-3,000 million years before the present, the process of photosynthesis evolved. Life was f confined to the oceans for a long time. 

Origin Of Life  

The record of life that existed on this planet in different periods is found in rocks in the form of fossils. The microscopic structures closely related to the present form of blue algae have been found in geological formations much older than some 3,000 million years. It can be assumed that life began to evolve sometime 3,800 million years ago.  

Share

& Comment

Tweet