Origin And Evolution Of Earth
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GEOGRAPHY

Origin And Evolution Of Earth 

A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth. This article is all about the origin and evolution of the earth.

Origin And Evolution Of Earth

Introduction

  • A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth.
  • The hypotheses are mainly divided into two groups:

Ancient theories: Gaseous mass theory, Laplace theory, Planetesimal hypothesis, Tidal theory

Modern theories: Big bang theory

  • The term "evolution" usually refers to the biological evolution of living things. But the processes by which planets, stars, galaxies, and the universe form and change over time are also types of "evolution”.
  • The origin of the earth and other planets in our solar system has been debated since 1755 when Immanuel Kant proposed the first theory.

Gaseous mass Theory

  • Presented by German Philosopher Immanuel Kant in the year 1755.
  • Kant claimed that his ‘gaseous hypothesis’ was based on the sound principles of Newton’s law of gravitation and rotatory motion.

Assumptions of Kant:

  • The supernaturally created primordial hard matter was scattered in the universe.
  • There was a primeval, slowly rotating cloud of gas (Nebula) and matter comprised of very cold, solid, and motionless particles.
  • The particles began to collide against each other under their mutual gravitational attractions.
  • Presented by German Philosopher Immanuel Kant in the year 1755.
  • Kant claimed that his ‘gaseous hypothesis’ was based on the sound principles of Newton’s law of gravitation and rotatory motion.

Assumptions of Kant:

  • The supernaturally created primordial hard matter was scattered in the universe.
  • There was a primeval, slowly rotating cloud of gas (Nebula) and matter comprised of very cold, solid and motionless particles.
  • The particles began to collide against each other under their mutual gravitational attractions.

Merits of Gaseous Hypothesis:

  • In spite of several criticisms, the hypothesis was considered a great step forward in the field of cosmogony.
  • Explains the basics of the formation of the sun, planets, and satellites.

Demerits of Gaseous Hypothesis:

  • Was based on several erroneous facts of science.
  • Not explained the source of origin of primordial matter i.e. nebula.
  • Did not explain the source of energy to cause random motion of the particles of the nebula which were cold and motionless in the initial stage.
  • The collision between the particles of primordial matter can never generate rotatory motion.
  • Kant’s assumption that the rotatory speed of the nebula increased with the increase of its size was against the law of conservation of angular momentum.

Nebular hypothesis

French mathematician Laplace propounded his ‘Nebular Hypothesis’ in the year 1796.

  • In some way modified version of Kant’s hypothesis.

Assumptions by Laplace:

  • Assumed that there was a huge and hot gaseous nebula in the space.
  • From the very beginning, this nebula was rotating on its axis.
  • The nebula was continuously cooling due to loss of heat from its outer surface and thus it was continuously reduced in size.

Theory:

  • Gradual loss of heat from the outer surface of nebula  through radiation due to its rotation
  • These processes i.e. gradual cooling and contraction resulted in a decrease in the size and volume of the nebula.
  • As the size continued to decrease the velocity of rotatory motion continued to increase.
  • The outer layer was condensed due to excessive cooling and could not rotate with still cooling and contracting the central nucleus of the nebula and got separated.
  • As per Laplace, only one such ring is separated not nine rings (as per gaseous hypothesis). 
  • This original ring was divided into nine rings thus nine planets were formed after cooling and condensation of these rings and the sun is the remaining part of the nebula.

Merits of Nebular Hypothesis:

  • Explains that each celestial body possesses acquired motion of rotation and evolution in one and the same sense.
  • Tried to remove the defects of the gaseous hypothesis.

Demerits of Nebular Hypothesis:

  • Did not describe the source of origin of the hot and rotating nebula as assumed by the Laplace.
  • Why did only nine rings come out from irregular rings detached from nebula? Laplace failed to explain this.
  • Unable to explain the peculiar distribution of the present-day angular momentum in our solar system.

Planetesimal Hypothesis

  • T.C.Chamberlin, geologist, in collaboration with Forest  Ray Moulton, an astronomer postulated a new hypothesis for solving the problem of the earth in 1905.
  • This theory is known as a Planetesimal hypothesis and unlike previous monistic concepts, it envisaged the origin of the solar system with the help of two heavenly bodies.
  • Initially, there were two heavenly bodies in the universe; Proto-sun and its companion star.
  • The behavior and properties of the proto–sun were not like other stars. It was formed of very small particles which were cold and solid and circular in shape.
  • There was another star, termed as ‘Intruding star’  of ‘companion star’  which was destined to pass very close to proto-sun.
  • An infinite number of small particles were detached from the outer surface of the proto-sun due to the massive gravitational pull exerted by the giant intruding star
  • Chamberlin termed these detached small particles as planetesimals.
  • Initially, the detached particles or planetesimals were just like dust particles. few planetesimals around the proto- sun were of fairly big size. These larger planetesimals became nuclei for the formation of future possible planets.
  • Gradually large planetesimals started attracting small planetesimals.  and ultimately these large planetesimals grew in the form of planets due to the continuous accretion of an infinite number of planetesimals.
  • With the passage of time, the remaining proto- sun changed into the present-day sun. The satellites of the planets were created due to the repetition of the same process and mechanisms.

Demerits of Planetesimal Hypothesis:

  • Does not offer any explanation for the type of arrangement of the planets in our present-day solar system.
  • The infinite space of the universe makes such a close encounter between the stars a remote possibility.
  • No explanation about the planets of the outer circle which are of very low densities and is in a gaseous state.

Tidal Hypothesis

  • Propounded by Sir James Jeans (British) to explain the origin of the earth in the year 1919.
  • Another British scientist, Harold Jeffreys, suggested modifications in the ‘Tidal Hypothesis’ in 1929 and thus made it more relevant and significant.
  • Sun was a gaseous mass; another star several times larger than the sun accidentally came close to it & pulled the gaseous mass away from the sun due to its gravitational pull.
  • The giant tongue of matter came out from the sun & planets were formed.

Assumptions:

  • The solar system was formed from the sun and another ‘Intruding Star’.
  • Sun was a big incandescent gaseous mass of matter.
  • Besides the sun, there was another star termed as ‘Intruding star’ in the universe. (much bigger in size than the’ Primitive Sun’).
  • The ‘Primitive Sun’ was stationary and rotating on its axis.
  • The ‘Intruding Star’ was moving along such a path in such a way that it was destined to come nearer to the ‘Primitive Sun’.
  • When an intruding star comes nearest to the ‘Primitive Sun’, its gravitational force becomes maximum, and thus a giant cigar-shaped tide, thousands of km in length and huge masses was created on the outer surface of the primitive sun and ultimately was ejected from the primitive sun.
  • On moving away, gravitational attraction became again minimum and thus a small amount of matter was ejected from the outer surface of the ‘Primitive Sun’. Thus, the shape of matter ejected from the ‘Primitive Sun’ became like a cigar.
  • The solar systems were formed due to the cooling and condensation of the incandescent mass of gaseous matter of the filament. The filament after being detached from the ‘Primitive Sun’, began to cool down. Thus, the filament started contracting in size on cooling.

Merits of Tidal Hypothesis :

Shape and ordering of planets - The arrangement of the planets formed of the filament clearly matches the present planetary arrangement of our solar system.

Ordering and arrangement of satellite - According to this hypothesis the satellites of  the planets were formed through the condensation of incandescent gaseous matter ejected from the planets in the same manner as the planets were formed

Number and size of satellites - A larger number of satellites were formed from bigger planets, because, bigger planets took a longer time to cooling and condensing.

Demerits of Tidal hypothesis

  • Stars and the Sun are so apart from each other that the passing of one star near the Sun does not appear to be convincing.
  • Could not elaborate the process and mechanism of the condensation of matter ejected from the ‘Primitive Sun’.
  • The planets of our solar system are largely formed of the elements having high atomic weight, but, the constituent elements of the sun are of lighter atomic weight e.g. hydrogen and helium. The tidal hypothesis fails to offer a convincing explanation for such an anomalous situation.

Big Bang Theory

  • Also called expanding universe hypothesis. 
  • Edwin Hubble, in 1920, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart.
  • Scientists believe that through the space between the galaxies is increasing, observations do not support the expansion of galaxies.

Considers the following stages in the development of the universe.

  • 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.
  • 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.
  • Within 300,000 years from the Big Bang, the temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter.
  • The expansion of the universe means an increase in space between the galaxies.
  • However, with greater evidence becoming available about the expanding universe, the scientific community at present favors the argument of expanding the universe.

The Star Formation

  • 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 the development of galaxies.
  •  A galaxy contains a large number of stars and spread over vast distances that are measured in thousands of light-years.
  • The diameters of individual galaxies range from 80,000-150,000 light-years. 
  • A galaxy starts to form by the accumulation of hydrogen gas in the form of a very large cloud called a nebula.
  • Eventually, the growing nebula develops localized clumps of gas.
  • These clumps continue to grow into even denser gaseous bodies, giving rise to the formation of stars. The formation of stars is believed to have taken place some 5-6 billion years ago.

Evolution of earth

  • The planet earth initially was a barren, rocky, and hot object with a thin atmosphere of hydrogen and helium.
  • Various processes changed it into a beautiful planet with an ample amount of water and a conducive atmosphere favoring the existence of life.
  • A period of 4,600 million years and the present, led to the evolution of life on the surface of the planet.
  • The earth has a layered structure.
  • From the outermost end of the atmosphere to the center of the earth, the material that exists is not uniform.

Evolution of lithosphere

  • The earth was mostly in a volatile state during its primordial stage.
  • Due to a 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.
  • With the passage of time, it cooled further and solidified, and condensed into a smaller size.
  • This later led to the development of the outer surface in the form of a crust.
  • 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.
  • Starting from the surface to the central parts, we have layers like the crust, mantle, outer core, and inner core.

Evolution of Atmosphere and Hydrosphere

  • Earth’s atmosphere is chiefly contributed by nitrogen and oxygen.
  • There are three stages in the evolution of the present atmosphere. 
  • The first stage is marked by the loss of the primordial atmosphere.
  • In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere.
  • Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis.
  • The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds.
  • This happened not only in the case of the earth but also in all the terrestrial planets, which were supposed to have lost their primordial atmosphere through the impact of solar winds.
  • During the cooling of the earth, gases and water vapor were released from the interior solid earth.
  • This started the evolution of the present atmosphere. The early atmosphere largely contained water vapor, nitrogen, carbon dioxide, methane, ammonia, and very little free oxygen. 
  • The process through which the gases were outpoured from the interior is called degassing.
  • Continuous volcanic eruptions contributed water vapor and gases
  • As the earth cooled, the water vapor 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 earth’s oceans were formed within 500 million years from the formation of the earth.
  • 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 got evolved.
  • Oceans began to have the contribution of oxygen through the process of photosynthesis.
  • Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere.

 Origin of life

  • The last phase in the evolution of the earth relates to the origin and evolution of life.
  • Initially, the earth or even the atmosphere of the earth was not conducive to the development of life. 
  • Modern scientists refer to the origin of life as a kind of chemical reaction, which first generated complex organic molecules and assembled them. 
  • This assemblage was such that they could duplicate themselves converting inanimate matter into living substance.
  • 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.
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