Geography: Origin of earth

Note: This topic is not part of UPSC syllabus. But you should have decent understanding about it. So, go through it once.

Origin of earth

• Galaxies:
  • Galaxies are sprawling space systems composed of dust and countless stars. The number of galaxies cannot be counted – the observable universe may alone contain hundred billion galaxies.
  • Name of our galaxy is milkiway. Its diameter is about 100,000 light years. It contains more than thousand million stars.
  • Our Sun with its solar system is about 30 thousand light years away from the center of the milkiway. Like other stars, the Sun with its solar system is revolving around the center of the milkiway. The period of revolution is about 224×106 years. Sun has thus made only two complete rounds around the center of the milki way till now. The solar system revolves around the milkiway with the speed of 285 kilometers per second.
• The Solar System:
  • The Solar System is believed to be formed nearly 4.6 billion years ago from the gravitational collapse of a giant molecular cloud. It consists of the Sun and the objects that orbit it, whether they orbit it directly or by orbiting other objects which orbits it directly.
  • Of those objects that orbit the Sun directly, the largest nine are the planets that form the planetary system around it, while the remainder is significantly smaller objects, such as dwarf planets and Small Solar System Bodies (SSSB’s) such as comets, and asteroids, etc.
  • All planets differ in size. The planets located in the middle of the solar system are bigger in size than those on the sides. Geophysicists called this arrangement of planets as ‘cigar shaped’.
  • The planets of the solar system can be classified into two groups:
    • Inner or terrestrial planets:
      • The inner circle consists of four planets including Mercury, Venus, Earth along with Mars and Asteroids.
      • These planets are called inner planets as they lie between the Sun and the belt of asteroids and are closer to Sun.
      • They are called terrestrial which means Earth like, as they are made up of rocks and metals. These planets are comparatively small in size and have relatively higher density. Their speed of rotation is less.
      • They either have no satellite or very less satellites for example the Earth has only one and Mars have two satellites.
    • Outer planets:
      • They are also known as the giant planets or Jovian planets (Jupiter like) – the outer circle consists of four planets namely Jupiter, Saturn, Uranus and Neptune.
      • These planets are bigger in size and are less dense. They have thick atmosphere mostly composed of helium and hydrogen. There are called the gas giants, are substantially more massive than the terrestrials.
      • Their speed of rotation and number of satellites are more compared to the inner planets.
      • The two largest namely, Jupiter and Saturn, are composed mainly of hydrogen and helium.
      • The two outermost planets, Uranus and Neptune, are composed largely of substances with relatively high melting points (compared with hydrogen and helium), called ices, such as water, ammonia and methane, and are often referred to separately as “ice giants”.
  • The vast majority of the system’s mass is in the Sun, with most of the remaining mass contained in Jupiter.
  • The Solar System also contains regions populated by smaller objects.
    • The asteroid belt, which lies between Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal.
    • Beyond Neptune’s orbit lie the Kuiper belt and scattered disc, linked populations of trans-Neptunian objects composed mostly of ices. Within these populations are several dozen to more than ten thousand objects that may be large enough to have been rounded by their own gravity. Such objects are referred to as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris.
    • In addition to these two regions, various other small-body populations, including comets, centaurs and Interplanetary dust, freely travel between regions.
    • Six of the planets, at least three of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed “moons” after Earth’s satellite Moon.
    • Each of the outer planets is encircled by planetary rings of dust and other small objects.
  • The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere. It extends out to the edge of the scattered disc.
  • The Oort cloud, which is believed to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
  • The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of interstellar wind.
  • The Solar System is located within one of the outer arms of the Milky Way, which contains about 200 billion stars.
• Asteroids and Planeteroids:
  • Asteroids are minor planets, especially those of the inner Solar System. The larger ones are called planetoids. These terms have specifically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet.
  • There are millions of asteroids, many thought to be the shattered remnant of planetesimal bodies within the young Sun’s nebula, which never grew large enough to become planets.
  • The large majority of known asteroids lie in the belt between the orbits of the Mars and Jupiter. However, the other orbital families exist with significant population including the near Earth asteroids.
  • Individual asteroids are classified by their characteristic spectra into three groups namely C type (carbon group) S type (stony) and M type (metallic). Only one asteroid 4 Vesta which has a relatively reflective surface is normally visible to the naked eye only but in very dark skies and when it is in favorable position.

Concepts behind the origin of the Earth:

• Origin of earth was part of origin of the solar system. From time to time various scientists have given their concepts, hypothesis and theories in order to explain the origin and evolution of the Solar System.
• Such views and concepts may be divided into two groups: religious concepts and scientific concepts.
• Religious concepts are discarded as they do not have logical and scientific base.
• The scientific concepts are generally based on hard sciences divided into two schools namely hot origin concepts and cold origin concepts.
  • According to hot origin concept, the planets are believed to have been formed from the matter which was either hot or was heated during the process of origin of the planets.
  • On the other hand, the school of the cold origin concept explains the Solar System originated from the matter which was either initially cold or always remained cold.
• On the basis of the number of heavenly bodies involved in the origin of the Solar System and the Earth, the scientific concepts are divided into three groups:
  • Monistic concept – one star hypothesis
    • According to this hypothesis, the Solar System originated from one star due to the gradual evolutionary process.
    • The hypothesis of Kant, Laplace, Roche and Lockear comes under this category.
    • Gaseous Hypothesis of Kant:
      • The German philosopher, Kant, put forward his hypothesis in 1755 claiming that his hypothesis was based on sound principles of Newton’s first law of gravitation and rotatory motion.
      • According to him, innumerable particles of primordial matter were scattered in the universe. And, these particles started colliding against each other due to gravitational attraction.
      • As a result of this collision, heat was generated. This changed the primordial matter from solid to liquid and from liquid to gaseous state.
      • Thus the original cold and motionless cloud of matter became in due course a vast hot nebula and started rotating around its axis and with continuous rise in the number of primordial particles, the nebula expanded in size.
      • Due to the continuous increase in size of nebula the speed of rotation became so fast that the centrifugal force exceeded the centripetal force. This created a bulge in the center of the gaseous mass.
      • When this bulge increased in size, the rings started forming one by one and were separated from the middle part of the nebula and were thrown off due to centrifugal force.
      • The residual central mass became the Sun and rest of the rings became the planets. By the repetition of the same process, the rings were separated from the newly formed planets.
      • And the material of each ring condensed to form satellites of the concerned planets.
      • Critical analysis:
        • Kant has not explained the source of the primordial matter.
        • Kant said that the particles of the primordial matter started colliding due to gravitation energy. He has not explained how the source of energy which caused motion of these particles (which were cold and motionless in initial state) suddenly became active.
        • According to science of law of motion, the collision of the particles can never generate rotatory motion.
        • Kant’s assumption that the speed of rotation of the nebula increased with the increase in the size of the gaseous matter is also against the law of science of law of motion.
      • Though in the beginning, Kant’s hypothesis received appreciation on a large scale. But, later it was disapproved as it was based on assumption and wrong application of Newton’s law of gravitation and invalid concepts.
        • But, then too, we cannot overlook the fact that it was the first scientific attempt ever made to solve the mystery of the origin of the Earth.
    • Nebular Hypothesis of Laplace:
      • Kant had postulated his hypothesis before Laplace therefore got the advantage of refining this Hypothesis by removing the inherent weak points and inaccurate concepts of Kant’s hypothesis. Thus, we may consider the Hypothesis of Laplace as the modified version of Kant’s hypothesis.
      • According to him, a huge and hot gaseous matter called nebula existed in the space which was continuously rotating on its axis.
      • This nebula was losing heat from its outer surface due to the process of radiation and was thus cooling and reducing in size and volume due to contraction on cooling.
      • As the size and volume of the nebula decreased, the velocity of rotatory motion began to increase. It increased so much that the centrifugal force became greater than the centripetal force.
      • As the size and volume of the nebula decreased, the velocity of rotatory motion began to increase. It increased so much that the centrifugal force became greater than the centripetal force.
      • The outer rings (layers) thus started separating from the nebula one by one. Each ring condensed at a point in the form of gaseous accumulation and started rotating around the nebula.
      • This gaseous mass later cooled and formed as the planets. The remaining part of nebula thus became the Sun and the nine rings became the planets.
      • The satellites were also formed by repetition of the aforesaid process. From this we can conclude that Laplace considered that the Solar System as well as the planets are all originated from the same source.
      • This hypothesis is of great importance:
        • The rings revolving around Saturn is an excellent example that supports the Laplace’s hypothesis. Besides this, there are many nebulas existing in the Universe which supports his view.
        • When the diameter of the revolving mass reduces, its speed of rotation increases. This view of Laplace is in accordance with the laws of motion science.
        • The presence of the same kind of elements in the formation of planets also proves his views right.
        • According to Laplace, all planets have been formed due to cooling of the gaseous mass. The upper layer of this gaseous mass became solid but the inner part is still in liquid state. The liquid lava erupting from the volcanoes supports his hypothesis. It is for this reason that this hypothesis commanded respect for more than fifty years. But as there are two sides of a coin, this hypothesis also has its demerits.
      • Critical analysis:
        • Laplace assumed that there existed a hot rotating nebula in the space. But he did not explain the source of origin of nebula and the source from where it received heat and rotation.
        • Laplace did not explain why only nine rings came out from the irregular ring detached from the nebula and why not more or less rings.
        • If the planets have been formed from the rotating nebula then the part of the nebula i.e. Sun should rotate at the highest speed due to decrease in size but it is not so.
        • Critics feel that if the Sun is the remaining part of the nebula, it should have a bulge in the middle, but it is not so.
        • According to Laplace’s hypothesis, all satellites should revolve in the direction of their father planet but it is not so as planets like Saturn and Jupiter revolve in the opposite direction of their father planets.
        • If we accept Laplace’s view that planets were formed from the nebula then the planets would have been in liquid state in the initial state and hence would not have been able to rotate around the Sun.
          • Only a solid matter can rotate or revolve along or near the circular path without losing its shape.
        • The British physicist James Clark Maxwell and Sir James Jeans showed that the mass of the rings was not enough to provide the gravitational attraction to form individual planets.
        • According to S.W. Wooldridge and R.S. Morgan, the small degree of cohesion between the particles of nebula would make the formation of ring a continuous not an intermittent process.
  • Dualistic concept (binary hypothesis) i.e. involving two Heavenly bodies
    • According to dualistic concept (binary hypothesis), the Solar System originated from two stars. The hypothesis of James Jeans, Chamberlain and Molten, Weitzacker’s, and Russell comes under this category.
    • The Planetesimal hypothesis of Chamberlin and Moulton
      • According to this hypothesis, the planets originated form Planetesimals. They believed that two big stars i.e. the Sun and a companion star, existed in the universe in the initial stage. The Sun was much bigger than the present Sun and was made of very small particles which were cold and solid.
      • The companion star was moving on its path and while doing so, it came closer to the Sun, and due to the gravitational pull exerted by the star, solar tide accrued and a large number of particles got detached from the outer layer of the Sun. They termed these particles as Planetesimals.
      • These Planetesimals could not combine with the moving star because by the time they reached it the star had moved ahead on its path and vanished in the space.
      • These Planetesimals were attracted by the proto Sun and started revolving around the Sun. These Planetesimals were of different sizes. The bigger Planetesimals served as the nucleus and attracted the smaller Planetesimals towards them. Gradually the bigger Planetesimals became bigger and became the present planets.
      • Limitations:
        • Jeffrey has criticized this hypothesis saying that such big planets cannot be formed by the material ejected from the Sun.
        • The assumption that the increase in the size of the nucleus due to collision of the Planetesimals is not trustworthy.
    • Tidal hypothesis of James Jeans and Jeffrey
      • According to this hypothesis, the Sun existed as a big mass of gas rotating around its own axis in the universe. Besides the Sun, there existed one more star called the intruding star which was many times bigger than the Sun.
      • As this star neared the Sun, tides started occurring on the outer surface of the Sun due to gravitational pull exerted by this star. When this intruding star came at its closest point to the Sun the height and the size of the tides increased.
      • As a result, huge amount of matter was ejected from the Sun and a cigar shaped tide filament which was thousands of kilometers in length was created. James Jeans named this ejected cigar shaped matter a filament as it was thicker in the center and thinner at the ends.
      • This filament got separated from the Sun and then came closer to the intruding star but by then the star moved ahead on its path.
      • Therefore, this filament could neither unite with the Sun nor with the star. This filament then started revolving the Sun due to the effect of gravitation. Due to the gravitational pull and condensation, knots started forming from the liquid matter of the filament. The knotted filament then condensed and formed different planets.
      • Due to the tidal effect, the filament remained thicker in the center and thinner at the ends. Hence the planets formed by this filament are bigger in the center and smaller at the sides.
      • Arguments in support:
        • If we arrange all the planets in a line, we will see that the bigger planets lie in the center and the smaller at the end. This cigar shaped arrangement supports his hypothesis.
        • The smaller planets comparatively took less time to cool, hence these planets either have very less or no satellite at all. The bigger planets remained hot for a longer period, hence they have more satellites.
        • In this hypothesis, it was assumed that all planets originated from the separated filament of the Sun. All planets are made of the same matter which again supports this hypothesis.
        • This hypothesis successfully justifies the fact that all the planets were formed at the same time.
      • Critical analysis:
        • According to the critics like Delevin, in the distance between different stars in the universe is very big. Hence there is a remote possibility of the star coming so close to the Sun that it can be affected by the gravitational force of the Sun.
        • According to Russell, there is no possibility that such a huge amount of material of filament could have come out of the Sun to form planets at such a greater distance.
        • Some scientists are of the view that planets cannot be formed due to the cooling of the gaseous filament. They instead feel that the gaseous filament might have disappeared in the universe due to the prevalence of extremely high temperature values.
        • Many astrophysicists are of the view that the angular momentum imparted by the star to the planet was not high enough to match the existing angular momentum of the planets of our Solar System.
    • Binary Star Hypothesis of Russell
      • Russell was of the view that there were two stars near the primitive Sun. These are known as the companion star and the approaching star. The companion star was revolving around the Sun. Later on, the approaching star came near the companion star and it too started revolving. The direction of the star was opposite to that of the companion star.
      • Russell assumed that there might have been a distance of 45 to 65 lakh kilometers between the stars. So, the approaching star might have been at a far greater distance from the Sun than the companion star.
      • Hence, there would have been no effect of the tidal force of the approaching star on the Sun. But the companion star would have certainly been attracted toward the approaching star because of the massive gravitational force.
      • As these two stars came closer, the gravitational and tidal force between them increased which created a bulge on the outer surface of the companion star. When the approaching star, came near the companion star huge amount of matter was ejected from it due to the gravitational force exerted by the approaching star.
      • The ejected matter started revolving in the direction of the approaching star i.e. in the opposite direction of the revolution of the companion star. The planets were formed from this ejected matter of the companion star and the satellites were formed from the ejected mater from the planets due to the mutual attraction.
      • Critical analysis:
        • Russell has explained the formation of the planets from the ejected matter of the companion star but he has not explained as to what happened to the remaining portion of the companion star.
        • He did not explain why the planets started revolving around the Sun after the giant approaching star moved ahead on its path.
  • Modern concept.
    • The Nova Hypothesis of Hoyle and Littleton
      • This hypothesis was based on nuclear physics. Energy which is emitted by any star in the form of light heat etc. is generated by the process known as nuclear fusion. According to them, the heavy elements played important part in the formation of planets.
      • These heavy elements are formed when atoms of lighter elements combined under intense heat and pressure released vast amount of energy. These heavy elements constitute 90% of the total mass of the planets. The main constituent in the formation of the stars is hydrogen. The planets on the other hand have less than 1% hydrogen.
      • The scientists F. Foyle and Littleton showed that the heavy elements originate even due to the burning of hydrogen. But an ordinary star like Sun can only form an element like helium. The formation of the heavy elements is possible only if the burning of hydrogen takes place at high temperature. Such high temperature is available only in supernova stars.
        • A star becomes supernova star when it is left with very less hydrogen which is not enough to burn. Hydrogen is the source energy which gets converted into helium and generates energy.
        • In the case of scarcity of hydrogen, the star has to shrink in order to produce energy. The speed of rotation of the star increases when it shrinks. The force at the center increases due to high rotational speed.
        • As a result, the star throws out first the lighter matter and then the heavy elements. The formation of the heavy elements in the Universe is possible in this state only.
      • The cosmic light which is many lakh times more than the light of Sun is visible in the center after the heavy metals are thrown at a distance. These stars with such huge light are termed as nova.
      • According to these scientists, the planets have been formed due to the explosion of one super nova star. The explosion of the super nova star generated intense heat equivalent to 5×10 -9 degree celsius which was sufficient enough to start the process of the nuclear fusion.
      • According to them, the two stars present there were the Sun and the super nova star. The distance between these two stars was the same as the distance between the Sun and Jupiter. The explosion of the super nova generated intense heat and pressure from which the primitive Earth was made.
      • Thus, the planets of our Solar System were formed due to the condensation of the matter of the disc form of the matter thrown out of the matter by super nova due to its explosion.
      • Critical analysis
        • This hypothesis does not support the origin of the pair of stars.
        • It fails to explain the peculiar arrangement of the planets on the basis of their direction of rotation, their size, plain of revolution, path of the planets and the lighter constituent elements of the planets of the outer circle of our Solar System.
    • The Big Bang Hypothesis
      • This theory was postulated by Limaitre in 1950-60 and validated in 1972.
      • According to this theory, all matter in the universe existed in the form of dense and huge primordial matter.
      • As violent explosion took place in this primordial matter. As a result of which the dust particles present in this matter were scattered in the Universe and formed the present Universe.
      • Two Indian scientists namely Govind Swaroop and Vijay Kapahi of Tata Institute of Fundamental Research (TIFR) are also working on this principle of big bang theory. In their view, the Universe originated about 20 billion years ago from the explosion of the big ball of fire which has been formed by the ejected matter of primordial matter.
      • In USA, a model has been prepared to generate the conditions of the big bang theory. The scientists of Berkeley University have used helium balloons to study the micro wave radiation and have supported the big bang hypothesis.
    • Cepheid Hypothesis of A.C. Banerjee:
      • In this hypothesis, some stars in the Universe keep contracting and expanding – This process is called pulsation of stars, and the stars undergoing this process are called Cepheid variable.
      • The group of stars in the universe also contains such stars. The brightness of these stars continuously keep changing. Such systematic change in the light of these stars is the result of the process of contraction and expansion.
      • Once an intruding star happened to pass closer to such Cepheid star, the pulsation in the Cepheid star increased due to the gravitational attraction of the intruding star. As a result, the intruding star attracted enormous amount of the matter of the Cepheid star towards itself.
      • The condensation of this matter formed the planets, the residual part became the Sun. The planet started revolving around the Sun. The intruding star had by then moved far away on its path.
  • Besides these theories, there are some more theories which have been recently postulated. Among them, a few significant ones are listed below:
    • Rossgunn’s rotational and tidal hypothesis
    • Kuiper’s hypothesis (1949)
    • Fosenkov’s globule concept
    • Voitkevich’s proto planetary
    • Chondrule’s concept (1971)
    • Jupiter Sun binary system hypothesis of E.M. Drobyshvski