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Unit II

THE EARTH

CHAPTER

ORIGIN

OF THE

EARTH

iewed from space, the planet earth appears as a round ball that shines bright and blue. Aristotle, a Greek philosopher, believed that the earth was at the centre of the universe and that the moon, sun, planets and stars orbited around it. Indian astronomer Aryabhatta, however, believed in heliocentric solar system. Today, we know that the sun is a star and the planets revolving around it came out of it in the distant past. The sun, its nine planets and the satellites of the planets constitute the solar system. Planets differ in size, constituent matter and temperature. All these characteristics are related to their respective distances from the sun. The earth is one of the small inner planets, along with Mercury, Venus and Mars. Inner planets comprise of heavy elements and are closest to the sun. The earth is, however, the only planet with conditions favourable for the sustenance of life. ORIGIN OF THE PLANETS Nebular Hypothesis : In 1755, Ger man philosopher Imanuel Kant hypothesised that slowly rotating cloud of gas, called Nebula, in some unspecified fashion condensed into a number of discrete and globular bodies. The great French mathematician Laplace also proposed, more or less, the same theory in 1796. According to Kant and Laplace, the original mass of gas cooled and began to contract. The rotational speed increased as a consequence of the law of conservation of angular momentum. Thus, successive rings of gaseous material were spun off from the central mass by centrifugal force. In the final stages the rings condensed into planets.

Collision Hypothesis : Sir James Jeans and Sir Harold Jeffreys, well-known scientists of England, came forward with the collision hypothesis. According to this hypothesis, gaseous material was pulled away from the pre-existing sun by the gravitational attraction of a passing star. Giant tongues of matter came out. These tongues broke into small chunks or planetesimals, which went flying as cold bodies into orbits around the sun in the plane of the passing star. By collision and gravitational attraction, the larger planetesimals swept up the smaller pieces, and thus, were formed the planets. THE BIRTH OF THE SOLAR SYSTEM The earth was formed at the same time as the sun and the other planets of the solar system. The entire solar system, consisting of the sun, nine planets, and their satellites form a very small part of the galaxy that consists of many stars (Appendix I). Innumerable such galaxies form the universe. Our galaxy is popularly, called the Milky Way. The pressure and temperature at the centre of the Nebula that produced the solar system became so great, that it triggered a nuclear reaction. Some of the hydrogen in the cloud fused into helium, releasing great amount of energy. The gaseous cloud exploded to form a supernova. The explosion caused shock waves that pushed the denser portions of cloud to collapse under their own gravity. The dense core grew larger and hotter as its gravity attracted more material. In the process, the hot core developed into a protostar. Finally, the protostar became the infant sun.

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FUNDAMENTALS OF PHYSICAL GEOGRAPHY

Homogeneous Accretion Hypothesis : The homogeneous accretion hypothesis maintains that the earth accreted from an intimate mixture of silicate particles and metal particles. The material was assumed to have been formed in the solar nebula by a complex series of chemical and physical processes which had occurred prior to the accretion of planets. According to hypothesis, accretion of the earth occurred over a sufficiently long period (107 108 years) so that its gravitational potential energy was efficiently radiated away and it formed in an initially cool and unmelted condition. Subsequently, heating by long-lived radioactive elements occurred, leading to melting of the metal parts and its segregation into the core. The major element composition of the earth and its depletions in volatile elements can be explained through this hypothesis. It also explains the approximate chemical uniformity of the mantle formed after the formation of the core. Heterogeneous Accretion Hypothesis : According to heterogeneous accretion hypothesis, the composition of the material from which the earth accreted changed with time giving rise to layered structure of the earth. According to this model, the earth formed inside out, with a cool, oxidised, and

volatile rich nucleus and more metal-rich and devolatilised outer rings. ORIGIN OF THE SATELLITES (MOON) A discussion on the origin of the earth will be incomplete without discussing the origin of the moon. In this case also there are many hypotheses. Radiometric dating of the rocks from the moon show that it was born along with the earth. It is not younger. Apparently then, there are two possibilities. It either came out of the sun in a gaseous form but being too small was attracted by the earth, or it flung out of the earth due to a huge meteorite falling on the earth. The area where the meteorite fell, a huge hollow was created, which is now filled up by the ocean and the landmass flung to the outer space created the moon. In all fairness, several possible courses for our planets evolution are presented here. In general, there is fair agreement in the course of events. Looking for the plausible explanation to the earths evolution is like making a house out of a variety of blocks. The constraint is that each block has to fit with the ones under it and over it and that the whole structure has to stand up, but there may be more than one way to build the house.

EXERCISES

Review Questions 1. Answer the following questions: (i) Why do planets differ in size, constituent matter and temperature? (ii) What is a Nebula? (iii) What are planetesimals? (iv) Who first proposed Nebular hypothesis? (v) Name our galaxy. (vi) What is a protostar? (vii) How is it known that the moon was born along with the earth? 2. Write short notes on: (i) Collision hypothesis; (ii) Homogeneous accretion hypothesis; and (iii) Origin of the moon. 3. Discuss various hypotheses for the evolution of planets. 4. Describe the evolution of the solar system.