ORIGIN OF EARTH

ORIGIN OF EARTH

             Earth is one of the 9 planets (8 excluding Pluto) orbiting the SUN in the Solar System.

           The Universe is composed of several Galaxies. Our solar system is part of Milky way galaxy which is disk shaped with about 1, 00,000 million stars of varying sizes. Our solar system consists of 9 planets and 31 satellites, a belt of asteroids

THEORIES

•      NEBULAR HYPOTHESIS

•      PLANETESIMAL HYPOTHESIS

NEBULAR HYPOTHESIS

•      Solar system formed through the condensation of a nebula which once encircled the Sun

•      The outer planets formed first, followed by Mars, the Earth, Venus, and Mercury. This hypothesis suggests a sequential origin from outermost planet to innermost.

•      As per this hypothesis, Mars evolved earlier than the Earth. This hypothesis is widely accepted to explain origin of different planets

PLANETESIMAL HYPOTHESIS

•      A catastrophic hypothesis, proposed by Thomas Chamberlin and Forest Moulton (1905), in which the planets of the Solar System are seen to arise from an encounter between the Sun and another star.

•      In this scenario, the gravity of the passing star tears a succession of bolts from the solar surface.

•      Bolts coming from the side nearer the star are thrown out to distances comparable with those of the giant planets, while those from the far side of the Sun are ejected less violently to the distances of the terrestrial planets.

 

Table 1.Radius, distance from sun and revolution period

S.No	Planet	Radius (km)	Distance from sun(m.k)	Revolution period (day)
1	Mercury	2439	57.9	88
2	Venus	6052	108.2	245
3	Earth	6378	149.4	365.25
4	Mars	3397	227.9	687
5	Jupiter	71398	778.3	4333
6	Saturn	60000	1427	10743
7	Uranus	23620	2870	30700
8	Neptune	24300	4496.5	60280
9	Pluto	1150	5970	90130

Table 2.Geological Time Scale of Earth – Development of Life

Era	Period	Age (m.Yr)	Organism
Archean	Archean	> 3500	Lifeless
Precambrian	Precambrian	-do-	Soft bodied plant & animals
Palaeozoic	Cambrian	500	Algae and shell bearing molluscus
	Ordovician	400	Molluscus & sea weeds
	Silurian	360	Land plants & breathing land animals
	Devonian	320	True fishes and  ampibians
	Carboniferous	280	Coal forming materials , non flowering plants
	Permian	246	Reptiles
Mesozoic	Triassic	235	Marine life & reptiles
	Jurassic	185	Dinosours
	Cretaceous	139	Many plants & fishes
Cenozoic	Tertiary             Eocine	20	Mammals
	Myocine	29	Mammals
	Pliocine	10	Mammals
	Quartanary
	Plistocine	01	Mammals
	Recent time		Mammals

Table 3.Average composition of Earth’s crust (% by weight)

Non - metallic	Oxygen	O2-	46.60%	74.32%  (3/4th)
	Silica	Si4+	27.72%
Metallic	Aluminium	Al3+	8.13%	1/4th  of the total
	Iron	Fe2+	5.00%
	Calcium	Ca2+	3.63%
	Sodium	Na+	2.83%
	Potassium	K+	2.59%
	Magnesium	Mg+2	2.09%
	Others	-	1.41%

Eight elements are abundant – 98.6%