THE ORIGIN AND EVOLUTION OF THE EARTH
Origin of the Earth
- Mathematician Laplace revised earlier and popular arguments was by German philosopher Immanuel Kant in 1796, known as Nebular Hypothesis. The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.
- 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.
- The arguments considered of a companion to the sun to have been coexisting. These arguments are called binary theories.
- In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany revised the ‘nebular hypothesis’, though differing in details. They considered that the sun was surrounded by 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 were formed through the process of accretion.
Origin of the Universe
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. As time passes, galaxies move further and further apart.
The Big Bang Theory 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 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 first three minutes from the Big Bang event, the first atom began to form.
- Within 300,000 years from the Big Bang, temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter. The universe became transparent. The expansion of universe means increase in space between the galaxies.
An alternative to Big Bang was Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time.
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 development of galaxies.
- A galaxy contains a large number of stars.
Origin of Galaxy:
- A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula.
- Nebula develops into localised clumps of gas.
- These clumps continue to grow into even denser gaseous bodies, giving rise to formation of stars.
- The formation of stars is believed to have taken place some 5-6 billion years ago.
Light travels at a speed of 300,000 km/second. Considering this, the distances the light will travel in one year is taken to be one light year. This equals to 9.4611012 km. The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.311minutes.
Formation of Planets:
The following are considered to be the stages in the development of planets:
- The stars are localised 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.
- In the next stage, 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.
- 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
- 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.
- Mercury, Venus, Earth and Mars are called as the inner planets as they lie between the sun and the belt of asteroids the other four planets are called the outerplanets.
- The first four are called Terrestrial, they are made up of rock and metals, and have relatively high densities.
- The rest four are called Jovian or Gas Giant planets. Most of them are much larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen.
- All the planets were formed in the same period sometime about 4.6 billion years ago.
- Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (2003 UB313)discovered in recent past may be called ‘dwarf planet’.
Why are the inner planets rocky while others are mostly in gaseous form?
The difference between terrestrial and jovian planets can be attributed to the following conditions:
- The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location.
- The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to the Jovian planets.
- The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.
- The moon is the only natural satellite of the earth.
- In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a
single rapidly rotating body. The whole mass became a dumb-bell-shaped body and eventually it broke.
- It was also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean.
- However, the present scientists do not accept either of the explanations. It is now generally believed that the formation of moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”.
- The distance between Earth and Mars is small when compared with Earth and Venus.
- Out of 8 planets Density of the Earth is highest.
- Saturn has the most number of natural Satellites in our solar system.
Evolution of Lithosphere
- The earth was mostly in a volatile state during its primordial stage. 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 centre of the earth and the lighter ones to move towards the surface. With passage of time it cooled further and solidified and condensed into a smaller size.
Evolution of Atmosphere and Hydrosphere
- The present composition of 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 primordial atmosphere. The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds. This happened to all the terrestrial planets.
- 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 largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen. The process through which the gases were outpoured from the interior is called degassing.
- Continuous volcanic eruptions contributed water vapour and gases to the atmosphere. 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.
- Oceans began to have the contribution of oxygen through the process of photosynthesis.
Origin 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.