The Earth

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Viewed 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, German 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. 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.

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 planet’s evolution are presented here. In general, there is fair agreement in the course of events. Looking for the plausible explanation to the earth’s 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.

About 11 to 15 billion years ago all of the matter and energy in the Universe was concentrated into an area the size of an atom. At this moment, matter, energy, space and time did not exist. Then suddenly, the Universe began to expand at an incredible rate and matter, energy, space and time came into being (theBig Bang). As the Universe expanded, matter began to coalesce into gas clouds, and then stars and planets. Our solar system formed about 5 billion years ago when the Universe was about 65% of its present size . Today, the Universe continues to expand.


Why do Most Scientists Accept the Big Bang Theory?

The acceptance of this theory by the scientific community is based on a number of observations. These observations confirm specific predictions of the Big Bang theory. In a previous section, we learned that scientists test their theories through deduction and falsification. Predictions associated with the Big Bang theory that have been tested by this process are:
If the Big Bang did occur, all of the objects within the Universe should be moving away from each other. In 1929, Edwin Hubble documented that the galaxies in our Universe are indeed moving away from each other.
The Big Bang should have left an "afterglow" from the explosion. In the 1960s, scientists discovered the existence of cosmic background radiation, the so-called "afterglow" after the Big Bang explosion. Our most accurate measurements of this cosmic radiation came in November 1989, by the Cosmic Background Explorer (COBE) satellite. The measurements from this satellite tested an important prediction of the Big Bang theory. This prediction suggests that the initial explosion that gave birth to the Universe should have created radiation with a spectrum that follows a blackbody curve. The COBE measurements indicated that the spectrum of the cosmic radiation varied from a blackbody curve by only 1%. This level of error is considered insignificant.
If the Universe began with a Big Bang, extreme temperatures should have caused 25 percent of the mass of the Universe to become helium. This is exactly what is observed.
Matter in the Universe should be distributed homogeneously. Astronomical observations from the Hubble Space Telescope do indicate that matter in the Universe generally has a homogeneous distribution.


How will the Universe End?

Cosmologists have postulated two endings to the Universe. If the Universe is infinite or has no edge, it should continue to expand forever. A Universe that is finite or closed is theorized to collapse when expansion stops because of gravity. The collapse of the Universe ends when all matter and energy is compressed into the high energy, high-density state from which it began. This scenario is of course called the Big Crunch. Some theorists have suggested that the Big Crunch will produce a new Big Bang and the process of an expanding Universe will begin again. This idea is called the oscillating Universe theory.

Early History of the Earth

Scientists believe the Earth began its life about 4.6 billion years ago. The Earth formed as cosmic dust lumped together to form larger and larger particles until 150 million years had passed.

At about 4.4 billion years, the young Earth had a mass similar to the mass it has today. The continents probably began forming about 4.2 billion years ago as the Earth continued to cool. The cooling also resulted in the release of gases from the lithosphere, much of which formed the Earth's early atmosphere. Most of the Earth's early atmosphere was created in the first one million years after solidification (4.4 billion years ago). Carbon dioxide, nitrogen, and water vapor dominated this early atmosphere. Table 5b-1 below describes the three major stages of development of the atmosphere.

As the Earth continued to cool, the water vapour found in the atmosphere condensed to form the oceans and other fresh water bodies on the continents. Oxygen began accumulating in the atmosphere through photo-dissociation of O2 from water, and by way of photosynthesis (life). The emergence of living organisms was extremely important in the creation of atmospheric oxygen and ozone. Without ozone, life could not exist on land because of harmful ultraviolet radiation.

Origin Time of major plant & animal groups

Most of the build up of oxygen in the atmosphere occurred between 2.1 and 1.5 billion years ago as a direct result of photosynthesis from ocean based plants like algae

At about 450 million years ago, there was enough oxygen in the atmosphere to allow for the development of a stratospheric ozone layer that was thick enough to keep terrestrial life protected from ultraviolet radiation. As a result, terrestrial life began its development and expansion at this time. Table 5b-2 describes the timing of the evolutionary development of some of the Earth's dominant forms of life before and after 450 million years before present (BP).