Earth is a small habitable planet moving in orbit around a medium size star which is one of hundreds of billions of stars composing our galaxy. The formation of the Earth was the greatest global change.
Our star, born about 4.5 billion years ago, formed a Solar System to get rid of its angular momentum or spin. If Jupiter, as the largest planet in our Solar System, to be an unlit star, the concept of the binary star revolving system is a reality. The sun contains most of the mass, the planets do virtually all of the spinning and the planets are grouped into the inner planets; Mercury, Venus, Earth, and Mars, and the other planets: Jupiter, Saturn, Uranus, Neptune and Pluto, the outermost planet.
The space between mass and Jupiter is filled with debris that is the source of many meteorites hitting the Earth, the asteroid belt. Comets describe orbits which indicate supply from a zone at the fringe of the Solar System, the Oort belt.
The sun formed a disc of matter extended from its equator in a plane described by the present day orbits of the planets. Collision of small particles in the swirling and rotating disc made larger particles. Finally, several large bodies formed attracting the debris encountered in each swing around the sun.
The outer planets were so cold that the ubiquitous gases, hydrogen and helium, which make up the sun, condensed into large bodies. The inner planets did not trap these gases and formed early on as the rocky, jagged planets we see today.
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Each of the planets in our solar system has a different kind of atmosphere. Earth has a comparatively mild climate and a life-giving atmosphere of oxygen and nitrogen. The planets Jupiter and Saturn have cold, heavy poisonous atmospheres of hydrogen and methane gas. The planet Mercury has no atmosphere at all, while Venus is very hot under clouds of carbon dioxide.
The gas in an atmosphere is held to the planet’s surface by gravity, that natural force or pull toward the center of the planet. The molecules of this gas are in constant motion. They bump into each other and bounce away repeatedly as if they were trying to get as far apart as possible.
Out in space between the planets, where there is very little gas, the molecules are so far apart that they bump into each other much less often. But close to the planet’s surface, the pull of gravity packs them closely together into a useful atmosphere. The larger the planet, the more gas its gravity can hold. The high planet Jupiter has a strong force of gravity, which holds a deep, heavy atmosphere. The small planet Mercury has such a weak force of gravity that it could not keep the gas molecules from bouncing off into space. As a result, Mercury has no atmosphere.
The planet Mercury has lost its atmosphere because of its high temperature.
On warmer planets, those close to the sun, the bouncing gas molecules move faster and are likely to escape. On colder planets, those farther from the sun, the gas molecules moves more slowly and are more easily held by gravity.
Many scientists believe that our solar system was formed from a great cloud of dust and gas that contracted to form the sun, the planets, and their moons. The gas contained a large amount of hydrogen, with some helium, the two lightest gases in the universe. Also included was water vapor, ammonia, and methane. See figure 1.
(figure available in print form)
Because Jupiter and Saturn have strong gravities and are very cold, these high planets were probably able to hold most of these original gases in their atmospheres.
Mars, Venus and Earth with their weaker gravities and warmer climates, quickly lost the lighter gases, hydrogen and helium, into space. The heavier gases that remained were slowly changed, according to one theory, by the sun and chemical reactions. See figure 2.
(figure available in print form)
Ultraviolet radiation poured into the atmosphere. These strong radiations broke molecules of water (H2O) down into molecules of hydrogen (H) and oxygen (O). The hydrogen quickly escaped into space, leaving the heavier oxygen in the atmosphere to combine with the other gases and with the materials of Earth’s crust.
This free oxygen (O) split apart the methane (CH4) molecules and then combined with the carbon (C) atoms to make carbon dioxide (CO2) and with the hydrogen (H) atoms to make water (H2O). See figure 3.
(figure available in print form)
The oxygen also split apart ammonia (NH3) and then combined with the hydrogen to make water, releasing the nitrogen (N) into the atmosphere. The methane and ammonia in the atmosphere of the small planets slowly changed, over billions of years, into carbon dioxide and nitrogen. More carbon dioxide and other gases were added to the atmosphere by erupting volcanoes. The free nitrogen slowly combined with minerals in the planet’s crust, leaving only carbon dioxide as the major gas in the atmospheres.
When the free oxygen had used up all the methane and ammonia, a small amount of free oxygen accumulated. The ultraviolet rays from the sun changed it into ozone. Ozone then blocked most of the ultraviolet rays from penetrating into the lower atmosphere, just as it does today. With only a small amount of ultraviolet left to split water molecules into hydrogen and oxygen, very little more free oxygen was produced. The atmosphere became stable, remaining mostly carbon dioxide, with only a very small amount of oxygen and other gases.
Mars and Venus are believed to have atmospheres of these gases today. The atmosphere of Mars is very thin because of its weak force of gravity; that of Venus is much heaver because of its greater gravity. Because carbon dioxide holds heat in the atmosphere of Venus makes this planet very hot.
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