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A History of Life on Earth - Written by Jon Erickson:
Chapter One:
Planet Earth:
Our sun, with its nine planets and their satellites, is a rarity among stars, one of only a small number of single, medium-sized stars in our galaxy. The inner rocky, or terrestrial, planets are much alike, with the notable exception of the Earth, the only planet with a water ocean and an oxygen atmosphere. It is also the only terrestrial planet with a rather large moon, a pairing that still defies explanation.
The atmosphere and ocean evolved during a tumultuous period of crustal formation, volcanic outgassing, and comet degassing. Numerous giant meteorites slammed into the Earth, adding unique ingredients to the boiling cauldron. Raging storms brought deluge after deluge and unimaginable electrical displays. Out of this chaos came life.
The Solar System:
Some 15 billion years ago, the universe originated with a force whose power is still hurling the farthest galaxies away from us at nearly the speed of light. The steady expansion of the beginning universe might have been temporarily sped up by a sudden inflationary bulge, as the new universe rapidly ballooned outward for an instant and then settled down to a steady growth. As the protouniverse expanded, it cooled sufficiently to allow basic units of matter to clump together to form billions of galaxies each containing billions of stars. The first galaxies evolved when the universe was about 1 billion years old and only about a tenth of its present size.
Our Milky Way galaxy has five spiral arms that peel off a central bulge. New stars originate in dense regions of interstellar gas and dust called giant molecular clouds. Several times a century, a giant star over 100 times larger than the sun explodes, producing a supernova a billion times brighter than an ordinary star. When a star reaches the supernova stage, after a very hot existence spanning several hundred million years, the previously stable nuclear reactions in its core become explosive. The star sheds its outer covering, while the core compresses to an extremely dense, hot body called a neutron star; this would be like squeezing the Earth down to about the size of a golf ball.
The expanding stellar matter from the supernova forms a nebula composed mostly of hydrogen and helium along with particulate matter that comprises all the other known elements. About a million years later, the solar nebula collapses into a star. Shock waves from nearby supernovae compress portions of the nebula, with gravitational forces causing the nebular matter to collapse upon itself, forming a protostar. As the solar nebula collapses, it rotates faster and faster, and spiral arms peel off the rapidly spinning nebula to form a protoplanetary disk. Meanwhile, the compressional heat initiates a thermonuclear reaction in the core, and a star is born.
A new star forms in the Milky Way galaxy every few years or so. About 4.6 billion years ago, our sun, an ordinary main-sequence star, ignited in one of the dusty spiral arms of the galaxy. Single, medium-sized stars like the sun are a rarity, and due to their unique evolution such stars appear to be the only ones with planets. Thus, of the myriad stars overhead, only a handful might possess a system of orbiting planets, and fewer yet might contain life.
During the sun's early developmental stages, it was ringed by a protoplanetary disk composed of several bands of coarse particles, called planetesimals, accreting from grains of dust cast off by a supernova. Some 11 trillion planetesimals orbited the sun during the Solar System's early stages of development. As they continued to collide and grow, the small rocky chunks swung around the infant sun in highly elliptical orbits along the same plane, called the ecliptic.
The constant collisions among planetesimals built larger bodies, some of which grew to over 50 miles wide, but most of the planetary mass still resided in the small planetesimals. The presence of large amounts of gas in the solar nebula slowed the planetesimals, enabling them to coalesce into planets. The planetesimals in orbit between Mars and Jupiter were unable to combine into a planet due to Jupiter's strong gravitational attraction and instead formed a belt of asteroids, many of which were several hundred miles wide.
The Solar System itself is quite large, consisting of nine known planets and their moons. The image of the original solar disk can be traced by observing the motions of the planets, all of which revolve sun in the same direction it rotates, all but one, Pluto, within 3 degrees of the ecliptic. Some 7 billion miles from the sun lies the heliopause, which marks the boundary between the sun's domain and interstellar space. About 20 billion miles from the sun is a region of gas and dust, probably remnants of the original solar nebula. Astronomers think a belt of comets lying on the ecliptic exists in this region. Several trillion miles from the sun is a shell of comets that formed from the leftover gas and ice of the original solar nebula.
Chapter One:
Planet Earth:
Our sun, with its nine planets and their satellites, is a rarity among stars, one of only a small number of single, medium-sized stars in our galaxy. The inner rocky, or terrestrial, planets are much alike, with the notable exception of the Earth, the only planet with a water ocean and an oxygen atmosphere. It is also the only terrestrial planet with a rather large moon, a pairing that still defies explanation.
The atmosphere and ocean evolved during a tumultuous period of crustal formation, volcanic outgassing, and comet degassing. Numerous giant meteorites slammed into the Earth, adding unique ingredients to the boiling cauldron. Raging storms brought deluge after deluge and unimaginable electrical displays. Out of this chaos came life.
The Solar System:
Some 15 billion years ago, the universe originated with a force whose power is still hurling the farthest galaxies away from us at nearly the speed of light. The steady expansion of the beginning universe might have been temporarily sped up by a sudden inflationary bulge, as the new universe rapidly ballooned outward for an instant and then settled down to a steady growth. As the protouniverse expanded, it cooled sufficiently to allow basic units of matter to clump together to form billions of galaxies each containing billions of stars. The first galaxies evolved when the universe was about 1 billion years old and only about a tenth of its present size.
Our Milky Way galaxy has five spiral arms that peel off a central bulge. New stars originate in dense regions of interstellar gas and dust called giant molecular clouds. Several times a century, a giant star over 100 times larger than the sun explodes, producing a supernova a billion times brighter than an ordinary star. When a star reaches the supernova stage, after a very hot existence spanning several hundred million years, the previously stable nuclear reactions in its core become explosive. The star sheds its outer covering, while the core compresses to an extremely dense, hot body called a neutron star; this would be like squeezing the Earth down to about the size of a golf ball.
The expanding stellar matter from the supernova forms a nebula composed mostly of hydrogen and helium along with particulate matter that comprises all the other known elements. About a million years later, the solar nebula collapses into a star. Shock waves from nearby supernovae compress portions of the nebula, with gravitational forces causing the nebular matter to collapse upon itself, forming a protostar. As the solar nebula collapses, it rotates faster and faster, and spiral arms peel off the rapidly spinning nebula to form a protoplanetary disk. Meanwhile, the compressional heat initiates a thermonuclear reaction in the core, and a star is born.
A new star forms in the Milky Way galaxy every few years or so. About 4.6 billion years ago, our sun, an ordinary main-sequence star, ignited in one of the dusty spiral arms of the galaxy. Single, medium-sized stars like the sun are a rarity, and due to their unique evolution such stars appear to be the only ones with planets. Thus, of the myriad stars overhead, only a handful might possess a system of orbiting planets, and fewer yet might contain life.
During the sun's early developmental stages, it was ringed by a protoplanetary disk composed of several bands of coarse particles, called planetesimals, accreting from grains of dust cast off by a supernova. Some 11 trillion planetesimals orbited the sun during the Solar System's early stages of development. As they continued to collide and grow, the small rocky chunks swung around the infant sun in highly elliptical orbits along the same plane, called the ecliptic.
The constant collisions among planetesimals built larger bodies, some of which grew to over 50 miles wide, but most of the planetary mass still resided in the small planetesimals. The presence of large amounts of gas in the solar nebula slowed the planetesimals, enabling them to coalesce into planets. The planetesimals in orbit between Mars and Jupiter were unable to combine into a planet due to Jupiter's strong gravitational attraction and instead formed a belt of asteroids, many of which were several hundred miles wide.
The Solar System itself is quite large, consisting of nine known planets and their moons. The image of the original solar disk can be traced by observing the motions of the planets, all of which revolve sun in the same direction it rotates, all but one, Pluto, within 3 degrees of the ecliptic. Some 7 billion miles from the sun lies the heliopause, which marks the boundary between the sun's domain and interstellar space. About 20 billion miles from the sun is a region of gas and dust, probably remnants of the original solar nebula. Astronomers think a belt of comets lying on the ecliptic exists in this region. Several trillion miles from the sun is a shell of comets that formed from the leftover gas and ice of the original solar nebula.