I am crazy about documantary these days, I want to share the follow article with you, It is about he origins of Earth. Good for improving our knowledge
The Origins of Planet Earth
In its infancy, Earth was a primeval hell, a lifeless planet bombarded by massive asteroids and comets. Instead of water, red hot lava streamed across the surface of our planet. Volcanoes spewed noxious gases into the primitive atmosphere. So how did Earth make such an astonishing transformation? How did it change from a raging inferno like this, to a place where we have air we can breathe, and water covering nearly three quarters of its surface? A place where life could take hold and evolve into complex organisms like you and me? Earth only became a habitable planet only after a series of devastating disasters in its early years
Earth was born 4.5 billion years ago, but its violent history began well before that, when huge ancient stars that had reached the ends of their lives exploded. These supernovas cooked up all the chemical elements we know today including iron, carbon, gold and even radioactive elements like uranium. Over time, gravity took hold, and this cloud of stardust collapsed into an enormous rotating disk: the solar nebula.
In the center of this disk, temperature and pressure rose, and a star, our sun, was born. Eventually, gases like hydrogen and helium would be swept to the far reaches of the disk, but closer to the sun were dust grains made of the heavier elements.
These dust grains circled around the early, and occasionally grains traveling nearby collided. If they collide slowly, they can add up to a larger object and gradually grow. With enough collisions, dust grew into pebbles and pebbles grew into rocks. And as the rocks grew larger, so did the collisions. Sometimes if they collided head on or at higher velocities then they'll actually broke apart, but other times, the rocks stuck together. And the larger they got, the stronger their gravity became.
Because of the gravitational attraction between these bodies, the particles and built bigger things, because gravity holds things together. In time, gravity shaped them into small, round planets. Eventually, some grew as big as our moon. And then they combined to form the four small, rocky planets closest to the sun: Mercury, Venus, Mars and Earth. At this point the Earth was a fiery ball covered with lava.
During the first few hundred million years, the Earth was so energetic and was recycling materials so vigorously and melting material, that rocks from that period have not survived. So to reconstruct the story of the Earth's infancy, we look for clues not from the ground but from outer space. More than a hundred million miles from Earth, between Mars and Jupiter, lies a region called the Asteroid Belt. Here, trillions of asteroids, enormous rocks left over from planet building, are held in orbit.
Every now and then, a fragment of one of these asteroids is knocked out of orbit and set on a collision course with Earth. Called meteors, they consist of chondrite, a carbon-rich meteorite formed from the very same stardust that built the Earth.
Meteorites are a window on the past, since radioactive elements within them decay at a precisely known rate, allowing scientists to calculate the meteorite's age. Since most meteorites formed at the same time as the planets, and from the same material, the age of the meteorite gives you the age of Earth and its neighbors. If you date meteorites, what you find is that almost all meteorites have the same age, about four and a half to five billion years old. And so what we do is take the oldest of the ages and use that as the initial age of the solar system.
Earth had barely taken shape before the first of several major disasters struck the young planet. Earth's gravity was pulling in huge quantities of debris from space, a continual bombardment that generated enormous amounts of heat on the surface. At the same time, radioactive elements trapped deep within the Earth were decaying, producing even more heat, roasting the planet from the inside. The combined effect was catastrophic turning the planet into a raging furnace. And when the temperature reached thousands of degrees, dense metals such as iron and nickel in Earth's rocky surface melted. The outer part of the Earth would have been completely molten.
When you have a totally molten object like this, the heaviest elements—and that includes things like iron—would sink to the center and the lightest elements—things rich in carbon and water for instance, or light elements—would float to the top. The global migration of the elements, known as the Iron Catastrophe, would have a profound effect on the future of our planet. The sinking iron accumulated at Earth's center where it created a molten core twice the size of the moon. The liquid iron is constantly swirling and flowing. And even today this motion generates electric currents which turn our planet into a giant magnet with north and south poles. The core is still in constant motion.
The magnetic field is constantly fluctuating, and one result of this is the fact that it causes the magnetic pole to actually move randomly over the course of a day. At the time of the most recent survey, the pole had moved 125 miles off the Canadian coast.
Without Earth's liquid iron core, life would be in trouble. This swirling ball of molten iron is what generates the magnetic field around our planet. And we need that magnetic field because every day a deadly stream of electrically charged particles bombards the Earth. These electrically charged particles are ejected by the sun in monstrous solar flares forming what is known as the solar wind. Some think that if the solar wind ever reached our planet, it would strip away the atmosphere. But Earth's magnetic field creates a protective shield that deflects these deadly particles.
And you don't have to travel far to see the fate of a planet that lost its shield. Four billion years ago, Mars had a liquid iron core and a magnetic field just like Earth's. Mars built up a thick atmosphere and supported liquid water on its surface. The planet may even have been home to primitive forms of life. But Mars is just a fraction the size of the Earth, so it cooled more rapidly. And as it cooled, its molten iron core hardened. As a result, Mars stopped generating its magnetic shield. And, according to one theory, this left its atmosphere to be scoured away by the solar wind.
Today, the surface of Mars is a barren desert. Mars is a stark reminder of what our world could have become if its iron core had cooled, because without a magnetic shield a planet is left prey to the solar wind, and life, as we know it, could never flourish.
But even when the Iron Catastrophe was over, and even with the formation of Earth's core and magnetic shield, our planet remained a hostile and alien world. Volcanoes spewed clouds of noxious gases and Earth was enveloped in a suffocating atmosphere of carbon dioxide, nitrogen and steam. With no oxygen to breathe and no ozone layer to block the lethal ultraviolet radiation, this was not a hospitable place for life, at least life as we know it.
Where did the moon come from and how did it get there? The Apollo astronauts collected hundreds of rocks from the moon's surface, and upon return scientists calculated their age using radioactive dating. They discovered that the moon was millions of years younger than Earth, and that the materials on the moon have exactly the same chemistry as the Earth.
Based on this information, scientists came up with a theory for the formation of the moon. This theory proposed that about 50 million years after Earth had formed massive rock about the size of Mars, slammed into our planet. The energy of that impact was so great it melted and fused the two colliding bodies together forming a new, larger Earth. At the same time, this enormous collision ejected into orbit vast amounts of molten rock. This debris eventually coalesced to form the moon. This collision that created the moon caused such an immense impact that it forced Earth's axis to tilt in relation to the sun, causing the familiar seasons.
Earth's hot molten surface took at least a billion years after the moon was created to cool and form a thick skin, or crust, soon after the moon was formed. And once Earth was cool enough to form solid ground water could collect on its surface eventually forming a landscape of islands and small continents, bathed by a primitive ocean. Eventually, water, which is the key to life, would cover nearly three quarters of the Earth's surface.
One theory as to how would Earth ended up with such vast quantities of water was that these great oceans may have been there from the very beginning, just hidden away. One key to the riddle was volcanoes, which, throughout Earth's infancy, pumped huge amounts of steam into the atmosphere. Then, as Earth cooled, that steam condensed into rain. Drop by drop, water collected in low-lying areas.
Another theory proposes that the water in our oceans might have come from outer space, delivered to the surface by massive ice-bearing comets. Perhaps the heat of an impact would have evaporated the ice within a comet, creating storm clouds over vast areas of the planet. These clouds produced a deluge of hot, possibly acidic rain that continued for millions of years. At first the rain would have formed lakes and rivers, and eventually water would cover almost the entire globe.
The young Earth was still very different from the planet we know today. It was a hostile and forbidding place, with an atmosphere full of poisonous gases. Yet, somehow, these harsh conditions set the scene for a crucial phase of Earth's development: the origin of life.
