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how did water form
The simple answer is we still don't know, but we have theories. One theory states that, nearly 4 billion years ago, millions of asteroids and comets slammed into the Earth's surface; a quick glance at the moon's crater-pocked surface gives us an idea of what conditions were like. The theory goes that these weren't normal rocks but rather the equivalent of cosmic sponges, loaded with water that was released on impact. Finally, researchers from the California Institute of Technology found that water from the comet Hale-Bopp contains much more heavy water (which has more deuterium) than Earth's oceans, meaning either the comets and asteroids that hit Earth were very different than Hale-Bopp, or Earth got its water some other way.Another recent theory states that a young Earth was bombarded by oxygen and other heavy elements produced within the sun. The oxygen combined with hydrogen and other gases released from the Earth in a process known as degassing, forming the Earth's oceans and atmosphere along the way.

A team of scientists from Japan's Tokyo Institute of Technology has devised yet another theory, which states that a thick layer of hydrogen may have once covered the Earth's surface, eventually interacting with oxides in the crust to form our planet's oceans.(From how stuff works)
 has described how the chemical building blocks of water, hydrogen and oxygen, were formed in the "big bang" and in the interior of stars by a process known as nuclear synthesis. In the last few years planetary probes have detected tantalizing evidence that water may exist on other bodies in our solar system, though in fact no other planet or moon in our solar system has the amount of liquid water present on the earth.

"The earth appears to be unique in our solar system in that it contains an enormous amount of water, and that water has existed in a form not too different from its present state for billions of years. Given that the laws of the nature operate everywhere in the solar system, we have to question why we are so privileged to have large bodies of liquid water on our planetary surface for so long a time. What makes the earth different from the other planets? To answer that question, we have to deal with two issues. 1) How did the earth acquire such a large amount of water in the first place, and 2) Once acquired, how was it retained ?

The first question has to do with how the earth was formed and the second involves the evolution of the earth and its atmosphere. As we shall see, the long term existence of our watery planet as a place hospitable for the evolution of life involves a considerable amount of good luck.

The most recent theories of of planet formation describe the process of planet formation as having two steps. First, gravitational collapse takes place forming small asteroid like bodies some as large as 1/500 of the mass of the earth. The planetesimals begin to collide and form the larger bodies of the planets. The rain of bodies on the surface of the earth generates large amounts of heat, enough to cause the heavier elements, such as iron to migrate to the center. A second factor has to do with the fact that when a meteor hits anything, some of it sticks and some is scattered back into space by the impact. The lower the density of the material, the more likely it is to escape. In the early stages, the earth collects heavier stuff more easily, leaving lighter stuff such as silicon and water still in orbit about the sun.

As the earth gets bigger, however, it more effectively traps the lighter material during the latter stages of planet formation.

The formation of the earth probably took a few hundred million years to be completed. That is to be compared with the time of about 3.5 billion years since the earth has developed a solid crust. About the time the earth was formed, the sun became large enough that the fusion reactions in the sun ignited. This didn't happen smoothly, but likely in sputtering way for a while. Each flaring up of the sun sent streams of particles sweeping out. If the earth had an atmosphere at this time, it would have been blown off leaving the earth as a rock with neither air or water on its surface. In fact, after the sun stabilized, the earth went through a process of releasing gases from its interior in a process called degassing. Over a relatively short time, something like a 100 million years, enough material had been released to form the oceans and to give the earth an atmosphere. There was no free oxygen in the atmosphere at this time, but it was a collection of gases, largely ammonia, methane and carbon dioxide, held to the earth by gravitational attraction. Fortunately, early in its history, the temperature of the earth dropped below 212 degrees Fahrenheit, and the water condensed into the oceans we know today.''
http://witcombe.sbc.edu/water/physicsearth.html The PHYSICS of WATER on EARTH
Professor George Lenz



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