A Sweet Briar College Learning Resource
The PHYSICS of WATER on EARTH
Professor George Lenz
Professor Hyman 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.
In fact, the mass of water present in the oceans, now about 10(24) grams, is about the same as the mass of water that was contained in the crust when the degassing process started. We can estimate the rate at which water is being lost today by estimating the rate at which water molecules in the atmosphere are dissociated into its constituent hydrogen and oxygen. The hydrogen is light enough that it easily moves off into space. The net effect of hydrogen loss decreases the amount of water vapor in the atmosphere. A good estimate is that 5x10(11) grams are lost this way each year. This amounts to a volume of a cube about 100 yards on a side. The total water lost to space since the beginning of the earth thus amounts to about 2x10(21) grams, about 0.2 percent of the water in the oceans.
This means that most of the water you see on the earth was the very same stuff that degassed from the crust when the earth was only a few hundred million years old.
Fortunately, the water lost to space is replaced by the same geologic processes that formed the oceans originally.
At the present time, about 70%of the surface of the earth is covered with water. The present coastlines are where they are because some of the water is locked up in the polar ice caps.
In terms of volume, the water on earth is distributed in the following way:
If the water locked up in polar ice were to completely melt, the oceans would rise about 240 feet above its present level.
The second question we raised, Why is the water still here on the earth?, is more difficult to answer. It has to do with the changing nature of the atmosphere due to evolution of life, specifically algae. The algae produced free oxygen by photosynthesis which destroyed ammonia and methane, so called greenhouse gases, just as the sun's luminosity was increasing by about twenty five percent. If that hadn't happened the oceans would have boiled away long ago. In fact, we are the beneficiaries of an incredible balancing act which allowed just enough heat to escape from the earth to keep the oceans from boiling, but not so much as to cause the earth to freeze solid.
With this as an introduction to the origins of our watery planet, I will turn in class to consider the role water plays in defining our weather (climate) on decadal or centennial time scales.
H20 - The Mystery, Art, and Science of Water
Chris Witcombe and Sang Hwang
Sweet Briar College