Evolution of the Earth and Moon

The collapse of the solar system nebula into the Sun and planets began the solar system evolution. Following the formation of planets each objective evolved in its own distinctive way.

The Formation and Early History of the Earth:

Once the countless thousands of planetsimals were formed, the formation of the planet quickly followed quickly. As planetsimals moved in their orbits, they collected smaller planetsimals through the process of gravitational attraction. Then these larger planetsimals collided and coalesced into the beginnings of a planet. As the process of accumulation went on, the growing planet gradually swept up all the debris that lay near its orbit.

If you had been standing on the surface of the newly forming Earth during this stage, you would have seen a spectacular display. A constant rain of debris left over from the initial period of planetary formation fell to the surface, consistently adding mass to the Earth. During this period, called the great bombardment, the large amounts of kinetic energy carried by the shower of stones were converted into heat, which was added to the newly forming planet. By some accounts, much of the Earth's surface would have glowed bright red from this accumulating heat, and each large impact would have been accompanied by a spectacular splash of molten rock. Although the addition of material to the Earth has slowed considerably since the beginning, it has not stopped. Every time you see a meteor (often called a shooting star) for example, you are seeing an object roughly the size of a grain of sand being added to our planet. Scientists estimate that the mass of the Earth grows by about 20 metric tons (20,000 kg, or two times 10 to the seventh power gm) per day by accretion of material falling from space.

When the nebular hypothesis was first proposed in the eighteen century, there seemed little chance that any direct observation evidence could be found to support it. In 1992, however, astronomers, using the Hubble Space Telescope were able to detect thick masses of dust encircling newborn stars in a region of space called the Orion nebula. It appears that in these cases we are seeing distant solar systems in the process of being born--observations that give us a measure of confidence in our model of how planets come into existence.

The Layered Structure of the Earth:

Each time another planetesimal hit the the early Earth, all of its kinetic energy and potential energy was converted into heat. That heat diffused through the planet, which glowed red hot, and reached temperatures of thousands of degrees in its deep interior. Eventually, the Earth either melted completely or else was heated to high enough temperatures so that it was very soft all the way through. Heavy, dense materials (like iron and nickel) sand under the force of gravity toward the center of the planet, while lighter, less-dense materials floated to the top. The result of this process called differentiation, is that the present-day Earth has a very definite layered structure. (page 380)

In a sense, while happened to the Earth long ago is not too different from what happened to a mixture of oil and water that is shaken up and then allowed to stand. Eventually, the lighter oil will float to the top and the heavier water will sink to the bottom under the influence of gravity. The Earth also separated into layers of different density when it underwent differentiation.

At the center of the Earth, with a radius of about 3400 kilometers (2000 miles), is the core, made primarily of iron and nickel metal. Temperatures at the center of the Earth are believed to exceed 5,000° C (9,000° F), but pressure are so high--about 3.5 billion grams per square centimeter (almost 50 million pounds per square inch)--that the iron-nickle core is solid. A little farther out the pressures are somewhat lower, so that the outer region of the iron-nickel core is actually a liquid.

The metal core is overlain by a thick layer, the mantle, that is rich in the elements oxygen, silicon, magnesium, and iron. Metallic bonding predominates in the core, but the mantel features minerals with primarily ionic bonds between negatively charged oxygen ions and positively charged silicon, magnesium, and other ions. Mantle rocks are similar in composition to some familiar surface rocks but the atoms in these high-pressure materials are packed together in much denser forms.

At the very outer layer of the Earth is the crust, which is made up of the lightest materials. The crust thickness ranges from less than 10 kilometers (6 miles) in parts of the ocean, to as much as 70 kilometers (about 45 miles) beneath parts of the continents. The crust is the only layer of the solid Earth with which human beings have had contact, and it remains the source of almost all the rocks and minerals that we use in our lives.

You might wonder how scientists can describe parts of the Earth that no human being has ever seen. Seismology, a branch of science that has provided ( among other things) our present picture of the Earth's interior.

Technology:

Producing World Record-High Pressures—— The force of gravity, pulling inward on all of the Earth's layers, results in immense internal pressures, exceeding 3 million times the atmospheric pressure at the Earth's center. What changes affect rocks and minerals at these extreme conditions? High-pressure researchers, who have learned to sustain laboratory pressures greater than those at the center of the Earth, are providing surprising answers.

Of all the materials from the Earth's deep interior, none hold more fascination than diamonds, the high-pressure form of carbon. This magnificent gemstone is also the hardest known substance and the most efficient abrasive for machining the tough metal parts of modern industry. Until the mid-1950, diamonds were available only from a few natural sources, but in 1954 scientists at General Electric discovered how to manufacture diamonds by duplicating the extreme temperatures and pressures that exist hundreds of kilometers beneath the Earth's surface. The researchers squeezed carbon between the jaws of a massive metal vise and heated their sample with a powerful electrical current. Early experiments yielded only a fraction of a carat of diamond, but large factories now produce dozen of tons of diamonds annually, am output exceeding the total amount of diamonds mined since Biblical times.

The Earth taught us how diamonds are made, and now scientists use diamonds to learn how the Earth was made. The highest sustained laboratory pressures available today are obtained by clamping together two tiny pointed anvils of diamond. Samples squeezed between the diamond-anvil faces are subjected to pressure of several million kilograms per square centimeter, greater than at the center of the Earth. At such extreme conditions, rocks and minerals compress to new, dense forms occupying less than half their original volume. Dramatic changes in chemical bonding are also observed, with many ionically and covalently bonded compounds transforming to metals at high pressure.