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The Origin of Earth

Agnieszka P. Baier and Michelle Hall-Wallace

Did you know that Earth is 4.556 billion years old?

Earth emerged from a swirling nebula of interstellar gas and dust that also gave rise to the Sun and other bodies in our solar system. Countless tiny mineral grains in the solar nebula grew to form rocks of increasing size and then merged into planetesimals 10 km or so across. Gravity pulled the planetesimals together causing them to collide and coalesce into even larger planets like Earth.

Earth and Moon as seen today from space. Image credit : NASA.

Researchers define the age of the Earth very precisely. They believe that planets completed their formation process very soon after the oldest components of primitive meteorites (objects which have fallen to Earth from space) condensed out of the solar nebula 4.56 billion years ago. H. Jay Melosh, Professor of Planetary Sciences at the University of Arizona in Tucson, explains: "A class of meteorites called carbonaceous chondrites is believed to be the most primitive material in the Solar System." These meteorites are mostly composed of silicate materials and are rich in water and carbon. Much of the carbon is in the form of organic compounds, including amino acids, which are the 'building blocks' of life. Inside some carbonaceous chondrites are mineral grains containing inclusions rich in calcium and aluminum. "The combination of elements found in these inclusions condenses at a very high temperature - higher than any other components of rocks. It is believed that these calcium-aluminum rich inclusions were produced at the time when the very hot solar nebula began to cool off. As the temperatures in the nebula were dropping, these grains condensed first," explains Jay Melosh. Later, they became part of larger meteorites. The planets formed about a hundred million years after this initial condensation. By 4.5 billion years ago, the Earth had most of its present mass.

These were very turbulent times. Earth grew in size as more and more planetesimals joined together. Through this process intense heat was generated inside the planet. Short-lived radioactive elements created during Earth's formation were decaying, giving off more energy. Many researchers believe that at one point the Earth was so hot that it may have been entirely molten.

These hot conditions allowed the heavier elements such as iron and nickel to sink into the Earth's center to form the core. The lighter elements remained in the mantle and some of them eventually formed Earth's rigid crust. As the planet was cooling off, various gases were released from Earth¹s interior, including water vapor. They enshrouded the planet in a dense cocoon that formed the primordial atmosphere. It contained little, if any, oxygen. When temperatures became cool enough, the water vapor rained out of the atmosphere and the ocean, as we know it today, formed.

Artistic vision of a cool early Earth 4.4 billion years ago rendered by Andree Valley and Mary Diman.

Four and a half billion years ago the Solar System was a crowded place. Many bodies of planetary size wandered in space and impacts among them were frequent. Earth was repeatedly involved in such cataclysms. "Impacts of planet-size objects generated enough energy to probably annihilate the atmosphere and vaporize the ocean many times. In these events, some of the surface rock would melt to form a magma ocean and some would even vaporize to form a very dense atmosphere. The planet would heat up to a few thousand degrees and radiate like a small star," explains Melosh. One of these impacts happened about 4.51 billion years ago; an object the size of Mars collided with Earth and jolted a lot of material from Earth¹s interior into orbit. "Debris from this collision coalesced to form our Moon," in only a few hundred years," says Prof. Melosh, who in the 1980s created computer models to determine how much material went into orbit to form the Moon. Although this event was truly catastrophic, Earth's environment settled down rapidly. There is evidence that by 4.4 billion years ago the planet was calm and cool enough to have established a rock cycle that continually creates and destroys rock today. Even then, however, Earth looked nothing like the planet we know.

Evidence of this formation process is present in the rocks geoscientists study every day. Through EarthScope, scientists will have a unique opportunity to learn much more about the formation of Earth by focusing on processes still active in the core and mantle. In addition, the technology of EarthScope will provide high quality 3D images of Earth from the surface to the core that will help us understand how Earth's past affects our future.



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Last update 21-November-2002
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