Origins of the Earth and Moon
History of Earth. (2008, September 4). In Wikipedia, The Free Encyclopedia. Retrieved 20:06, September 9, 2008, from http://en.wikipedia.org/w/index.php?title=History_of_Earth&oldid=236195965. Edited for content by B.Rick.
The Earth formed as part of the birth of the Solar System: what eventually became the solar system initially existed as a large, rotating cloud of dust, rocks, and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas.
Then, as one theory suggests, about 4.6 billion years ago a nearby star was destroyed in a supernova and the explosion sent a shock wave through the solar nebula. The solar nebula cloud began to rotate. Gravity and inertia flattened it into a protoplanetary disk. Most of the mass concentrated in the middle and began to heat up.
The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic heat at the center of the cloud. Its inability to transfer that energy away resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, ignited to create the Sun.
Meanwhile, gravity caused matter to condense around the objects outside of the new sun's gravity grasp. Dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately destined to become protoplanets. These included one collection approximately 150 million kilometers from the center: Earth. The solar wind of the newly formed star cleared out most of the material in the disk that had not already condensed into larger bodies.
The origin of the Moon is still uncertain, although much evidence exists for the giant impact hypothesis. Earth may not have been the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from both the Sun and the Earth.
This planet, named Theia, is thought to have been smaller than the current Earth, probably about the size and mass of Mars. Its orbit may at first have been stable, but destabilized as Earth increased its mass by the accretion of more and more material. Theia swung back and forth relative to Earth until, finally, an estimated 4.533 billion years ago, it collided at a low, oblique angle.
The low speed and angle were not enough to destroy Earth, but a large portion of its crust was ejected into space. Heavier elements from Theia sank to Earth’s core, while the remaining material and ejecta condensed into a single body within a couple of weeks. Under the influence of its own gravity, this became a more spherical body: the Moon.
The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons. It may also have sped up Earth’s rotation and initiated the planet’s plate tectonics.