History of Earth

The history of Earth covers approximately 4.55×10⁹ years, from its formation out of the solar nebula to the present. This article presents a broad overview of this period, summarizing the leading scientific theories for each time period. Due to the difficulty of grasping very large amounts of time, an analogy to a clock will be used, with midnight beginning at the formation of Earth and the next midnight occuring now. Each second on this "clock" represents approximately 53,000 years, and the Big Bang and origin of the universe took place almost three days ago.

Origin

Main articles: Solar system § Origin and age of the solar system, and Planet § Planetary formation

The formation of Earth occurred as part of the formation of the solar system. It existed initially as a large rotating cloud of dust and gas, composed of hydrogen and helium produced in the Big Bang, as well as heavier elements produced in numerous supernovas from stars long gone. Then, about 4.6×10⁹ years ago, maybe fifteen minutes or half an hour before midnight, a nearby star probably exploded as a supernova. This sent a shock wave towards the solar nebula, causing it to contract. Due to inertia, as the cloud rotated, it became a flat disc perpendicular to its axis of rotation. Most of the mass concentrated in the middle and began to heat up. Meanwhile, the rest of the disc began to break up into rings, with gravity causing matter to condense around dust particles. Small fragments collided to become larger fragments, including one collection approximately 150 million kilometers from the center; this would later be named Earth by some of the life forms that arose on its surface. As the Sun condensed and heated, fusion initiated and the solar wind cleared out most of the material in the disc that had not condensed into larger bodies.

Moon

Main articles: Moon § Origin and history, and Giant impact hypothesis

The origin of the Moon is still uncertain, although considerable evidence exists for the giant impact hypothesis. Earth was probably not the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from from both the Sun and the Earth, at the fourth Lagrange point; 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 have initially stable, but destabilized as Earth increased its mass. It swung back and forth, relative to Earth, until finally, 4.553×10⁹ years ago (perhaps 12:33 a.m. on our clock), 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. Heavier elements from Theia sank to Earth's core, while the remaining material and ejecta condensed into a single body within a couple weeks; it would become a more uniform, spherical body probably within a year. The impact also probably knocked the Earth's axis to produce the large 23.5° axial tilt that is responsible for Earth's seasons (a simple ideal model of the planets' origins would have axial tilts of 0° and no recognizable seasons). It may have also sped up Earth's rotation and been instrumental in the origins of Earth's plate tectonics.

References

• http://www.nasa.gov/worldbook/earth_worldbook.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_1.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_2.html