The Moon -(post 3 of 6)
The giant impact hypothesis is currently the most accepted scientific hypothesis for the formation of the Moon.
According to modern theories of planet formation in the solar system, Theia was part of a population of Mars-sized bodies that existed in the Solar System 4.5 billion years ago. It orbited the Sun in about the same orbit as the Earth, about 60° ahead or behind (at the L4 or L5 Lagrangian points relative to Earth), similar to a Trojan asteroid.
However, the stability of Theia's orbit was affected when its growing mass exceeded a threshold of approximately 10% of the Earth's mass some 20-30 million years later.
In this scenario, gravitational perturbations by planetesimals caused Theia to depart from its stable Lagrangian location, and subsequent gravitational interaction with the proto-Earth locked it into an ultimate collision course.
In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle when the Earth was nearly fully formed.
Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s. Theia's iron core would have sunk into the young Earth's core, and most of Theia's mantle accreted onto the Earth's mantle, however, a significant portion of the mantle material from both Theia and the Earth would have been ejected into orbit around the Earth. This material quickly coalesced into the Moon (possibly within less than a month, but in no more than a century). Estimates based on computer simulations of such an event suggest that some twenty percent of the original mass of Theia would have ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon.
The Earth would have gained significant amounts of angular momentum, and mass from such a collision. Regardless of the speed and tilt of the Earth's rotation before the impact, it experienced a day some five hours longer after the impact, and the Earth's equator shifted closer to the plane of the Moon's orbit in the aftermath of the giant impact.
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