Theia

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File:TheiaCGIpromo.PNG Theia (protoplanet)

Theia /θiːə/ is a hypothesized ancient planetary-mass object in the early Solar System that according to the giant impact hypothesis collided with the Early Earth around 4.4 billion years ago (BYa) and one of the main characters in How the Universe Works.^[1] According to the hypothesis, Theia was an Earth trojan about the size of Mars; if the impact were not glancing, it could have destroyed Earth. Models of the impact propose that Theia's debris gathered together around Earth to form what was the early Moon. After the debris from the collision flew into space, some scientists think that it originally formed two moons^[2][3] which later merged to form the single Moon we know today. However, the two-moon hypothesis is not necessary to explain the difference in the faces of the near and far sides of the Moon. The theory also explains the reason that Earth's core is larger than it would be for a body its size, since according to the theory, Theia's core merged with Earth's.^[4]

Orbit

Theia was thought to have orbited in the L4 or L5 spots in the Earth's orbit. It grew to a Mars-like size and, through gravitational perturbation from Venus, was moved around and eventually set on a collision course with Earth.^[5]

Name

The name for Theia came from the Greek titaness, Theia. In mythology, Theia was the mother of Selene, the goddess of the moon, just as the planet Theia's collision with the early Earth is theorized to have created the Moon. An alternative name, Orpheus, has been used in the past but has largely been discarded.

Collision

According to the Giant Impact Hypothesis Theia orbited the Sun at around the orbit of Earth at the L4 or L5 Lagrangian points,Template:Fact but was perturbed by Jupiter and Venus into a collision with the proto-Earth.Template:Fact Theia struck Earth with a glancing blow^[6] and ejected many pieces of both the proto-Earth and Theia. These pieces either formed one body that became the Moon, or formed two moons that eventually merged to form the Moon.^[2] Had Theia struck the proto-Earth head-on, it would have led to the destruction of both planets, creating a short-lived asteroid belt between the orbits of Venus and Mars.

Theories

Main article: Origin of the Moon

From the beginning of modern astronomy, there have been at least four hypotheses for the proposed origin of the Moon: that a single body somehow divided into Earth and Moon; that the Moon was captured by Earth's gravity (as most of the outer planets' smaller moons were captured); that Earth and Moon formed at the same time when the protoplanetary disk accreted; and the Theia scenario. The lunar rock samples retrieved by Apollo astronauts were found to be very similar in composition to Earth's crust, and so were likely removed from Earth in some violent event.^[7][8]

References