Jul 8, 2020
Image: An artist's depiction of the hypothetical impact of a planet like Theia and the Earth - wikipedia.com
Theia /ˈθiːə/ is a hypothesized ancient planet in the early Solar System that, according to the 'giant-impact hypothesis', collided with the early Earth around 4.5 billion years ago says this article from wikipedia.com. According to one hypothesis, Theia was an Earth trojan about the size of Mars, with a diameter of about 6,102 km (3,792 miles). Models of the impact indicate that Theia's debris gathered around Earth to form the early Moon.
The Theia hypothesis also explains why Earth's core is larger than would be expected for a body its size: according to the hypothesis, Theia's core and mantle mixed with Earth's. Additional evidence published in 2019 suggests that Theia might have formed in the outer Solar System rather than the inner Solar System, and that much of Earth's water originated on Theia.
Name
Theia was named for the titaness Theia, who in Greek mythology was the mother of Selene, the goddess of the moon, which parallels the planet Theia's collision with the early Earth that is theorized to have created the Moon.
An alternative name, Orpheus, is also used.
Orbit
Theia is thought to have orbited in the L4 or L5 configuration presented by the Earth–Sun system, where it would tend to remain. In that case, it would have grown, potentially to a size comparable to Mars. Gravitational perturbations by Venus could have eventually put it onto a collision course with the early Earth.
Collision
According to the giant-impact hypothesis, Theia orbited the Sun, nearly along the orbit of the proto-Earth, by staying close to one or the other of the Sun–Earth system's two more stable Lagrangian points ( i.e., either L4 or L5). Theia was eventually perturbed away from that relationship by the gravitational influence of Jupiter and/or Venus, resulting in a collision between Theia and Earth.
Computer simulations suggest that Theia was traveling no faster than 4 km/s (8,900 mph) when it struck Earth at an estimated 45 degree angle.
Originally, the hypothesis supposed that Theia had struck Earth with a glancing blow and ejected many pieces of both the proto-Earth and Theia, those pieces either forming one body that became the Moon or forming two moons that eventually merged to form the Moon. Such accounts assumed that if Theia had struck the proto-Earth head-on both planets would have been destroyed, creating a short-lived second asteroid belt between the orbits of Venus and Mars.
In contrast, evidence published in January 2016 suggests that the impact was indeed a head-on collision and that Theia's remains can be found in both the Earth and the Moon.
Hypotheses
From the beginning of modern astronomy, there have been at least four hypotheses for the origin of the Moon:
A single body split into Earth and Moon
The Moon was captured by Earth's gravity (as most of the outer planets' smaller moons were captured)
The Earth and Moon formed at the same time when the protoplanetary disk accreted
The Theia scenario described above.
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
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