Origins
One hypothesis is that Theia formed at a Lagrangian point relative to
Earth, that is, in about the same orbit and about 60° ahead or behind. When the protoplanet Theia had grown to about the size of
Mars, it became too massive to reside stably in a Trojan orbit. As a
result, its angular distance from Earth fluctuated, with the
fluctuations growing larger until it hit the Earth. This is calculated
to have occurred 4.533 billion years ago (4.533 Ga); Theia is thought to
have struck the Earth at an oblique angle, destroying Theia and ejecting
most of Theia's mantle and a significant portion of the Earth's mantle
into space, while Theia's core sank into Earth's core. Current estimates
based on computer simulations of such an event suggest that some two
percent of the original mass of Theia ended up as an orbiting ring of
debris, about half of which coalesced into the Moon between one and 100
years after the impact. Regardless of the rotation and inclination the
Earth had before the impact, after the impact it would have had a day
some five hours long, and the Earth's equator would have shifted closer
to the plane of the Moon's orbit.
Evidence
Indirect evidence for this impact scenario comes from rocks collected
during the Apollo Moon landings, which show oxygen isotope compositions
that are nearly the same as the Earth. The highly anorthositic
composition of the lunar crust, as well as the existence of KREEP-rich
samples, gave rise to the idea that a large portion of the Moon was once
molten, and a giant impact scenario could easily have supplied the
energy needed to form such a magma ocean. Several lines of evidence show
that, if the Moon has an iron-rich core, it must be small. In
particular, the mean density, moment of inertia, rotational signature,
and magnetic induction response all suggest that the radius of the core
is less than about 25% the radius of the Moon, in contrast to about 50%
for most of the other terrestrial bodies. Impact conditions can be found
that give rise to a Moon that formed mostly from the mantles of the
Earth and impactor, with the core of the impactor accreting to the
Earth, and which satisfy the angular momentum constraints of the
Earth-Moon system.
A belt of warm dust in a zone between 0.25AU and 2AU from the young star
HD 23514 in the Pleiades cluster appears similar to the predicted
results of Theia's collision with the embryonic Earth, and has been
interpreted as the result of planet-sized objects colliding with each
other.[4] This is similar to another belt of warm dust detected around
the star BD+20 307 (HIP 8920, SAO 75016).
Difficulties
Even the dominant lunar origin
theory has some difficulties which have yet to be
explained. These difficulties include:
- Ratios of the Moon's
volatile elements are not consistent with the giant
impact hypothesis.
- There is no evidence that
the Earth ever had a magma ocean (an implied result
of the giant impact hypothesis), and some material
was found which may never have been in a magma
ocean.
- Iron oxide (FeO) content
of 13% of the bulk Moon properties rule out the
derivation of the proto-lunar material from any but
a small fraction of Earth's mantle.
- If the bulk of the
proto-lunar material had come from the impactor, the
Moon should be enriched in
siderophilic
elements, when it is actually deficient of those.
- Certain simulations of the
formation of the Moon require about twice the amount
of angular momentum that the Earth-Moon system has
now. However, these simulations do not take into
consideration Earth's rotation before impact. Some
researchers consider this as insufficient evidence
for disregarding the giant impactor theory.
Source:
Wikipedia
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