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 Space
Impacts and the Origin of the Moon
The currently accepted theory about the origin of the Moon
is that it was formed from matter ejected from the Earth after a
collision with a large body.
However so far all studies have failed to determine what kind of
object this might be, but a highly accurate model has now found
that a collision with a body the size of Mars could account for
the features of the Earth and the Moon.
According to Robin Canup and Erik Asphaug, the discovery further
strengthens the impact theory because objects of this size were
reasonably abundant in the early solar system (R Canup and E Asphaug
2001 Nature 412 708).
The most accepted theory on the origin of the Moon is as a consequence
of an impact. Modeling of this impact has however proven very hard.
In particular, two considerations limit the kind of impacts that
could have given rise to the Moon:
1) The large angular momentum of the Earth-Moon system suggests
that the Earth rotated once every five hours before the Moon formed.
2) Also it is widely accepted that the Moon contains far less iron
at its core than the earth.
Earlier models required a second impact - for which there is little
evidence - to justify the current state of the system. Such impact
is needed to remove angular momentum from the system. Furthermore
this required the two bodies to keep on gathering similar matter
after the collision, which would not explain the different concentration
of iron in the two bodies.
New simulations carried out at the Southwest Research Institute
in Colorado and the University of California at Santa Cruz show
that a collision with a body about the size of Mars could lead to
an iron-poor Moon and the current dynamics of the Earth and the
Moon. The calculations were based on 'smooth particle hydrodynamics',
which modelled the dynamics of different impacts and the tens of
thousands of ejected fragments.
This result provides a major support to the impact theory for the
formation of the Moon.
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