|How did the Moon form?
astronauts went to the Moon there were several main theories about how
the Moon formed. One of the goals of the Apollo missions was to try and
choose which theory fit best, but in the end, a completely new theory
was born. Before we look at all the different theories, we need to look
at some of the differences and similarities between the Earth and Moon:
Earth has a large iron core, but the Moon does not.
has an average density of 5.5 g/cm3 and the Moon
has an average
density of 3.3 g/cm3 (because it is depleted in
Moon has exactly the same oxygen isotope composition as the Earth,
whereas Mars rocks, and meteorites from other parts of the Solar System
(e.g. the asteroid belt) have much different isotope compositions.
original Moon-forming theories, and why they can’t be what
really happened, are as follows:
is about four times larger than the Moon
capture theory says that the Earth and Moon formed in two very
different parts of the Solar System but that at some point the Earth
‘captured’ the Moon with its gravity as it passed
the Earth. This can’t have happened as the oxygen isotopes of
both the Earth and the Moon showed they formed in pretty much exactly
the same place in the Solar System.
theory says that the Earth and Moon formed (accreted) in the same part
of the Solar System from similar building blocks, and that the Moon is
a sister planet to the Earth. This theory failed because it
explain why the Moon is depleted in iron compared to the Earth.
describes the somewhat crazy idea that the young Earth was spinning so
fast that it spun a lump of material off that went on to form the Moon.
This theory fails because it defies the laws of physics!
already heard about the Giant Impact theory, which is the current
favoured formation mechanism for the Moon. This theory accounts for
both the similarities and the differences between the Earth and
Moon’s composition. It assumes that the Earth’s
already drained into the core by the time the impact happened, and that
the impactor also contained an iron core. The majority of debris thrown
out by the giant impact came from their rocky mantles, and the core of
the impactor melted on impact and merged with the iron core of the
There is one thing this theory
however, and that is why only one Moon formed, or why we
see evidence for this happening elsewhere in the Solar System!
of the Moon's features >