Introduction
This is the first of the Moon Origin posts. This post
explains the Giant Impact Theory, which at the moment in scientific circles, is
the theory that it is most favoured. It is worth noting that this theory has
some holes, that this there aspects about the Moon that this theory can not
account for.
The following excerpt comes from the book “Modern Mysteries
of the Moon” by Vincent S. Foster.
Giant Impact Hypothesis (GIH)
“The collision of two
protoplanetary bodies during the early accretional period of Solar System
evolution is the most widely accepted explanation for the origin of the Moon.
This theory became popular in 1984. It satisfies the orbital conditions of
Earth and the Moon and can account for the relatively small metallic core of the
Moon. Collisions between planetesimals are now recognized to lead to the growth
of planetary bodies. In this framework it is inevitable that large impacts will
sometimes occur when the planets are nearly formed. The theory is thought to
have originated in the 1940s with Reginald Aldworth Daly, head of the geology department
at Harvard (Fig. 3.1 )
.
The colliding body is
called Theia , the mother of Selene , the Moon goddess in Greek mythology. The
hypothesis requires a collision between a body about 90 % the present size of
Earth, and another the diameter of Mars (half of the terrestrial radius and a tenth
of its mass). This size ratio is needed in order for the resulting system to
possess sufficient angular momentum to match the current orbital configuration.
Such an impact would have put enough material in orbit around Earth that could
accumulate and eventually form the Moon.
Computer simulations
show a need for a glancing blow, which would cause a portion of the colliding
body to form a long arm of material that then would shear off. The asymmetrical
shape of Earth following the collision would then cause this material to settle
into an orbit around the main mass. The energy produced by this collision would
have been impressive: trillions of tons of material would have been vaporized
and melted. The temperature would have risen to 10,000 °C (18,000 °F).
The relatively small
iron core of the Moon is explained by Theia ’s core accreting into Earth’s. The
lack of volatiles in the lunar samples is also explained in part by the energy
of the collision. The energy liberated during the re-accretion of material would
have been sufficient to melt a large portion of the Moon, leading to the generation
of a magma ocean.
The newly formed Moon
orbited 90 % closer than it does today. It became tidally locked with Earth,
where one side continually FIG. 3.1 The giant impact theory suggested that
growing smaller planetary body (Mars-sized protoplanet) hit Earth about 4.5
billion years ago, blowing out rocky debris that was captured into orbit around
Earth and coalesced into the Moon. NASA illustration faces toward Earth. The geology of the Moon
has since become more independent of Earth. Although this hypothesis explains many
aspects of the Earth-Moon system, there are still unresolved problems with it,
such as why the Moon’s volatile elements were not depleted as expected from
such an energetic impact.
Comparisons of lunar
and Earth isotopes is another issue. In 2011, the most precise measurement yet
of the isotopic signatures of lunar rocks was published. Surprisingly, it
showed that the Apollo lunar samples carried an isotopic signature identical to
Earth rocks, but different from other Solar System bodies. Since most of the
material that went into orbit to form the Moon was thought to come from Theia ,
this observation was unexpected. In 2007, researchers from Caltech found that
the likelihood of Theia having an identical isotopic signature to Earth was
very small (<1 %).
An analysis of
titanium isotopes in Apollo lunar samples in 2012 revealed that the Moon has
the same composition as Earth, which conflicts with the Moon forming far from
Earth’s orbit. To help explain problems with this, a new theory was published
by R. M. Canup in late 2012 which posits that two bodies five times the size of
Mars collided, then re-collided, forming a large disc of debris that eventually
formed Earth and the Moon.
Released at the same
time was another study on the depletion of zinc isotopes on the Moon, which
supported the giant impact origin for Earth and the Moon. In 2013, a study was
released indicating that water in lunar magma was ‘indistinguishable’ from
carbonaceous chondrites and nearly the same as Earth’s, based on the composition
of isotopes. Another challenge was issued September 2013, with a growing sense
that lunar origins are much more complicated than can be fully explained by the
GIH theory.”
End (65).
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