ASTROGEOLOGY 8 (ADVANCED LEVEL)

The origin of the moon

            The following are some of the most prominent hypotheses that have been put up to explain the origin of the moon;

1)      The Fission hypothesis:

This was proposed by George Darwin (son of the famous biologist Charles Darwin) in the 1800s and retained some popularity until Apollo.  This is the idea that an ancient, rapidly spinning Earth expelled a piece of its mass.  The Austrian Geologist Otto Ampherer in 1925 also suggested the emerging of the moon as cause for continental drift.  It was proposed that the Pacific Ocean represented the scar of this event. 

The only problem is that there is absolutely no known method by which this can occur. If the Earth ever spun fast enough to through off a fragment, it would have completely fallen apart. Even mathematicians couldn't come up with convincing arguments to support the fission idea.

However, today it is known that the oceanic crust that makes up this ocean basin is relatively young, about 200 million years old and less, whereas the Moon is much older since it does not consist of oceanic crust but instead of mantle-material, which originated inside the proto-earth in Precambrian. 

Nevertheless, the assumption that the Pacific is not the result of lunar creation does not disprove the fission hypothesis.  This hypothesis also cannot account for the angular momentum of the Earth-Moon system.

2)      The capture hypothesis; 

This hypothesis states that the Moon was captured by the Earth.  This was popular until the 1980s, and some things in favour of this model include the Moon’s size, orbit, and tidal locking.

One problem is understanding the capture mechanism.  A close encounter with Earth typically results in either collision or altered trajectories. For this hypothesis to function, there might have been a large atmosphere ex-tended around the primitive Earth, which would be able to slow the movement of the Moon before it could escape.

In addition, this hypothesis has difficulty explaining the essentially identical oxygen isotope ratios of the two worlds. 

3)      The hypothesis of accretion (or the twin formation hypothesis):

The hypothesis of accretion suggests that the Earth and the Moon formed together as a double system from the primordial accretion disk of the Solar System.  The problem with this hypothesis is that it does not explain the angular momentum of the Earth-Moon system or why the Moon has a relatively small iron core compared to the Earth (25% of its radius compared to 50% for the Earth).

4)      The Giant Impact hypothesis:

This is the most widely accepted explanation for the origin of the Moon.  It involves a collision of two proto-planetary bodies during the early accretional period of Solar System evolution.  This “giant impact hypothesis”, which became popular in 1984, satisfies the orbital conditions of the Earth and Moon and can account for the relatively small metallic core of the Moon. 

It is thought to have originated in the 1940s with Reginald Aldworth Daly, a Canadian professor at Harvard.  The hypothesis requires a collision between a body about 90% the present size of the Earth, and another the diameter of Mars (half of the terrestrial radius and a tenth of its mass).  The colliding body has sometimes been referred to as Theia, the mother of Selene, the Moon goddess in Greek mythology.

This size ratio is needed in order for the resulting system to possess sufficient angular momentum to match the current orbital statistics.  Such an impact would have put enough material into orbit about the Earth to have eventually accumulated to form the Moon. 

The newly formed moon orbited at about one-tenth the distance that it does today, and became tidally locked with the Earth, where one side continually faces toward the Earth.  The geology of the Moon has since been more independent of the Earth.  While this hypothesis explains many aspects of the Earth-Moon system, there are still a few unresolved problems facing it, such as the Moon’s volatile elements not being as depleted as expected from such an energetic impact.

Another issue is Lunar and Earth isotope comparisons.  In 2001, the most precise measurement yet of the isotopic signatures of lunar rocks was published.  Surprisingly, the Apollo lunar samples carried an isotopic signature identical to Earth rocks, but different from other Solar system bodies.  Other isotopic comparisons (like the 2012 study on the  rate of depletion of zinc isotopes) also support the above hypothesis.

A new theory was published in late 2012 which explained that two bodies five-times the size of Mars collided, then re-collided, forming a large disc of debris that eventually formed the Earth and Moon.

The paper was called “Forming a Moon with an Earth-like composition via a Giant Impact,” by R.M Canup.

In 2013, a study was released that indicated water in lunar magma was 'indistinguishable' from carbonaceous chondrites and nearly the same as Earth’s, based on the composition of isotopes.

In 2011, it was theorized that a second moon existed 4.5 billion years ago, and later had an impact with the Moon, as a part of the accretion process in the formation of the Moon.

One hypothesis, presented only as a possibility, was that the Earth took the Moon from Venus.