Giant Impact Theory – Moon Was Formed 4.5 Billion Years Ago After Theia Collided With Earth

by Sankalan Baidya
The Giant Impact Theory of Moon's Origin

Did you ever wonder exactly how our moon was created? If you have actually come across this question then you must have heard of different theories and the Giant Impact theory must have been one of the theories. However, did you know that this theory of fiery planetary fender bender is very new (relatively speaking)? Interestingly however, the Giant Impact theory has taken a big leap in the recent years, overshadowing the previous theories that attempted to explain how moon was formed.

Old Theories of Moon’s Origin

Before we get deep into the Giant Impact theory, let us take a quick look at the alternative theories that were put forward to explain moon’s origin.

The Spun Off Matter

This theory came from George Darwin. He was the son of famous Charles Darwin. According to George, in its early days, the Earth lost a chunk of its matter. It is this matter that George proposed, actually spun off and formed our beautiful and mystical moon. This theory actually came during the late 1800s. So, it is pretty old.

Earth’s Gravity Captured the Moon

This is another theory that originated around the same time as the previous one. According to this theory, it (moon) was created separately and was hanging around somewhere in the solar system. Luckily, it managed to get close enough to earth when earth’s gravitational tug overpowered the celestial object and robbed it of its freedom and jailed it forever as Earth’s own satellite. Unfortunately, this theory never says how the moon was actually formed elsewhere.

Moon Originated in the Nebula

This theory was pretty interesting. It said that there was a nebula from which the whole of the solar system originated. According to this theory, the moon and the earth are both love child of the same material cloud inside the nebula from where the solar system came into existence. This was indeed a pretty good theory because it manages to explain how both earth and moon have the exact same oxygen composition.

The Giant Impact Theory of Moon’s Origin

This theory popped up somewhere in 1970s – towards the middle of the decade. Once this theory was proposed, it stole the limelight and became the most popular theory explaining the origin of moon. This theory that goes by the name Giant Impact Theory states that about 4.5 billion years ago, a smaller planet-sized meteorite, about the size of our current neighbor Mars, collided with Earth. The next thing that happened because of this massive celestial accident is that both the planets lost some matter which shot off in free space as debris of partially molten rock. This debris was caught up within Earth’s gravitational tug but its distance from our home orb didn’t allow it to fall back on Earth and neither could is escape the gravity. Eventually over the 4.5 billion years of time frame, the molten debris coalesced and created today’s enigmatic moon.

This smaller planet that collided with Earth has been called by the scientific community as the Theia. Theia is actually a deity in Greek Mythology who gave birth to Moon Goddess.

Problems With Giant Impact Theory

There is absolutely no doubt that the Giant Impact Theory is indeed fascinating but at least two problems with this theory were identified. They are:

  1. The composition of our Moon has a very close resemblance with our planet Earth. Scientists have not found a different composition that can go on to establish that there was indeed a different planet in action.
  2. Isotopic fractionation is absent in moon rocks that have been studied. Isotopic fractionation is actually a biochemical process which invariably shows up along side the massive evaporation that takes place during any massive collision.

Yet another major problem was that computer simulations of the Giant Impact suggested that majority of the moon should have been made of material from Theia. Earlier analysis of lunar samples gave exactly the opposite results, i.e. the moon was made of the same material that made up Earth. This was pretty baffling for scientists.

New Testings to Validate Giant Impact Theory

Scientists decided to go for another round of testing and this time, they carried out these testings more sensitively. The testings revealed that the moon and the Earth indeed have some key differences.

The new testings focused on ratio of Oxygen-16 and Oxygen-17. Now, Oxygen-16 and Oxygen-17 are two different isotopes of the same element – Oxygen. Isotopes differ in terms of number of neutrons present in an atom of an element. In case of Oxygen-16, there are 8 neutrons and in case of Oxygen-17, there are 9 neutrons. It is a well known fact that two different planets can have the same element but different isotopic composition for the same element.

What the scientists did is that they examined lunar rocks that were brought back from the moon by Apollo Missions instead of looking at lunar rocks that landed on our planet as lunar meteorites. The problem with lunar meteorites is that when they reach Earth, they end up collecting oxygen isotopes from water present on Earth and thus, the lunar samples get contaminated.

These tests were conducted by Daniel Herwartz (from University of Göttingen, Germany) and a few of his colleagues. Herwartz, who is an isotope geochemist, found, along with his other research colleagues that the rocks from moon have higher Oxygen-17 isotope than rocks of Earth which are high on Oxygen-16 isotopes. Though in absolute terms it is not a big difference, the difference is quite significant.

According to Herwartz, the small differences are quite difficult to detect but they are present. Based on the new findings, Herwartz said that not only the scientific community now has a fair idea of Theia’s composition but the new data also points to the fact that 40% of the moon is made up of matter from Theia. Based on the data, Herwartz and his team believe that Theia was similar to enstatite chondrites – a class of meteorites with very Earth-like isotopic composition. According to Herwartz, since Theia’s isotopic composition was similar to that of Earth, earlier studies failed to detect differences in oxygen isotopes or any other element’s isotopes between moon and Earth.

Herwartz said that there may be future studies which will look for differences in isotopes of other elements like titanium in order to find out the composition difference between moon and Earth. He also stated that the data he and his colleagues found may also lead to new findings that carbonaceous chondrites (another class of meteorites that are low in Oxygen-17 isotopes) created a coating of material on Earth after the moon was formed, which could also possibly explain why Earth has low Oxygen-17 level compared to moon rocks.

In order to further confirm Herwartz’s and his teams findings, more lunar samples need to be analyzed and this will require collection of rock samples from underneath the lunar surface to ensure that external factors such as space debris or solar winds that lead to weathering or lunar surface can be ruled out. Getting access to those rocks will mean manned missions to the moon. Several space programs like ISRO (Indian Space Research Organisation), ESA (European Space Agency), ISA (Iranian Space Agency), Roscosmos (Russian Federal Space Agency), CNSA (China National Space Administration) and JAXA (Japan Aerospace eXploration Agency) have proposed sending manned missions to moon but none of them have received any funding so far. Until such studies take place, Herwartz’s finding validates the Giant Impact Theory of formation of moon.

Sources: 1, 2, 3

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