Moon Formation - Evidence Backs Up Giant Impact Theory
October 17, 2012

Scientists Claim Moon Was Result Of Earth Collision With Planetary Body

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Lee Rannals for - Your Universe Online

Scientists are backing up a theory, claiming that the Moon was created when a planetary body the size of Mars collided with Earth.

Planetary scientists found evidence that backs up the Giant Impact Theory, which was first proposed at a conference in 1975.

The Giant Impact Theory says that Earth´s moon was created in an apocalyptic collision between a planetary body called Theia and the early Earth.

According to the theory, the planet smashed up against Earth, releasing so much energy that it melted and vaporized Theia and much of the proto-Earth´s mantle.

The Moon then condensed out of the cloud of rock vapor, some of which also re-accreted to the Earth, according to the Giant Impact Theory.

Several papers have been published in the journals Nature and Science this week that provides new insight into this decades old theory.

One challenge to the theory has been that the Earth and Moon have identical oxygen isotope compositions, even though models indicated they should differ substantially.

A new model by Southwest Research Institute (SwRI) accounts for this similarity in composition, while also yielding an appropriate mass for the Earth and the Moon.

Earlier models found that most of the disk material that helped formed the Moon would have originated from the Mars-sized impacting body. However, the new models are a little different.

The models developed by Dr. Robin M. Canup, an associate vice president in the SwRI Space Science and Engineering Division, involve much larger impactors than what were previously considered.

In the new simulations, both the impactor and the target are of comparable mass, each containing 4 to 5 times the mass of Mars. The near symmetry of the collision causes the disk's composition to be extremely similar to that of the final planet's mantel over a relatively broad range of impact angels and speeds, consistent with the Earth-Moon compositional similarities.

The new impacts would produce an Earth that rotates 2 to 2.5 times faster than the current angular momentum of the Earth-Moon system. However, another paper published in Science by Dr. Matija Ćuk of the SETI Institute and Dr. Sarah T. Stewart of Harvard University, found a solution.

These researchers show that a resonant interaction between the early Moon and the Sun could have decreased the angular momentum of the Earth-Moon system by this amount after the Moon-forming impact.

"By allowing for a much higher initial angular momentum for the Earth-Moon system, the Ćuk and Stewart work allows for impacts that for the first time can directly produce an appropriately massive disk with a composition equal to that of the planet's mantle," Canup said.

She says the ultimate likelihood of each impact scenario will need to be assessed by improved models of terrestrial planet formation, as well as a better understanding of the conditions required for the evection resonance mechanism.

Another research team looked at lunar rocks to prove the Giant Impact Theory, rather than just use a model.

Moon rocks brought back to Earth show that they are very poor in sodium, potassium, zinc and lead, said Frédéric Moynier of Washington University in St. Louis.

“But if the rocks were depleted in volatiles because they had been vaporized during a giant impact, we should also have seen isotopic fractionation,” Moynier said. Isotopes are variants of an element that have slightly different masses.

Moynier said that when a rock is melted and then evaporated, the light isotopes enter the vapor phase faster than the heavy isotopes, leaving a vapor enriched in the light isotopes and a solid residue enriched in the heavier isotopes.

“If you lose the vapor, the residue will be enriched in the heavy isotopes compared to the starting material,” explained Moynier.

The scientists who originally analyzed the first moon rocks were unable to find isotopic fractionation, but this didn´t stop Moynier from believing the Giant Impact Theory to be true.

“When you find something that is new and that has important ramifications, you want to be sure you haven´t gotten anything wrong,” he said. “I half expected results like those previously obtained for moderately volatile elements, so when we got something so different, we reproduced everything from scratch to make sure there were no mistakes because some of the procedures in the lab could conceivably fractionate the isotopes.”

The team analyzed 20 samples of lunar rocks, including ones brought back from the Apollo 11, Apollo 12, Apollo 15 and Apollo 17 missions. They also analyzed 10 Martian meteorites for comparison.

They found that the lunar rocks have much lower concentrations of zinc, but are enriched in the heavy isotopes of zinc. Earth and Mars have isotopic compositions like those of chondritic meteorites, which are thought to represent the original composition of the cloud of gas that helped form the solar system.

The isotopic homogeneity of the lunar materials suggests that isotopic fractionation resulted from a large-scale process rather than one that operated just locally.

The team said the most likely large-scale event is wholesale melting during the formation of the Moon. The zinc isotopic data supports the theory that a giant impact gave rise to the Earth-Moon system.

“The work also has implications for the origin of the Earth,” Moynier says, “because the origin of the Moon was a big part of the origin of the Earth.”

Without the moon, planetary scientists believe the Earth would spin more rapidly, making days shorter, weather more violent, and climate more chaotic, which they believe would have also resulted in humans not coming to existence.