Thursday, April 5, 2012

Titanium Testing Reveals Moon Is Earth’s Baby

Even though it has been some four decades since the Apollo mission returned to Earth with their precious cargo of lunar soil and rock samples, these old samples are still teaching us new things. According to a recent study of Moon matter, the chemical signature blows away the common theory that the Moon was a product of a collision. According to the giant impactor scenario, the Moon may have formed when Earth and a hypothetical Mars-sized body referred to as “Theia” collided some 4.5 billion years ago. However, graduate student, Junjun Zhang of the University of Chicago, may prove otherwise. By analyzing the titanium content of lunar samples, Earth and meteorites, she speculates the Moon’s composition comes solely from Earth.

What makes her so confident? Then speculate on what happens when you combine two ingredients. “Just like in humans, the Moon would have inherited some of the material from the Earth and some of the material from the impactor, approximately half and half,” said Nicolas Dauphas, associate professor in geophysical sciences at University of Chicago, and co-author of the study which was published in Nature Geoscience. “What we found is that the child does not look any different compared to the Earth,” Dauphas said. “It’s a child with only one parent, as far as we can tell.”

Their work focused on the minute subatomic variations of titanium isotopes. The research team chose titanium for its refractory properties – properties that allow it to remain in a solid or liquid state instead of becoming a gas when encountering extreme heat. Because titanium isotopes don’t dissipate, this makes it unlikely they would be present in both Earth and Moon samples in equal amounts. Another signature of titanium occurs during supernovae. These differing isotopic fingerprints could have been absorbed in various ways, leaving clues which allow scientists to determine where the material originated.

“When we look at different bodies, different asteroids, there are different isotopic signatures. It’s like their different DNAs,” Dauphas said. Meteorites, which are pieces of asteroids that have fallen to Earth, contain large variations in titanium isotopes. Measurements of terrestrial and lunar samples show that “the Moon has a strictly identical isotopic composition to the Earth,” he said.
“We thought that the Moon had two parents, but when we look at the composition of the Moon, it looks like it has only one parent,” Zhang said.

During Zhang’s initial investigation of both terrestrial and lunar material, she found variations in the titanium isotopic composition – variations which were adjusted to account for the effect of cosmic rays. This is an important consideration since Earth’s magnetic field prevents contamination, while the Moon takes it full force. “We compared the titanium isotopic composition with samarium and gadolinium since those two systems are very sensitive to the cosmic-ray effect,” Zhang said. The only compositional differences the scientists expected to see in samarium and gandolinium between Earth and Moon would be the result of cosmic rays. “We found a very nice linear correlation between titanium and samarium or gadolinium.”

Zhang’s findings give credence to previous studies done with both the Earth’s and Moon’s oxygen isotopes. They are also less refractory and prone to elevating to a gaseous state during an impact. To clarify, titanium is refractory and it might have reached a gaseous state during a major impact and became part of the signature materials which eventually coalesced into our Moon. In this scenario, it may have eradicated the presence of Theia’s titanium, validating the University of Chicago team’s findings. However, there remains the issue that the proto-planetary disk could have been absorbed by Earth if too much material was swapped between the two forming bodies.

No matter which lunar origin theory you choose, each one has distinct problems. The fission theory leaves much to be desired because a concentrated mass couldn’t reach a high enough rotational speed to split. The collision theory of an impact with an icy body sans titanium doesn’t work either. There simply isn’t any pure ice bodies in the solar system that we know of. “They would always have a significant fraction of solid material, so you would still expect the object to deliver some titanium,” Dauphas said. Sure, it’s also possible that Theia had the same composition as Earth – but it’s highly unlikely. Just think of all the different materials Mother Earth collected from small impacts during its formation!

“We thought we knew what the Moon was made of and how it formed, but even 40 years after Apollo, there is still a lot of science to do with those samples that are in curatorial facilities at NASA,” Dauphas said.

Written by Tammy Plotner. About the image: Nicolas Dauphas, UChicago associate professor in geophysical sciences, holds vials of material collected from the Moon during the Apollo 14 mission. He and graduate student Junjun Zhang also worked with samples from the Apollo 15, 16 and 17 lunar missions in their new study on the origin of the Moon. Photo by Lloyd DeGrane

Original Story Source: - University of Chicago Press Release. For Further Reading: - The proto-Earth as a significant source of lunar material.

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