New Theories Arise to Explain Creation of the Moon
October 20, 2012 11:03 AM
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The Giant Impact Theory
Two new studies attempt to prove the Giant Impact Theory in different ways
Two different studies say they've found evidence supporting the theory that
was created from a collision between Earth and a Mars-sized body.
The Giant Impact Theory, which was proposed back in 1975, suggests that the early Earth and a Mars-sized planet called Theia collided with one another. This completely obliterated Theia, and its composition created a ring around Earth and eventually came together to create the moon.
While scientists have been able to explain how this event occurred, one major thing didn't add up: the composition of the moon.
Scientists long believed that for this theory to be true, the moon would have to be composed mainly of Theia's elements. Moon rocks from that were brought to Earth were studied, and surprisingly, they had the same types and amounts of elements that the Earth had, including titanium, silicon and oxygen. This didn't make sense, and has left the theory wide open for the last 30+ years.
However, scientists from Washington University in St. Louis managed to measure a small excess of a heavier variant of zinc in moon rocks from that were brought to Earth in the 1970s. They believe the excess is due to
heavier zinc atoms
condensing out of the collision's cloud faster than the lighter zinc atoms, and the vapor that remained escaped before it condensed.
This bit of sorting by mass is called isotopic fractionation, and it's what scientists have been looking for all along. This shows that the moon rocks were depleted of easily evaporated elements called volatiles, and a large collision could explain this depletion while other theories can't.
"The magnitude of the fractionation we measured in lunar rocks is 10 times larger than what we see in terrestrial and Martian rocks," said Frédéric Moynier, PhD, from Washington University in St. Louis. "So it's an important difference."
But the Washington University team isn't the only one to bring new evidence to the table. Robin Canup, a planetary scientist from the Southwest Research Institute Colorado, used Harvard scientists' findings to create a theory of her own related to the collision.
According to the Harvard team, which used computer simulations to create its theory, ancient Earth had to have been spinning too fast for today's 24-hour rotation. They suggested that early Earth and a body half the size of Mars could have collided where both were obliterated and combined elements to create both
the moon and Earth's
heavy iron core/lighter rock layers.
At a later point, Earth's rotation could have slowed due to the moon and sun aligning in a way that changed Earth's orbit.
Canup used the idea that Earth's rotation was slowed and came up with the theory that two bodies similar in size collided at a slow speed, and their materials merged to create the Earth and moon.
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10/20/2012 10:04:06 PM
I wish I still had it, but my grandfather (retired librarian) had a book from the late 1800's about astronomy. In it, it said that they once thought the moon was formed from a collision between two objects a long time ago (as we now believe); but that that theory had been discarded as nonsense and the earth instead trapped the moon by its gravity.
I love how the state of 'truth' changes over time!
(thats a good thing, btw, changing ones opinion based on new evidence/facts)
10/21/2012 12:41:08 PM
That would be awesome to have. The big difference between now and then (and why I'd say our claims today are much stronger) is that we have evidence of the composition of each body, whose similarity is best explained by the giant impact hypothesis.
10/22/2012 8:52:56 AM
The "truth" doesn't change. All we have are hypothetical scenarios that could lead to the formation of the moon we have. The trick is finding one which fits all the observations and known composition of the moon so it graduate a little bit closer to "truth".
So the article above is a bit of a mess. We've known about the broad strokes for many decades - either we captured the moon or it formed in orbit from Earth debris, i.e. most likely from a collision with another planet. Capture seems improbable so collision is not the frontrunner. The new information provides a scenario where we end up with a moon closer to the composition we expect - that's why it's big news.
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