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When Moons Collide They Form One

| 4 Aug 2011 17:02
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Astronomers believe that Earth once had two moons and they collided to form the satellite we see today.

The moon has long been a source of fascination for druids, night elves and just about everyone else. Ancient humans attributed god-like - or goddess-like - qualities to the silver orb and modern man walking upon its surface still evokes patriotic feelings. The dark side of the moon has mystified both Pink Floyd and astronomers alike because once they saw pictures of it 1959, its high mountain peaks were very different from the low plains of volcanic rock that make up the hemisphere that faces the Earth. Scientists now believe that disparity derives from the fact that Earth once had two satellites and the pair of moons crashed together to form what we see in the sky today.

The study published in the August 4th issue of the scientific journal Nature by a collaboration between Erik Asphaug from the University of California, Santa Cruz and Martin Jutzi at the University of Bern in Switzerland reports that computer simulations reveal that the Moon would not only survive such an impact but it would account for the different makeup of both hemispheres.

But how would two moons suddenly collide in orbit without falling to Earth? Well, the answer to that question involves a concept called Trojan points. Imagine a equilateral triangle with the Earth and the Moon at two points. The third point a Trojan point and it is possible for a body to orbit the Earth in that position without crashing into either of its neighbors. For a while.

"It is entirely plausible for a Trojan moon to have formed in the giant impact [that created the Moon to begin with], and for it to go unstable after 10 million to 100 million years and leave its imprint on the moon," said Asphaug. He said to think of the impact as "a ball of Gruyere colliding into a ball of cheddar."

The second moon would have had to have been much smaller, about 750 miles in diameter and a mass about 4 percent of the Moon. Because the orbit decayed over time, the second moon would impact traveling relatively slowly - 4,500 to 6,700 miles per hour - and the result would essentially cram the rock and minerals of the second moon onto the Moon's surface like smashing two pieces of soap together to form one bar.

The theory of a second moon does has some detractors, even from Asphaug's own colleagues at UCSC. Francis Nimmo thinks that just gravity is powerful enough to create the highlands of the dark side of the moon. "As further spacecraft data and, hopefully, lunar samples are obtained, which of these two hypotheses is more nearly correct will become clear," Nimmo said in reply to Asphaug's research.

Sounds like an astronomy gang fight is in order. Someone close the observatory for the night and sharpen your telescopes. The only way to see which theory reigns supreme is to settle this like men. Two astronomers enter, one astronomer leaves! Two astronomers enter, one astronomer leaves!

Source: Nature via Space.com

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