Scientists find clues to moon's creation
Some 4.5 billion years ago, scientists believe the moon formed following a violent collision between Earth and another object about the size of Mars.
That, theory known as the Giant Impact Hypothesis, held up for decades, based on the size of the moon and its orbit around Earth. But doubts were raised because of confounding similarities between the chemical compositions of the two bodies.
Now, several papers in Nature this week have helped bolster the long-held theory around the collision.
The authors of one study suggest that their results may explain the compositional similarities between the Earth and the Moon - and why their compositions differ from those of other planets in the solar system.
Simulating collisions between protoplanets, Alessandra Mastrobuono-Battisti and her colleagues concluded that there was a 20 percent to 40 percent chance that the collision that produced the moon came from planets with similar compositions of chromium, silicon, titanium and tungsten. That is about 10 times higher than previous estimates.
Two other papers focused on the aftermath of a collision. Comparing rocks on Earth and the moon, the two teams of researchers looked at tungsten. They found that Earth had much less of the isotope tungsten-182, which would have resulted from the debris and dust left over from collision.
"The small, but significant, difference in the tungsten isotopic composition between Earth and the moon perfectly corresponds to the different amounts of material gathered by Earth and the moon post-impact," said Richard Walker, a professor of geology at the University of Maryland and a co-author of one study. "This means that right after the moon formed, it had exactly the same isotopic composition as Earth's mantle."
This finding supports the idea that the mass of material created by the impact, which later formed the moon, must have mixed together thoroughly before the moon formed and cooled. The collision would have melted the earth and the debris cloud that formed as result of the impact would have exchanged atoms with Earth.
"The impact would have melted a good portion of Earth," Walker said. "It was a vigorous event. It wouldn't have been a good day to be around."
This would explain the overall similarities in isotopic fingerprints in the early days. But as more meteoric material bombarded the Earth compared to the the moon, a slight difference in the proportion tungsten-182 emerged that was higher on the moon.
And though his paper doesn't focus specifically on the impact, Walker said the jury is still out on whether the object that collided with earth was similar or different. One way to determine that would be to test materials from Venus, although this is unlikely to happen any time soon.
"This result brings us one step closer to understanding the close familial relationship between Earth and the moon," Walker said. "We still need to work out the details, but it's clear that our early solar system was a very violent place."