UCSD Finds New Information About Formation of Moon

Scripps geochemist James Day and his colleagues, Randal Paniello and Frédéric Moynier of Washington University in St. Louis, studied the composition of volatile elements (elements that evaporate very easily) found in moon samples in hopes of discovering more clues behind the moon’s history.

“We wanted to know how the moon formed,” Day said. “The moon is really important —it’s one of the best examined objects out there but we still don’t know how it was formed.”

To answer this question the scientists decided that they would focus on zinc, what Day refers to as a “motherly, all-powerful element,” and collected 20 rock samples and one lunar meteorite from the Houston Johnson Space Center. These samples included three isotopes, or forms of an element with different masses, of zinc: zinc-60, 64 and 68.w Using a mass spectrometer — an instrument which measures the mass to charge ratio of charged particles – the scientists separated the zinc isotopes after they digested the moon rocks in acids and purified them in zinc. Then they compared the moon’s zinc composition to that of the earth.

“Think of it as, imagine you’re evaporating some water, and let’s say that water contains beef soup,” Day said. “If you over-boil it, what you’ll end up with is a gunge of soup and in the same way, if you measure that soup, that organic material in the soup is your heavy isotope – it’s rich, relative to the water.”

In the same way, evaporating zinc isotopes left only the heavier isotope, zinc-68. The comparison between moon rock and earth rock zinc composition showed that the moon lacks zinc. Day and his research team concluded that this occurred as a result of a single event that caused a large amount of zinc to be evaporated from the moon.

Prior to this discovery, a 1975 theory known as the giant impact theory, which stated that the earth and a planetary body known as Theia collided to form the moon, was never put under the zinc test.

Day and his colleagues discovered that a catastrophic collision probably caused vapor from a collision, similar to the one described in the GIT, to escape before heavier zinc atoms could.

From this vapor, the moon was created.

Day’s findings open the path to further questions about the moon that will inevitably result in future discoveries.

“[This research] is important because it shows the moon is volatile and doesn’t have much water or zinc, which are important parts of our own planet,” Day said. “We have oceans and atmospheres, and the moon and earth are so different but so close to each other, why? That’ll probably be our next line of inquiry.”

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