Inner Moon as Wet as Earth


Contrary to popular belief, the early moon could have been as wet as Earth’s mantle, new analysis from an Apollo lunar sample shows.

The discovery stems from sophisticated analysis of tiny bits of ancient magma sealed inside solid crystals. The so-called “melt inclusions” are no bigger than the width of a human hair.

They were found serendipitously by a Brown University freshman tasked by his teacher to look for anything unusual in a thimbleful of material brought back to Earth during the last Apollo moon mission in 1972.

“When he found it, I just couldn’t believe it. He said to me, ‘Why didn’t you tell me about this?’ I never even explained to him about melt inclusions because I thought that in the sample he was looking at there was very little chance that he would find it. And he found it,” Alberto Saal, associate professor of geological sciences at Brown, told Discovery News.

Analysis of material inside seven tiny glass beads revealed an even bigger surprise. If the samples are representative of the ancient interior moon, then it had as much water as Earth’s mantle, a startling find that challenges currently held theories about how the moon formed.

Scientists believe material that became the moon was blasted into space after a Mars-sized body smashed into baby Earth. However, computer models currently can’t account for how water would have survived that transformative experience.

“It puts serious constraints on the formation of the moon by a giant impact,” Saal said.

Perhaps the impact wasn’t as powerful as believed, or perhaps water molecules reformed after the impact, he suggests.

The research also calls into question the source of water found by NASA’s 2009 lunar impact mission LCROSS, which smashed into a permanently shadowed crater near the moon’s south pole, sending up a plume that was analyzed for signs of water.

Extrapolating from the data, scientists estimate the crater contains about 1 billion gallons of water.

If the rest of the moon was as flush with water as what the new sample studies show, then it had a billion times more than that, said geochemist Erik Hauri at the Carnegie Institution for Science.

“Even if only one percent of this water is released by volcanic eruption and trapped in the lunar surface, it’s still many millions of times more water than the amount of water that’s in that crater,” Hauri told Discovery News.

That amount of water makes it unlikely crashing comets and meteorites delivered the water to the moon, he added.

“The prevailing theory for ice at the lunar poles is that it came from degassing of comets and meteorites as they impacted the lunar surface,” he said.

A sample of the ice would resolve the question since the geochemical makeup and isotopic arrangement of molecules would be very different for comet-delivered water over the past few hundred million years and 3.5-billion-year-old water released during lunar volcanic explosions.

The work, published in this week’s Science, follows a 2008 study of lunar volcanic crystals that estimated water concentrations of 750 parts per million.

One part per million is about the equivalent of a drop of water in compact car’s 50-liter gas tank.

“We just found 1,200 parts per million, which is almost double what we thought it would be,” Saal said.

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