Scientists investigating the source of the moon's internal water have lifted telltale fingerprints from Apollo-era lunar rocks showing that the water may have originated from meteorites bombarding Earth.
The surprisingly "wet" volcanic rock, described online Thursday in the journal Science, contradicts the theory that lunar water first came from comets. It also throws a wrench into the commonly held story about the moon's origin.
"It is as if you made the moon by just plucking a piece of the Earth and putting it in orbit," said David Stevenson, a planetary scientist at Caltech who was not involved in the new research.
Until very recently, scientists believed the moon was bone dry, said study leader Alberto Saal, a geochemist at Brown University. Established theories of how the moon came to be seemed to back it up.
Planetary scientists suspect that a Mars-sized body collided with Earth about 4.5 billion years ago, when our planet was still being formed. The collision knocked loose molten debris that coalesced to form the moon, but after any water in that debris had escaped into space, scientists thought.
And yet, a 2008 study by Saal and his colleagues found what he called "unequivocal evidence" for water in lunar magmas. In 2011, his team reported that lavas on the moon had once held essentially as much water as some lavas on Earth.
The latest study was an effort to answer two questions: From where did the water come, and when?
Recent work had suggested that the moon's water was delivered by comets. But Saal was skeptical ? he suspected that the isotopic fingerprint in those samples had been warped as the lava traveled through the moon's crust and slowly cooled on the surface, allowing most volatile molecules to escape. Millions of years of cosmic-ray bombardment would have further altered the samples, he said.
So Saal turned to rocks that were probably ejected from the moon's interior in a violent lava eruption; if so, they would have cooled so quickly afterward that water and other volatile compounds would have been trapped inside the rock, forming bits of glass.
These rocks, brought back to Earth by the Apollo 15 and 17 missions, had been deemed pristine by a barrage of chemical tests.
Saal and his colleagues studied tiny glass bubbles trapped between crystals of olivine ? drops of magma whose water hadn't been able to escape when the rock crystallized around them, sealing them in. These bits of trapped magma could have contained as much as 1,200 parts per million of water, the highest yet seen in a primitive lunar lava, Saal said.
Then they looked at the ratio of hydrogen to its heavier isotope, deuterium, in the moon rock samples. Planets, comets and asteroids all have distinct, individual isotopic fingerprints that reflect their proximity to the sun's warming rays and other environmental conditions.
It turned out that the deuterium-to-hydrogen ratios in the lunar lava did not match the comets that were the suspected source of water. Instead, the ratio synced up to primitive meteorites known as carbonaceous chondrites, which originate between the orbits of Jupiter and Mars.
What's more, the moon's isotopic fingerprint was a very close match with Earth's.
The simplest explanation, Saal said, is that meteorites brought the water to Earth while it was still forming, and that the water somehow remained even after chunks of that proto-Earth were flying into orbit.
Of course, even if the moon had its own internal source of water ? a parting gift from Earth after a violent separation ? additional water could have been brought to the lunar surface from icy comets that bombarded the moon later.
"I'm inclined to think that the measurements, and the primary interpretation, make sense," Stevenson said. But, he added, "this is a piece of the puzzle rather than a solution to the puzzle."
Saal's proposal raises as many questions as it attempts to answer. Among them: How could the debris fragments from Earth hold on to all that water before coalescing to form the moon? And shouldn't there be evidence of the third body that smashed into Earth?
"We're always counting on science to tell us where we came from," said David Paige, a UCLA planetary scientist who was not involved in the study. "But the exact set of processes that led to our origin is a tough problem to crack."
amina.khan@latimes.com
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