Moon and Earth May Share a Watery Past
Tiny beads of green glass from the moon may hold the answers to the Earth's watery past. Photo by NASA.
It took three years for Alberto Saal of Brown University, Erik Hauri of the Carnegie Institution for Science, and James Van Orman of Case Western Reserve University -- all friends since graduate school -- to secure a thimble-sized sample of soil from the moon for analysis.
The precious lunar sample, which was collected by astronauts during the Apollo 15 and 17 missions, is kept in a locked case at the Carnegie Institution for Science in Washington, D.C. It is comprised of minerals, rocks, and greenish-tinted beads of volcanic glass. All of the beads combined could fit on a pinky nail.
It's within these tiny glass beads that Hauri and his colleagues have found signs of water. They believe that water originated on the planet Earth, and was transferred to the moon during a massive collision 4.5 billion years ago.
"We kept checking and rechecking," said Saal, a geochemist, on finding evidence of water in 2008. "We were afraid there was contamination."
Using tweezers and a microscope, they picked out tiny bits of green glass from the lunar dust, and then analyzed them for water. And they found water molecules-- 20 to 30 grams of water per million grams of volcanic glass -- melted into the beads.
Before 2008, many scientists believed the moon was a dry, arid rock. Then in 2009, NASA's LCROSS mission slammed a booster rocket traveling nearly 6,000 miles per hour into the moon, blasted out a hole and found ice on the surface. But no one knew where any of the moon's water came from.
It's thought that the moon formed when a Mars-sized asteroid crashed into the earth 4.5 billion years ago, blowing hot magma into space. That hot magma coalesced to form the moon. When the lunar rocks from the Apollo 11 mission appeared totally dry, scientists assumed that the heat from the impact caused any water to evaporate and release gas into space, Hauri said. But his team found that tiny inclusions of magma trapped inside volcanic crystals had as much water as magmas from Earth. That finding was published in a 2011 paper in the journal Science.
"At that point, people began to come to grips with the fact that the lunar interior had as much water as the Earth's mantle does today. So next was to say, 'Okay, what's the isotopic composition of this water? We can use that to trace the source of water to the moon,'" Hauri said.
Water molecules from Earth contain a specific proportion of hydrogen isotopes (with one proton) to deuterium isotopes (a proton plus a neutron). Each object in the solar system, like a comet or an asteroid, has characteristic proportions of hydrogen and deuterium. It's kind of like looking at an atom's DNA, Hauri explained.
That atomic DNA was analyzed in 2011 by James Greenwood, a geochemist at Wesleyan University, who studied lunar minerals called apatites. He concluded that based on the isotopic ratio, the interior water of the moon must have come from comets colliding into the moon.
Erik Hauri, left, and Alberto Saal in the NanoSims lab where they have been studying lunar samples for signs of water.
But Hauri, Saal and Van Orman's research showed that the isotopes found in the lunar water were much closer to the atomic DNA of water found on Earth, which, they say, rules out the comet theory. Simply, Earth was a wet planet at the time of the impact, and the water survived the moon's formation, Saal said.
"They're pretty close, but there is a small difference that is probably due to volcanic release of hydrogen vapor. We're both blessed and cursed that we can measure these isotope ratios very precisely," Hauri said.
Their research concludes that the moon and the Earth got their water from the same source, and that the interior water (water trapped in the rocks on the moon) originated from the Earth at the time of the moon's formation. The findings were published online in the journal Science last week.
But Greenwood said it's not a close enough match to conclusively prove the water is terrestrial. While his data was similar, he maintains that the water arrived later and probably from a comet.
"They are arguing that they have the same source of water and the source survived the impact event without changing the isotope signature," Greenwood said. "It's still similar to cometary water."
And given the isotopic ranges we currently have for comets, they're pretty close to Earth's water, he added. "If comets have a deuterium to hydrogen ratio the same as the Earth's oceans, then comets could have delivered all the water."
Saal sticks to his findings, saying that the volcanic glass samples are more representative of the primitive lunar deuterium-hydrogen ratio. The isotopic ratios are so close that water must have come from the Earth, he says, and the authors maintain that hydrogen gas release during volcanic eruptions can explain Greenwood's data.
"Most of what we call terrestrial planets have been believed to form without water, and the water came later," Saal said. "The moon is telling us that the water that came to Earth was here from the beginning."
Peter Schultz, a lunar scientist from the LCROSS mission says it's still not a consensus, and that there is much more work to be done. Scientists only have data from a handful of comets, so he can't rule out that comets could have placed some of the water on the surface of the moon.
"It's still possible that the deep interior could be comet if we don't know what the ratio was way back when," Schultz said. "We need to look at more comets. We need to make more observations, and we need to peer into these samples. We left the moon so long ago, and we left behind all these unanswered questions...I'm still anxious to find out the answers."