Earth’s Great Gift to the Moon: Water

Tiny samples from Apollo moon rocks help solve an ancient myatery

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The moon is home to some of the most lyrically named bodies of water that never existed. The Sea of Tranquility is familiar enough,  but what about the Ocean of Storms, the Sea of Nectar, the Lake of Forgetfulness, the Bay of Rainbows? Altogether, the lunar map features 20 seas, 14 bays, 20 lakes and one ocean. That’s both poetic and ironic, because a world that’s positively drenched in aquatic names has not a drop of actual water.

Or that’s what we always thought. In recent years, however, evidence for a watery moon has been mounting—particularly from NASA’s Lunar Reconnaissance Orbiter and its sister probe, the Lunar Crater Observation and Sensing Satellite. The moon is hardly a sodden place—it’s wetter than the Sahara Desert, but that’s about it. Still, that’s huge compared to what we once believed. Now,  a paper in Science explains in greater detail than ever how water got to so unlikely a place and how it survived the fiery violence that gave birth to the moon in the first place.

The prevailing—and now all-but universally accepted—theory for how the Earth-moon system came to be was first proposed in 1975 and quickly became known as the Big Whack theory. About 4.5 billion years ago, a Mars-sized planetesimal careered through the solar system and collided with the infant Earth, sending up a massive cloud of molten and vaporous debris. The hit-and-run world sailed on its way and the cloud of debris slowly accreted into the moon, eventually retreating to a stable orbit roughly 239,000 mi. (385,000 km) away, where it has remained ever since.

“The angular momentum, the distance, the size of the moon,” says Brown University geochemist Alberto Saal, the lead author of the Science paper, “there is no other model that can explain the physics of the Earth-Moon system than the Big Whack.”

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Physics isn’t chemistry, however, and none of this explained with any precision the elements and compounds—particularly water—that would have survived aboard the early moon. The inner solar system as a whole ought to be a very dry place. When the planets and moons that orbit the sun were first accreting, it took a very particular environment for water molecules to form. Anything too close to the sun would simply be too hot and energetic for H2O molecules to assemble themselves. You’d have to get about 3 astronomical units away (1 AU is the distance from the sun to the Earth) to reach what Saal calls the “snow line,” the place water could form. That boundary is roughly between Mars and Jupiter.

Nothing, however, would prevent water from being imported from the wet zone into the dry zone aboard comets or water-rich meteors known as carbonaceous chondrites. In recent years, comets have gained traction as the  source of Earthly water. They’re mostly made of water ice and rock after all, and we now understand just how many billions—perhaps trillions—of them there are surrounding the solar system in the formations known as the Kuiper Belt and the Oort Cloud.

But there’s a good case to make for the carbonaceous chondrites too. They’re hardly the sodden snowballs comets are, and the little bit of water they do contain is integrated deep in their matrices. But there are a great, great many of the meteors flying around out there, and there’s nothing to say a downpour of rocks, rather than two or three comets, could not have watered us too. The growing study of exoplanets is helping to explain just how this could happen.

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“In 2011,” Saal says, “there were observations in other solar systems of large planets like Jupiter that probably migrated in towards their suns and then migrated back out, due to gravitational resonance. This would affect overall gravitational fields causing materials from deeper out to move in.” That material would likely include meteors. That same scenario could have played out here too, and there’s a relatively straightforward way to prove it.

Water comes in more than one form. So-called heavy water is not simple H2O, but 2H2O, containing a hydrogen isotope, known as deuterium. Unlike ordinary hydrogen, with just a proton in its nucleus, deuterium also contains a neutron. Heavy water and ordinary water are present on comets, carbonaceous chondrite meteors and Earth, but in different ratios, known as their deuterium/hydrogen, or D/H ratio. Studies since 1998 have shown that Earth’s D/H ratio is more similar to that of the meteors than of the comets, making a strong case for meteoric origins for Earth’s water. That case is not closed: a 2011 study of Comet Hartley 2 drew the opposite conclusion. Still, at the moment, the meteor camp is winning.

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What Saal and his colleagues succeeded in doing was analyzing the D/H ratios in tiny beads of volcanically cooked glass in lunar samples brought back by Apollo 15 and Apollo 17 to see how closely they parallel Earth’s. The answer: exceedingly closely. “The moon’s D/H rations are similar to Earth’s and they’re both similar to the chondrites’,” says Saal. “What difference there is is probably due to the impact.”

So the moon’s water came from the Earth and maybe the impactor. But the fact that any water at all survived that collision is something of a surprise, since water’s extreme volatility should have caused it simply to vanish into space. Saal, however, cites research by Caltech astrophysicist David Stevenson and others that the blast might have created a sort of envelope of hot gasses, protecting the water while the moon accreted.

Of course, just as the Earth and moon drew apart in distance over time, so too did they part ways developmentally. The Earth’s greater gravity allowed it to retain an atmosphere, protecting the water that was there, as well as any that barreled in later. The moon could keep enough only to achieve its decidedly humbler wetter-than-the-Sahara status. Lunar gravity may turn the tides in our distant oceans, but the moon’s own seas and lakes and bays were—and always will be—mere phantasms.

(FROM THE ARCHIVES: Moon: From the Good Earth to the Sea of Rains)