New lunar research answers old questions, poses others

by Gabriel Popkin
Sunday, October 5, 2014

From its close perch in the sky, the moon has long mystified artists, lovers and scientists. Now a quartet of recent papers — all of which touch on the moon’s violent birth and turbulent infancy — may help scientists put two long-standing lunar mysteries to rest, while deepening a third enigma and suggesting a speculative solution to a fourth.

Most scientists think the moon formed from the rubble of a planet-sized object that slammed into Earth during the solar system’s early days. This giant impact theory explains several unusual features of the moon, including why its metal core is relatively small and why the moon lacks lightweight, volatile elements.

But recent studies of the moon’s composition have confronted the giant impact theory with challenges. Among the most perplexing is that scientists have failed to find theorized differences in the isotopic compositions of moon rocks versus those of rocks on Earth. Most computer simulations of the moon-forming impact suggest that the moon should be made largely of the remains of the obliterated planet, commonly called Theia. Given that every solar system body has its own unique set of isotope ratios, Theia presumably would have too. But studies of the moon’s oxygen, titanium, calcium, silicon and tungsten isotopes have yet to turn up a unique signature.

In recent years, scientists have proposed several ways to reconcile giant impact models with available isotopic evidence. Theia and Earth could have formed in an unusually similar environment, for example. Or, if Earth had been spinning fast enough before impact, it could have flung enough material into space to account for the moon’s present-day resemblance to its host planet.

To probe precisely how similar the two bodies are, a team of German scientists reanalyzed samples collected during the Apollo missions. The researchers applied a new method to extract and concentrate the oxygen from these rocks to a higher purity than previous studies had achieved. The team reported in Science that moon rocks have higher oxygen-17-to-oxygen-16 ratios than do Earth rocks. But the difference is tiny: only 12 parts per million. By contrast, Mars and Earth rocks differ by several hundred parts per million.

Still, the new findings “fit very well into the general concept of having a giant impactor form the moon,” says study author Daniel Herwartz, a geochemist at the University of Cologne in Germany. “I don’t think we need to assume a large difference” between Earth and Theia, Herwartz says. For instance, Theia might have been composed of materials similar to enstatite chondrites, a rare type of meteorite whose composition is similar to Earth’s.

Isotopic differences between Earth and moon rocks “is something that people like me, who have been working on the origin of the moon from the theoretical side, had expected, or hoped for,” says Matija Çuk of the SETI Institute in Mountain View, Calif., who wasn’t involved in the study. “And this is the first such claim, so it’s quite exciting.”

But Çuk, who along with colleague Sarah Stewart at Harvard proposed a moon formation model involving a fast-spinning Earth, cautions that it’s too early to tell if Herwartz’s team actually found the isotopic signature of an impactor. A “late veneer” of meteorites striking Earth after the moon’s formation could have also changed the planet’s oxygen isotope ratios enough to account for the differences the scientists found, Çuk says. The study authors also acknowledge this possibility, and Herwartz says high-precision studies of other isotopes may help resolve this dilemma.

A watery conundrum

Along with Theia’s isotopic fingerprint, scientists also have to reconcile the giant impact theory with recent findings about the moon’s surprisingly abundant water. Initial analyses of moon rocks returned by Apollo astronauts revealed no water, leading scientists to conclude that the moon was bone dry. (A few contradictory findings were chalked up to terrestrial contamination.) The lack of water and other volatile compounds bolstered the giant impact theory, as many simulations indicated that an impact would have vaporized the material that eventually formed the moon, boiling off most volatiles.

But recent discoveries of water in moon rocks have upended that conventional wisdom. Beginning in 2008, researchers reported finding substantial amounts of water in moon rocks. These rocks were coughed up from the moon’s mantle billions of years ago when the satellite was still volcanically active, so they provide glimpses into the moon’s now-frozen interior.

Taken together, the findings show that the concentration of water varies within the moon, University of Hawaii planetary scientists Katharine Robinson and Jeffrey Taylor wrote in a Nature Geoscience review article. In particular, certain volcanic glass beads seem to contain amounts of water similar to those found in volcanic glasses here on Earth. Even though on Earth such glasses come from mid-ocean ridge basalts — among the mantle’s driest known regions — the presence of such objects on the moon suggests it could contain anywhere from 1 to 10 percent as much water, per weight, as Earth, Taylor says.

These findings don’t upend the giant impact theory, Taylor says, but they suggest the material forming the moon may not have entirely vaporized and homogenized, as some models predict. “Some of the moon probably formed promptly from big blobs of melt, and there wasn’t enough time for the water to actually leave these big blobs,” Taylor says.

But, he notes, the water could also have come from asteroids and comets that collided with the moon after formation. More research is needed to pin down exactly where the moon’s water came from, Taylor says, but the answer will certainly paint a clearer picture of the moon’s birth and early life.

The moon’s strange shape

A third lunar mystery lingers from early moon missions that showed that the moon’s far side looks very different from the side humans had always seen. The far side largely lacks the dark “maria,” or ancient lava flows, common on the near side. More recently, the GRAIL mission confirmed that the far-side crust is several kilometers thicker than that of the near side, which could help explain why less magma made it through the crust there to form maria. But no one knows why the far-side crust became thicker in the first place.

Now, a team of astrophysicists from Penn State University has suggested in Astrophysical Journal Letters that the thick crust, also known as the lunar highlands, could have resulted from events that occurred when the infant moon was close to a hot Earth.

The scientists' reasoning goes like this: Just after the moon formed (presumably coalescing from a giant impact), both it and Earth were very warm — thousands of degrees Celsius — and entirely molten. The two bodies were also much closer together, so the moon was bathed in intense radiation from the glowing Earth, which loomed far larger in the lunar sky than it does today. Critically, the team argues, the moon quickly became tidally locked. In other words, within a few hundred days of formation, the moon’s rotation became synchronized with its period of revolution around Earth, after which it has only ever pointed one face toward Earth.

Under these circumstances, the moon’s far side would have cooled faster than its near side, the scientists argue. Aluminum and calcium, the first elements to condense out o" a vapor, would have begun precipitating on the far side. As the moon continued to cool, crust would have begun forming around these elements, leading eventually to a thicker crust on the far side.

But Ian Garrick-Bethell would have an effect, because of the long timescale it takes to form a crust," he says. “The moon was essentially just a ball of liquid for so long.”"

The team’s proposal is more of a rough hypothesis than a detailed model, notes study co-author Jason Wright. But new findings should allow modelers to fill in some of the missing details, he says. “Now is the time to get ideas out there, because the data are going to start disproving models pretty soon.”"

Garrick-Bethell, meanwhile, proposed in Nature an answer to another long-standing mystery about the moon’s shape. If the moon formed from a spinning mass of liquid, models predict it should have frozen out as a sphere with a slight equatorial bulge. But studies have shown that in addition to this bulge, the lunar crust is several kilometers thinner at the poles than at the equator, and that the moon bulges out even “urther in certain places, “like a lemon,” Garrick-Bethell says.

Attempts to determine why the moon has these features have been complicated by the body’s many craters and maria, which make it hard to determine the moon’s original shape. To solve this problem, Garrick-Bethell’s team used gravity data from GRAIL and laser-gathered topography data from NASA’s Lunar Reconnaissance Orbiter to reconstruct the moon’s primordial shape more precisely than any previous group ha”. Through a detailed mathematical analysis, the scientists concluded that approximately 80 percent of the moon’s “lemon-ness” resulted from tidal heating. The idea is that during the satellite’s hot, early days, powerful tidal forces from a much-closer Earth squeezed and stretched the lunar crust, which was flexible because it floated on a magma ocean. The distortions heated crust at the poles, leaving them thinner, and cooled crust in areas on the moon pointing toward and away from Earth, creating thicker crust there. As the moon receded, the tidal distortions petered out, leaving the lemon-like protrusions.

“he rest of the moon’s shape originated later, as the body cooled and migrated away from Earth, Garrick-Bethell says. When the moon was hot and de"ormable, tidal forces created a slight bulge along the axis pointing toward and away from Earth, similar to how the moon pulls on Earth’s water to create tides. Eventually the moon cooled enough that this tidal bulge froze in place, creating what is known as a “fossil bulge.”

“If you went to the beach at high tide and the"whole ocean froze, you’d be stuck with this permanent high tide,” Garrick-Bethell says. Similarly, “the moon … just froze in whatever final tide it was feeling.”

The last mystery is that the long axis of the “lemon” no longer points toward Earth; instead it is offset by roughly 36 degrees. This occurred, Garrick-Bethell says, because volcanoes and crater impacts redistributed some of the moon’s mass, causing the moon’s poles to wander over time.