As scientific mysteries go, the moon has long seemed a closed case. Astronauts have bounded across it and returned with vials of its rocky soil. Machines outfitted with sophisticated instruments have circled and crash-landed on it. Been there, done that.
But a resurgence of interest in lunar science, led in part by local scientists, is chipping away at seemingly settled theories and raising new questions about our closest neighbor: how much water is up there, for example, and did the Earth once have two moons?
These aren’t merely matters of astronomical curiosity. The moon holds a frozen record of the early history of the solar system, and understanding the details of its formation and composition may provide valuable insight into the conditions under which life emerged on Earth.
“Look at the battered surface of the moon” said Maria Zuber, a geophysicist at the Massachusetts Institute of Technology. “Earth used to look like that. That record isn’t preserved on Earth anymore.”
Zuber heads GRAIL, a NASA mission in which two washing-machine-sized spacecraft have been circling low over the moon’s surface. They use radio signals to measure slight disturbances in the other’s orbit caused by gravitational changes as they fly over different terrain. The measurements will be used to make a finely detailed map of the moon’s internal structure.
“We don’t expect to find life,” Zuber said, “but we will learn things . . . that [will] help us build a better model of what the Earth was going through” when life emerged.
After the Apollo missions of the late 1960s and early 1970s provided a flood of new samples, data, and measurements, a neat portrait of the moon began to emerge. Roughly 4.5 billion years ago, a Mars-sized object slammed into the nascent Earth, flinging out hot, molten debris that aggregated rapidly into the moon. It was a violent event that would have caused volatile content, including water, to be lost.
But now modern technologies are being applied to samples brought back by astronauts and decades-old data, generating surprising findings and new ideas about our familiar neighbor. Recently, it has become clear that the moon has a core similar to the Earth’s. Long thought to be bone-dry, the moon contains as much water as portions of the Earth’s interior.
“The attitude was Apollo solved most of these questions and now it’s time to invest in other planetary bodies,” said Chip Shearer, a senior research scientist at the University of New Mexico. “But in science, things aren’t stagnant, and we’re always looking for how to improve a model based on new observations and when to throw one model out because it doesn’t explain enough.”
Over the last few years, a team led by Alberto Saal, a Brown University geologist, reanalyzed lunar soil samples taken from late Apollo missions and found minute amounts of water within tiny drops of lava trapped in crystals. Parts of the moon’s interior are about as moist as the upper mantle of the Earth, a layer between the crust and the core, Saal and colleagues reported in the journal Science last year.
Saal’s work has sparked many more teams to study the moon’s chemistry in greater detail and spurred efforts to tweak the theory of how the moon formed to take into account the surprising presence of water.
Meanwhile, the GRAIL satellites may help explain why the far side of the moon is studded with craters and mountains, whereas the near side is smooth and flat.
The striking geographical difference has long been a mystery, and scientists proposed last year in the journal Nature that the Earth may have originally had two moons, one much smaller than the other, both created in the giant collision with Earth. That companion moonlet might have drifted into the moon in slow motion, leaving a pancake-like splat, the mountainous terrain on the far side.
GRAIL, which recently finished its first round of gravity measurements, will begin at the end of August to collect a second phase of data, swooping even lower over the landscape. The mission’s high-resolution gravity maps, combined with topographical measurements made by another mission in which Zuber also plays a leading role, could gather evidence to support or disprove the second-moon theory.
Zuber also hopes it will dig into questions about the 3.6 billion-year-old Shackleton crater on the moon’s south pole, which has long been of intense interest because its shadowy interior might harbor ice. In a study published in Nature on Wednesday, she led a team that reported possible evidence for small amounts of ice in a very thin layer at the bottom of the crater. When GRAIL resumes taking data, Zuber said, the mission will search for anomalies that could help flesh out whether there is ice under the surface, and how much.
Knowledge of the moon’s interior, which is crucial for understanding its origins, is evolving, thanks in part to the work of Benjamin Weiss, a planetary scientist at MIT who studies magnetism on the moon. Study of moon rocks by Weiss and colleagues suggests that until 3.7 billion years ago, its core was a hot, churning liquid, which created a magnetic field that left distinctive magnetic traces in rocks brought back by Apollo astronauts. A molten core is consistent with the idea that the moon arose from a massive collision.
Information on the makeup of the core has also come from instruments installed to record seismic activity on the moon between 1969 and 1972. Last year a team of scientists reported that reanalyzing the data revealed that the moon’s core has a solid inner layer and a fluid outer one.
Weiss said the moon’s proximity and extensive data gathered about it provides an unprecedented opportunity to build a detailed understanding of the early solar system.
One pressing question, for example, is knowing more about the “late heavy bombardment,” a time when the early solar system was pelted with asteroids or comets. The record of that bombardment on Earth was buried by weather and geological activity, including earthquakes and volcanic eruptions, but it is available on the moon.
“We’re in the phase where each discovery just catalyzes more discoveries,” Weiss said.
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