Credit: Bill Anders/NASA
In 1994, Maria Zuber, then a geophysicist at Johns Hopkins, was working on a research paper when her 4-year-old son walked into her office and asked what she was up to.
"I'm writing a paper on the shape of the Moon,'" Zuber told him.
"Mom," he said, "it's round."
Scientists have known for centuries that the Moon isn't round. Rather, the Moon is a flattened sphere—like a football—and is elongated on the side that faces the Earth.
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Despite this knowledge, scientists have been mystified that the Moon's distorted dimensions don't match their predictions, given its current orbit and distance from Earth. The Moon, mathematician Pierre-Simon Laplace noted in 1799, is too deformed and too flat.
Scientists tried to develop models of the Moon's early orbit that could explain how the distortions formed, but they always failed—no matter how close they moved the Moon's orbit to the Earth, or how fast they made it spin. No solution matched the Moon's exact dimensions.
Now, in the Aug. 4th issue of Science, Zuber, now at MIT, teams up with two colleagues to provide the first defensible answer to the 200-year-old puzzle of how the Moon got its figure.
The insight came when Ian Garrick-Bethell, a third-year graduate student in Zuber's group, decided to try a radically new orbit.
The Moon's orbit around the Earth today is roughly circular, so scientists trying to puzzle out its origins simply imagined that it had once had the same circular orbit, but closer in.
It occurred to Garrick-Bethell that the orbit didn't have to be circular.
Using a computer model, he constructed scenarios in which the Moon was in a much more eccentric, or elliptical, orbit. Combining a highly eccentric orbit with a faster spin and less distance from the Earth, the MIT team found that they could completely account for the Moon's unique shape.
"This is the first time that, by putting the Moon into an eccentric orbit, we're able to reproduce what we see on the Moon today," said Stan Peale, a now-retired physicist at the University of California, Santa Barbara.
When it first formed, the Moon wasn't hard rock, but a molten sphere. Most researchers now believe it accreted close to the Earth and then began traveling outward, gradually moving farther away every year.
When the Moon was molten, its rotation alone would have distorted its shape. A sphere of liquid that spins on its axis flattens: the faster the spin, the greater the flattening. The Earth's gravitational field also pulls on the Moon's near side, further distorting the satellite.
This has led scientists to propose that the deformations are remnants of the Moon's youth. When the Moon resided much closer to the Earth, the Earth's gravitational field would have exerted a stronger pull, causing greater distortion.
"We know the Moon at one point was very warm and mushy and liquidy, and it was probably closer to the Earth," said Garrick-Bethell. "If it froze in its shape when it was closer to the Earth, maybe that's why we get these deformations."
Over the years, a number of different scientists had hypothesized that the young Moon may have once spun faster than it does now. Today, the Moon and the Earth are said to be synchronous, meaning the Moon spins one time on its axis for every one full orbit around the planet. But if it spun faster during the phase when it was a molten body, it would have become flatter.
When the Moon eventually hardened, these deformations would have been preserved in the rock, giving it the appearance it has today.
"This is so cosmic for me," said Zuber, about her group's accomplishment. "I've been trying to figure out the shape of the moon for a large part of my career."
In addition to possibly solving this 200-year mystery, the researchers examined geophysical data from the Moon and used a series of well-established equations to estimate when the Moon may have had this fast-spinning, eccentric orbit—putting the date at about 100 million years after the satellite's formation.
The researchers acknowledge that their study is not the last word on the Moon's history.
"We don't know that this is the right answer," Zuber said. "We only know that this is the first real explanation for the data. It's the first solution that works."