Mysteries of the Moon -DAWN Science; August 19, 2006

The Moon is slightly squashed, as if someone had held it at the poles between thumb and forefinger and squeezed, flattening it around its equatorial midsection. That is not surprising. The Moon spins, and the outward centrifugal force should indeed have generated a bulge as the molten magma of a young moon cooled to solid rock eons ago.

But as far back as 1799, the mathematician Pierre-Simon Laplace noticed a back-and-forth wobbling because of the Moon’s deformed shape. Although the flattening was slight — the Moon’s width, 2,159 miles, is about 2.5 miles greater than its pole-to-pole height — it was still greater than would be expected for its current rotation period of 27 days 7 hours 43 minutes and 11.5 seconds.

“The puzzle had been that the Moon was too flat,” said Maria T. Zuber, a professor of geophysics and planetary sciences at the Massachusetts Institute of Technology. Space probes of the 1960’s and 1970’s found a second deformity of the Moon: it is slightly elongated along the Moon-Earth axis. That is, if the Moon were sliced in half along its equator, the cross-section would not be a circle, but more like a football, with one of the narrow ends pointing towards Earth.

But no one could come up with a completely convincing explanation for the Moon’s current shape. That is not its only mystery. Another is why is its near side always facing Earth, which is so different in material and appearance from the far side. The Moon’s origin still holds some uncertainties, although many scientists believe that it formed out of the debris when a Mars-size object struck Earth 4.5 billion years ago.

“Quite a lot of the darned thing is still quite mysterious,” said Kimmo Innanen, a professor of astronomy at York University in Toronto. In a recent issue of Science, Dr Zuber, with Jack Wisdom and Ian Garrick-Bethell, say they have a possible answer to the problem of the Moon’s shape. Actually, they say they have several.

What Laplace did not know is that the Moon is moving away from Earth and slowing down. Years of bouncing laser beams off mirrors left on the lunar surface by the Apollo astronauts show that each year the Moon is another 1.5 inches farther from Earth. The Moon now orbits in what astronomers call a 1:1 resonance with Earth, its orbital period equal to its rotation time so that the same side of the Moon always faces Earth.

Thus, in the past, the Moon was much closer and took less time to orbit. With the 1:1 resonance, the Moon spun faster as well, possibly explaining the bulge. But those calculations did not come with the correct answer for observed distortion along the Moon-Earth axis. “They ought to be self-consistent,” Dr Zuber said.

One suggestion has been that the Moon by chance cooled into this somewhat odd configuration. Other solid planets like Earth are not exactly in their predicted shape either.

The MIT scientists, however, say the observed distortions are larger than would be expected for chance. Instead, they suggest that in the Moon’s early history, it travelled in an elliptical rather than a circular orbit and that it was in a 3:2 resonance, spinning three times for every two orbits.

It would have been in this resonance for only a few hundred million years at most, the scientists said, before tidal forces slowed its spin and it fell into the current 1:1 circular resonance. Their calculations show that such an orbit would provide the necessary forces to produce the Moon’s shape. “There’s a 200-year-old problem, and we’ve got the first solution that works,” Dr Zuber said.

They also found that orbits with higher resonances, like two spins for every orbit, could produce the same lunar shape. “It’s a bunch of families of solutions,” Dr Zuber said.

In an accompanying commentary in Science, Dr Innanen described the proposed solution as “ingenious.” But Peter M. Goldreich, an emeritus professor of astrophysics and planetary physics at the California Institute of Technology, said the MIT team did not explain how the Moon was caught in the 3:2 resonance.

“That is a real weakness,” he said. While an elliptical orbit and a 3:2 resonance may explain the shape, they would not explain the disparities between the Moon’s sides. The crust on the near side is much thinner, with vast plains of dark basalt, called maria (pronounced MAH-ree-uh), that erupted long ago. The crust of the far side is thicker, with many more craters; there are few maria.

Partly because of the denser basalt of the mare, the Moon’s centre of mass is not at the centre, but shifted by more than a mile. But how that shift occurred is unknown. “That would be a good research project,” Dr Innanen said.

Many of the mysteries depend on a better understanding of the Moon’s early history. Under the prevailing theory that the Moon formed after something the size of Mars slammed into Earth, scientists believe that the Moon initially orbited very close, perhaps just 16,000 miles from Earth (compared with roughly 238,000 miles today) before moving outwards. David J. Stevenson, a professor of planetary science at Caltech, said the evidence for the collision theory was solid. Then he stopped. He said instead, “The argument against the alternatives is strong.”

In the past, scientists have suggested that the Moon is somehow captured gravitationally or that it formed out of material spun off from the Earth. The mysteries have persisted because little new data have been collected. In the three decades since the Apollo Moon landings, Nasa has sent only one probe, a bargain-basement one, to the Moon, the Lunar Prospector that orbited it in 1998 and 1999.

Nasa’s push to send astronauts back has led to robotic missions as well, including the Lunar Reconnaissance Orbiter, scheduled for launching in 2008. India, Japan and China are also planning robotic missions.

And that, scientists say, could lead to a scientific renaissance in the study of the Moon. “Talk to me in five years,” Dr Zuber said. — Dawn/The New York Times News Service

Deeper meaning of rumblings

IN THE category of “good research takes time”, scientists have just discovered more than 100 quakes that occurred on the Moon three decades ago. Seemingly combining archaeology and modern computer science, several groups of researchers have exhumed data from the Apollo landings and taken a closer look at them to improve their understanding of what the Moon is made of and why its inside trembles.

“It’s a very exciting data set,” said Dr Yosio Nakamura, a geophysicist at the University of Texas who was among the scientists who analysed the seismographs in the 1970’s and continues to work on them today. The lunar landers for Apollo 11 through 16 carried seismometers, the same type of instruments that measure earthquakes on Earth, designed to be left behind and to continue to radio back data after the astronauts left.

Apollo 11 left a prototype that failed after a month, and Apollo 13, which was almost lost when its oxygen tank exploded, never made it to the lunar surface. But the four other seismometers, from Apollo 12, 14, 15 and 16, recorded some 12,500 seismic events through 1977, when Nasa turned the network off.

As expected, the Moon shuddered when hit by small meteors. There were a handful of deliberate tremors when discarded parts of the Apollo spacecraft were sent crashing onto the lunar surface.

The seismometers also recorded 28 shallow “moonquakes” over eight years, the largest about magnitude 5.0. But more unexpected and still not well understood were numerous tiny moonquakes, several a day on an average that occurred far below, 500 to 750 miles down, about halfway to the Moon’s centre.

“That’s deeper than any earthquake we see on the Earth,” said Dr Catherine L. Johnson, a geophysicist at the Scripps Institution of Oceanography in San Diego. “It tells you something about the ability of the middle of the Moon to fracture.”

Nearly identical squiggles emerged from the Moon seismographs over and over, indicating that certain parts of the Moon’s interior broke repeatedly in the same way, almost like someone cracking his knuckles. Dr Nakamura and other scientists counted 108 of these regions, which they called nests.

The rate of moonquakes seemed to ebb and flow on every 27 days, the time it takes the Moon to circle the Earth, suggesting that they were caused by the pull of tidal forces. Almost all of the deep moonquakes originated on the near side of the Moon, the side that always faces the Earth.

That meant either the structure of the far side is different and does not experience moonquakes or moonquakes do occur, but the waves dissipate when they hit a still molten core.

That was the unsatisfying, unresolved conclusion to Dr Nakamura’s initial studies 20 years ago. He also could not figure out the sources for more than 9,000 of 12,500 events in the seismographs at that time.

The difficulty then lay in the limits of early computers. The four seismometers on the Moon were advanced for their era, recording the data digitally and radioing the information to Earth in real time. But mainframe computers were incapable of processing that much data.

Instead, Dr Nakamura and his colleagues took the computer-friendly digital data and printed them on old-fashioned rolls of paper, then placed the paper on light tables and identified moonquakes by eye. “We still have those pieces of paper,” he said. Now he has re-analysed the 9,000 unidentified events using computers. In a paper published last month in The Journal of Geophysical Research, Dr Nakamura reported that 5,885 of them turned out to be deep moonquakes and that he had located about 250 new nests. — Kenneth Chang/The New York Times