For some unknown reason, the side of the moon which never faces Earth is higher and far more blasted than the familiar near side.
Now there is a simple model that may partially explain the long mystery by calling on the same tidal processes that are seen at work today on Jupiter's icy moon Europa and even the ocean tides of Earth. All it takes is a once-hot moon with a molten innards that was being yanked by the tidal forces of Earth.
The mystery came into focus in the 1960s, when earthlings got their first good look at the moon's far side. Right away it was clear something was different: The far side is about six kilometers (3.7 miles) higher than the near side and it's a vast field of craters. There are none of the broad, dark, low-lying basins seen on the near side.
"Since then, we've come to understand that that area is high because the crust is thicker," explained Ian Garrick-Bethell, who recently moved from Brown University to the University of California at Santa Cruz.
The crust of the moon, like the crust of the Earth, is composed of less dense stuff than the rocks below it, so it "floats" on the surface, rather like an iceberg floats atop denser ocean water. The bigger the iceberg -- or in this case crust -- the greater the height it stands above water.
"No one's had a really good explanation for this crust," said Garrick-Bethell. So he and his colleagues looked back to the time when the moon was a blob of molten rock with a new, thickening crust to see how the simple tidal effects of the Earth affect the process.
Not too surprisingly, they found that the gravitational tug of the Earth would have caused the molten moon to bulge around the middle. That constant tug from Earth would have caused flexing and heating of the moon's crust and then-molten mantle.
The majority of that flexing and heating would have been near the moon's poles, where it would have slowed the cooling and thickening of that part of the crust. So, this model predicts that the moon's crust should be thinnest at the poles, which it is.
The model also predicts that the crust should be thicker at the equator, which it is, at least on the far side.
"This should have happened on the nearside as well, because the heating is symmetric," Garrick-Bethell told Discovery News. The problem is, the crust isn't thicker on the near side. "We don't see evidence of it."
It could be, he said, that the near side used to be a lot more like the far side, but subsequent events have changed it.
"It's very probable that the nearside is more geologically evolved,” said Garrick-Bethell, who is the lead author of a paper on the study with appears in the Nov. 12 issue of Science. That would imply that the far side is a relict of the very early Moon.
"This kind of thing chips away at the question: why is there so much volcanics on the (near) side," Garrick-Bethell said. "We don't understand that."
"This problem of the shape of the moon has been with us for decades," said lunar researcher David Stevenson of Caltech, who agrees the near side may have undergone more recent changes that obliterated its original, more far-side-like structure and appearance. "The central thing they have done is collected data on the moon and shown that it fits."
And this is where the connection with other objects in the solar system fits into the story, said Stevenson.
Europa and Titan are both moons of big planets. Both are subjected to powerful tidal forces that keep their insides warm and liquid, much like the early moon. The only difference is that the moon's liquid was molten rock and a moon like Europa is thought to have a crust of ice with a mantle of liquid water.
"What's often important in science is when somebody says something surprising," said Stevenson. "This is a surprising result."