Findings by a team at Ohio State University suggest that a large object actually hit the far side of the moon about 4 billion years ago, and it sent a shock wave through the Moon's core and all the way to the Earth-facing side. When the crust recoiled, the Moon's surface displayed the markings of the encounter.

The "Man in the Moon" is a collection of dark plains on the Earth-facing side of the Moon, where magma from the mantle flowed out onto the surface and flooded lunar craters. The Moon has long since cooled, but the dark plains that help to provide the appearance of a man's face remain from that early active time.

Scientists have known since the Apollo Moon missions between 1969 and 1972 that the Moon isn't perfectly spherical. Rather, its surface is warped in two spots. It has an Earth-facing bulge and a large depression on the opposite side. When the Apollo-era surveys found the anomalies, scientists first hypothesized that the surface features were caused by Earth's gravity tugging on the Moon early in its existence, when the surface was still molten and malleable.

According to team members Laramie Potts and Ralph von Frese, however, the features more likely are remnants from ancient impacts.

Potts and von Frese came to this conclusion after they used precise gravity fluctuations measured by NASA's Clementine and Lunar Prospector satellites to map the Moon's interior. They reported the results in a recent issue of the journal Physics of the Earth and planetary Interiors.

The researchers said they expected to see defects beneath the Moon's crust that corresponded to craters on the surface, because old impacts would have left marks down to the mantle, the thick rocky layer between the Moon's tiny metallic core and its thin outer crust.

Potts said a cross-sectional image of the Moon created using the Clementine data shows that the crust on the far side looks as though it was depressed and then recoiled from a giant impact. Beneath the depression, the mantle dips down as if it had absorbed a shock, and a piece of the mantle, located some 700 miles directly below the point of impact, still juts into the Moon's core today.

"People don't think of impacts as things that reach all the way to the planet's core," von Frese said, adding that what they saw from the core all the way to the surface on the near side of the Moon was even more surprising. The core bulges, he explained, as if core material was pushed in on the far side and pulled out into the mantle on the near side. Above that, there is an outward-facing bulge in the mantle and, on the Earth-facing side of the moon, there is a bulge on the surface.

The way the features line up strongly suggests a large object such as an asteroid hit the far side of the Moon early in its history and sent a shock wave through the core that emerged on the side facing Earth. In addition, a similar but earlier impact occurred on the near side.

These events happened during a period when the Moon was geologically active, with its core and mantle still molten and magma flowing. Back then, the Moon also was much closer to Earth than it is today, so the gravitational interactions between the two bodies were stronger. When the impacts freed magma from the Moon's deep interior, Earth's gravity took hold of it and wouldn't let go.

So the warped surfaces on the near and far sides of the Moon, and the interior features that connect them, essentially are signs of ancient injuries that never healed.

"This research shows that even after the collisions happened, the Earth had a profound effect on the Moon," Potts said.

The research also might help scientists determine the origins of a curious earthbound phenomenon: geologic hot spots - sites where magma bubbles to the surface periodically in volcanic eruptions that sometimes are catastrophic in nature.

"Surely Earth was peppered with impacts, too," von Frese said. "Evidence of impacts here is obscured, but there are hot spots like Hawaii (which is responsible for the periodic formation of the Hawaiian Islands). Some hot spots have corresponding hot spots on the opposite side of the Earth. That could be a consequence of this effect."

The two researchers said they are exploring the idea by studying gravitational anomalies under the Chicxulub Crater on Mexico's Yucatan Peninsula. A giant asteroid struck the spot some 65 million years ago, and is thought to have set off an environmental chain reaction that extinguished the dinosaurs.

Meanwhile, they said, further research is necessary because scientists still don't know what the Moon is made of entirely. "We don't fully understand the way these minerals settle out under temperature and pressure, so the exact composition of the moon is difficult to determine," Potts said.

"We have to use gravity measurements to calculate the density of materials, and then use that information to extrapolate the likely composition." Humans need to establish a lunar base before scientists can answer these questions more completely, von Frese said.

Related Links
Nakhla Paper
Lunar And Planetary Science Conference 2006
OSU