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onCampus--Ohio State's faculty/staff news

Vol. 38, No. 18

By: Pam Frost Gorder

Ancient impacts scarred moon to its core

Ohio State planetary scientists have found the remains of ancient lunar impacts that may have helped create the surface feature commonly called the "man in the moon."

Their study suggests that a large object hit the far side of the moon and sent a shock wave through the moon's core and all the way to the Earth-facing side. The crust recoiled and the moon bears the scars from that encounter even today.
The finding holds implications for lunar prospecting and may solve the mystery about how past impacts on Earth affect its geology today.

The early Apollo missions revealed that the moon isn't perfectly spherical. A bulge on the near side is complemented by a large depression on the far side. Scientists have wondered whether these features were caused by Earth's gravity tugging on the moon early in its existence, when its surface was still malleable.

According to Laramie Potts and Ralph von Frese, a postdoctoral researcher and professor of geological sciences, respectively, these features are actually remnants from ancient impacts.

Potts and von Frese used gravity fluctuations measured by NASA's Clementine and Lunar Prospector satellites to map the moon's interior, and reported the results in the journal Physics of the Earth and Planetary Interiors.

They expected to see defects beneath the moon's crust. Old impacts should have left marks only down to the mantle, the thick, rocky layer between the moon's core and its outer crust. And that's exactly what they saw, at first.

But a cross-sectional image of the moon created using Clementine data shows that beneath the depression, the mantle dips down as if it had absorbed a shock.

Evidence of the ancient catastrophe should have ended there. But some 700 miles directly below the impact, a piece of the mantle still juts into the moon's core today.

That was surprising enough. "People don't think of impacts as things that reach all the way to the planet's core," von Frese said.

But what they saw from the core all the way to the surface on the near side of the moon was more surprising. The core bulges, as if material was pulled out into the mantle. Above that sits an outward-facing bulge in the mantle, and above that - on the Earth-facing side - sits a bulge on the surface.

To Potts and von Frese, the way these features line up suggests that a large object such as an asteroid hit the far side of the moon and sent a shock wave through the core that emerged on the near side.

They believe that a similar, but earlier impact occurred on the near side, and suspect that these events happened about four billion years ago, when the moon was geologically active - with its core and mantle still molten and magma flowing.
Back then, the moon was much closer to the Earth than it is today, Potts explained, so the gravitational interactions between the two were stronger. When magma was freed from the moon's deep interior by the impacts, Earth's gravity took hold of it and wouldn't let go.

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

The "man in the moon" is a collection of dark plains on the Earth-facing side of the moon, where magma from the mantle once flowed out onto the surface and flooded lunar craters. The moon has long since cooled, von Frese explained, but the plains are a remnant of that active time - "a frozen magma ocean."

How that magma got there is a mystery, but if he and Potts are right, giant impacts could have created a geologic "hot spot" on the moon - a site where magma bubbled to the surface.

To investigate whether other impacts could have formed hot spots on Earth, they are studying gravitational anomalies under Chicxulub Crater in Mexico. A giant asteroid struck the spot 65 million years ago and is believed to have set off an environmental chain reaction that killed the dinosaurs.

Funding for this research was provided by NASA.

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