[meteorite-list] Kaboom! Ancient Impacts Scarred Moon To Its Core, May Have Created 'Man in the Moon'

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Feb 9 12:38:55 2006
Message-ID: <200602091737.k19HbEY02308_at_zagami.jpl.nasa.gov>

Research Communications
Ohio State University

Contact:
Laramie Potts, (614) 292-7365
Ralph von Frese, (614) 292-5635

Written by Pam Frost Gorder, (614) 292-9475

2/8/06

KABOOM! ANCIENT IMPACTS SCARRED MOON TO ITS CORE, MAY HAVE CREATED "MAN IN
THE MOON"

COLUMBUS , Ohio -- Ohio State University 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 a
mystery about how past impacts on Earth affect it's geology today.

The early Apollo missions revealed that the moon isn't perfectly
spherical. Its surface is warped in two spots; an earth-facing bulge on
the near side is complemented by a large depression on the Moon's far
side. Scientists have long wondered whether these surface features were
caused by Earth's gravity tugging on the moon early in its existence, when
its surface was still molten and malleable.

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

Potts and von Frese came to this conclusion after they used 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.

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

Potts pointed to a cross-sectional image of the moon that the scientists
created using the Clementine data. On the far side of the moon, the crust
looks as though it was depressed and then recoiled from a giant impact, he
said. Beneath the depression, the mantle dips down as he and von Frese
would expect it to do if it had absorbed a shock.

Evidence of the ancient catastrophe should have ended there. But some 700
miles directly below the point of 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 even more surprising. The core bulges, as if core
material was pushed in on the far side and pulled out into the mantle on
the near side. Above that, an outward-facing bulge in the mantle, and
above that -- on the Earth-facing side of the moon -- sits a bulge on the
surface.

To the Ohio State scientists, 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.

The scientists believe that a similar, but earlier impact occurred on the
near side.

Potts and von Frese suspect that these events happened about four billion
years ago, during a period 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.

So the warped surfaces on the near and far sides of the moon and the
interior features that connect them are all essentially signs of 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 impacts may have created conditions that led to a prominent lunar
feature.

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

How that magma made it to the surface 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 bubbles to the surface. Some time between when
the impacts occurred and when the moon solidified, some magma escaped the
mantle through cracks in the crust and flooded the nearside surface and
formed a lunar "hot spot".

A hot spot on Earth forms the volcanoes that make the Hawaiian island
chain. The Ohio State scientists wondered: could similar ancient impacts
have penetrated the Earth, and caused the hot spots that exist here today?
von Frese thinks that it's possible.

"Surely Earth was peppered with impacts, too," he said. "Evidence of
impacts here is obscured, but there are hot spots like Hawaii . Some hot
spots have corresponding hot spots on the opposite side of the Earth. That
could be a consequence of this effect."

He and Potts 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 believed to
have set off an environmental chain reaction that killed the dinosaurs.

NASA funded this research. The space agency has been charged with
returning astronauts to the moon to prospect for valuable gases and
minerals.

But even today, scientists don't entirely know what the moon is made of --
not down to the core, anyway. They can calculate where certain minerals
should be, given the conditions they believe existed when the moon formed.
But impacts like the one Potts and von Frese discovered have since
shuffled materials around. Gravity measurements, they said, will play a
key role as scientists figure out what materials lie within the moon, and
where.

"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. We have to use gravity measurements to calculate
the density of materials, and then use that information to extrapolate the
likely composition," Potts said.

von Frese said a lunar base would be needed before scientists can more
completely answer these questions.

Potts agreed. "Once we have more rock samples and soil samples, we will
have a lot more to go on. Nothing is better than having a person on the
ground," he said.

[NOTE: Images supporting this release are available at
http://researchnews.osu.edu/archive/moonboompix.htm ]
Received on Thu 09 Feb 2006 12:37:13 PM PST