[meteorite-list] Scars of a Lunar Comet Impact?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:52 2004
Message-ID: <200403191649.IAA10386_at_zagami.jpl.nasa.gov>


Scars of a lunar comet impact?
March 18, 2004

Did a comet create the biomorphic swirl on the Moon known as Reiner Gamma?

In western Oceanus Procellarum on the Moon lies a curious feature named
Reiner Gamma. Its light-colored, semi-symmetrical swirls look mysterious
enough to startle any backyard lunar astronomer who stumbles across it

The feature, measuring about 20 by 40 miles (30 by 60 km), has been ascribed
to many causes. The most widely mentioned cause is that buried and magnetized
material that has created a magnetic field - in effect, a micro-magnetosphere -
that shields the mare surface from weathering (darkening) by the impact of
ionized particles from the Sun. Supporters of this theory point to the site's
strong magnetic anomaly, which was first mapped during the Apollo 15 and 16
missions in the early 1970s.

Patrick Pinet (CNRS/University of Toulouse, France) and a group of coworkers
used Clementine imagery to derive the photometric properties of the top-most
lunar surface across the southern half of the feature. These data, they
report, demonstrate that the feature is "optically immature," meaning that
its properties appear to have been exposed to solar darkening for only a
geologically brief time, perhaps 10 million years.

This apparent newness of the feature led the researchers to suggest it was
caused by the impact of a collection of icy fragments - a comet, in other
words. According to Pinet, the best candidate is a comet nucleus that had
been disrupted by Earth's gravity, much as Jupiter's gravity broke up Comet
Shoemaker-Levy 9 a decade ago. In any case, Pinet says, the impacting object
was made of a cloud of small particles. This could have produced swirls on
the Moon's surface when the impacting gas and dust interacted with the
uppermost layer of lunar soil, affecting only its uppermost layer. The
magnetization might have resulted from compression of the comet's magnetic
field, he suggests.

But where is the crater? Pinet says the impacting fragments were small and
fluffy, and detecting any craters will call for sharper images than what has
been achieved so far. "We are planning to use high-resolution images taken
by the SMART-1 spacecraft when it reaches the Moon early in 2005," he says.
These images will let the researchers examine the surface of Reiner Gamma
closely and perhaps reveal details that will tell an unambiguous story of
the feature's origin. There are two Japanese missions headed for the Moon
as well, Lunar-A (to be launched this summer) and Selene (slated for 2006).
Imagers and other instruments on these spacecraft also may make observations
that will help settle the question.

"This is a puzzling problem that hasn't yet been solved," Pinet concludes.
"And its solution will be applicable to all bodies with airless surfaces
that have developed a regolith." Besides the Moon, he points to Mercury
and asteroids.
Received on Fri 19 Mar 2004 11:49:07 AM PST

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