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December 15, 1994
CONTACT: Emil Venere
EMBARGOED FOR 6 P.M. EASTERN TIME DEC. 15
Hopkins, Goddard Scientists Probe Lunar
The Clementine spacecraft has revealed that the moon's evolution
was more complex than previously thought, leading to a central
mystery: Why would parts of the moon have quickly cooled off
early in its development, preserving craters and peaks that are
much deeper and higher than scientists had expected?
Mysteries with Clementine
"These data are telling us that we don't know as much about the
shape and evolution of the moon as we thought we did," said Maria
Zuber, a geophysicist at The Johns Hopkins University and Goddard
Space Flight Center. "Many fundamental questions about our
nearest neighbor remain to be addressed after the rich legacy of
the Apollo missions."
A paper on the latest findings about the moon's shape and
internal structure will be published in the Dec. 16 issue of the
journal Science. Clementine, a low-cost spacecraft launched Jan.
25, 1994, orbited the moon for two months and collected an
unprecedented amount of information about Earth's companion.
"There are data now available that will permit our perceptions of
the internal makeup and evolution of the moon to be clarified
considerably," said Dr. Zuber, a member of a research team that
includes geophysicists David E. Smith and Frank G. Lemoine, at
Goddard, and Gregory Neumann, a post-doctoral fellow at Johns
Throughout its early life the moon generated heat by the decay of
radioactive elements in its interior. It also was pummeled by
space-borne debris in collisions that produced intense heat,
melting its surface and forming deep craters and high peaks. But
then parts of the moon apparently cooled off quickly enough to
freeze many surface features in place. Clementine's evidence of
rapid cooling runs counter to the traditional view of lunar
evolution, which theorizes that lunar basins caused by meteor
impacts relaxed into a smoother surface shape. This slow cooling
would have permitted the peaks and deep impact basins to flow
like molasses into a natural state of equilibrium, or low stress,
since the moon's mass would have settled evenly around the lunar
But Clementine is painting a different picture, a scenario of
uneven strength of the moon's outer shell and an uneven
distribution of mass in the interior.
"It was thought that the moon was much like a billiard ball, not
a lot of topography, that originally deep impact basins would
have shallowed," said Dr. Zuber, associate professor in the
Hopkins Department of Earth and Planetary Sciences and a senior
research scientist at Goddard.
The geophysicists are using two types of data to learn more about
the moon. Laser light was bounced from Clementine to the lunar
surface, providing precise measurements for a topographic map.
Another crucial tool is a record of the moon's gravity field,
which was determined by tracking the spacecraft's speed around
The topographic map, which can resolve changes in height as small
as 10 meters, revealed that the moon's variation from the highest
peaks to the deepest craters is much greater than previously
thought. That overall range of topography was found to be 16
kilometers, or almost 10 miles -- about 30
percent greater than scientists had expected.
Based on analyses of lunar samples taken by the Apollo
astronauts, many scientists had thought that the entire outer
part of the moon was at one time molten, creating a "lunar magma
ocean." But the Clementine data are forcing scientists to rethink
that idea. The most simple explanation for why portions of the
moon might have cooled rapidly is that those areas never
completely melted to begin with. The data raise the question of
whether the lunar magma ocean was really many separate, globally
distributed "magma seas," Dr. Zuber said.
So far the research has resulted in several important
discoveries. The team has found that the South Pole-Aitken Basin,
a massive structure on the moon's far side first identified by
images taken in previous lunar research, is 12 kilometers (7.4
miles) deep and 2,500 kilometers (1,550 miles) wide. Those
measurements confirm that it is the largest known impact basin in
the solar system, and on Earth it would stretch from the East
Coast to the Rocky Mountains.
The size of an object needed to cause such a huge hole would
depend on the object's velocity, but a good estimate is that it
was about 120 miles in diameter, Dr. Zuber said.
The team also confirmed the existence of another major basin. It
measures about 700 kilometers wide and is located on the near
side of the moon. Clementine's data are enabling scientists to
clear up any doubt that many structures suspected of being impact
basins were created from bombardment with debris left over after
the creation of the solar system, about 4.6 billion years ago.
But perhaps the best is yet to come. The volumes of information
collected by the spacecraft will keep researchers busy for a
decade, as scientists interpret the data to learn about the
moon's origin and development.
EMBARGOED FOR 6 P.M. EASTERN TIME ON DECEMBER 15
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