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Office of News and Information
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December 15, 1994
CONTACT: Emil Venere


Hopkins, Goddard Scientists Probe Lunar
Mysteries with Clementine

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?

"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 Hopkins.

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 globe.

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 moon.

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.


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