News Release

The overall range of topography, from the deepest to the highest formations around the lunar globe, is greater than 20 kilometers, or about 12 and a half miles, said Maria Zuber, a geophysicist at The Johns Hopkins University and Goddard Space Flight Center. Scientists had previously thought the height variation was only about half that. The data suggest that the lunar surface was cooler, and therefore stronger, earlier in its history than previously believed. Features caused by impacts into the lunar crust were, therefore, preserved to a higher degree than expected.

"If it was hot, all this high and low topography would have flowed away like molasses," 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.

One example of lunar landscape extremities is the South Pole-Aitken Basin. Scientists had thought the crater was 7 kilometers deep, but it actually extends 12 kilometers, or about 7 and a half miles, below the moon's surface and is now believed to be the deepest impact crater in the solar system. The basin, located on the side of the moon that perpetually faces away from Earth, would swallow Mount Everest and is comparable to the deepest Earth features, such as ocean-floor trenches. It dwarfs the Grand Canyon, which is about 1.6 kilometers deep.

Dr. Zuber presented preliminary findings May 26 during a meeting of the American Geophysical Union in Baltimore. Clementine, a low-cost spacecraft launched Jan. 25, orbited the moon for two months, gathering unprecedented volumes of information about Earth's companion.

The scientists have used Clementine data to draw the first precise and comprehensive topographic map of the moon, measuring the depths and peaks to an accuracy of 100 meters, or about 330 feet. That's 10 times better than the previous knowledge of the global topography of the moon. The data allows 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. By taking a precise inventory of the quantity and sizes of impact basins, scientists will be able to learn more about how much heat was produced by large impacts during the moon's early evolution.

"Impacts are the geologic process that were most important in shaping the moon's early history," Dr. Zuber said. Analysis of Clementine tracking data also support the possibility that the moon may contain a small amount of partially molten material deep in its interior. But another mission would be needed to fully answer that question.

Clementine promises to aid scientists who are trying to solve several mysteries about the moon's structure and evolution. In its early history most, if not all, of the lunar surface and near surface was molten, and the melted material ultimately formed the lunar crust. Scientists believe that the side of the moon that perpetually faces Earth has a thinner crust than the back side. This effect is so large that the moon's center of mass is about 2 kilometers closer to Earth than is its geometric center, Dr. Zuber said.

But before scientists can understand why the moon has such a feature, and ultimately, how the moon formed, they must measure globally the thickness of the lunar crust and understand details about the moon's mass and shape, said Dr. Zuber, an associate professor who holds the Second Decade Society Chair in the Hopkins Department of Earth and Planetary Sciences. Researchers are using Clementine data from laser measurements to determine the height of the spacecraft above the lunar surface, and radio signals to calculate changes in Clementine's orbital velocity, which provide information on the moon's gravitational field. One of the means of understanding the lunar interior is to make a detailed record of how gravitational strength fluctuates from one part of the moon to another. Areas of greater density exert a stronger gravitational pull on the space probe than areas of less density. As the space craft orbited the moon, it dipped down closer to the surface in places where the gravitational field was stronger. Scientists recorded the spacecraft's ups and downs as it circled the moon by measuring small changes in its velocity. Using a phenomenon known as the Doppler effect, they calculated minute differences in velocity by analyzing changes in radio wavelengths. As an object moves toward you, its radio waves are compressed. As an object moves away from you, its radio waves are stretched into longer frequencies. The faster an object moves, the more pronounced its Doppler shift.

The radio wave and laser data can be dovetailed to create a picture of lunar topography and mass distribution in the moon. Knowledge of the sub-surface mass will contribute to an understanding of what the moon is made of, since different substances have different densities. This information may help to shed some light on how the moon was formed. One theory holds that it was created in a collision shortly after the birth of the solar system, when a Mars-size object possibly collided with Earth, sending debris from the mantles of both the Earth and the object into space. The debris possibly coalesced into the moon.