Data from the lunar-orbiting Clementine spacecraft suggest that the moon's crust is much lumpier and stiffer than scientists believed.
Giant asteroids billions of years ago appear to have plowed huge craters that never fully smoothed over again, creating deeper basins and higher mountain peaks than previously thought.
Clementine's sensors found that one of them, the South Pole-Aitken Basin on the moon's far side, is 7.4 miles deep and 1,500 miles wide, blasted by an object about 155 miles wide.
It is the biggest and deepest known impact basin in the solar system. On Earth, it would stretch from the Atlantic Ocean to the Rocky Mountains.
"The fact that the moon turned out to be more rigid and strong than we thought means we will have to rethink our ideas about the early impact history of the Earth," said Dr. Maria T. Zuber, 36, a Johns Hopkins University geophysicist at the Goddard Space Flight Center and member of the Clementine team.
The Clementine findings were published today in the journal Science by Dr. Zuber, David E. Smith and Frank G. Lemoine at Goddard, and Gregory Neumann at Hopkins.
Scientists had believed from the Apollo moon mission and other data that the moon was once almost completely molten and that it had cooled evenly. Its crust formed as it cooled, "like the scum on a pond," she said.
After its launch in January, however, Clementine began to measure the thickness of that crust and found that it is 30 percent more variable than had been thought.
For example, the crust is thicker on the far side of the moon, averaging 42 miles, compared with an average of 37 miles on the near side.
But the thicknesses vary from just 2.5 miles to 66.5 miles. The val- leys are also deeper and the mountains higher than expected.
Also, said Dr. Zuber, "there are big variations in the rigidity of the moon, how strong it is in certain places."
These sharp variations could be the result of huge impacts, where asteroids blasted rock from one place and piled it up in others.
Or, it's possible that chunks of the moon never were completely molten, or that they cooled at different rates.
"Something very heterogeneous was going on," Dr. Zuber said. "Instead of a magma ocean, there may have been seas of magma on the moon" separated by cooler, denser regions.
Most scientists no longer believe that the Earth and moon formed in the same manner. That idea was discarded after they recognized that the moon is mostly rock, and lacks an iron core like the Earth's.
In a search for answers, scientists working with computer simulations in 1982 concluded that "if you hit the Earth at an angle with something the size of Mars, you only strip out mantle, and you can form a moon without iron," Dr. Zuber said.
That sort of impact is now believed to have formed the moon 4.6 billion years ago.
The $75 million Clementine mission was launched in January to test "star wars" anti-missile sensors and tracking gear. Defense Department scientists asked the instruments to find, track and fly the craft to the moon and a near-Earth asteroid. Scientists for the National Aeronautics and Space Administration were invited to mount some lunar studies along the way.
During two months in lunar orbit, the craft scanned most of the moon's surface with laser range finders, compiling the first accurate topographic maps.
Clementine later malfunctioned en route to a near-Earth asteroid and was parked in Earth orbit, where it still provides data on radiation.