The party’s over. Clementine, touted as the first in a new breed of faster, cheaper and better space missions, ended in technical failure May 7. The program’s managers are jubilant.
“It was fun. It was just fun to be on this mission,” said Eugene Shoemaker, the head of the science team responsible for analyzing the scientific information collected by Clementine.
Just before it failed, Clementine completed the most extensive map of the moon ever made. Planetary scientists discussed their preliminary analyses of that map last month at a Baltimore meeting of the American Geophysical Union.
Clementine’s failure prevented its completing the second stage of its mission, a flyby of the near-Earth asteroid Geographos. The encounter would have been the third between a human-built spacecraft and an asteroid, and the first to produce a video of such an encounter.
But enthusiasm for their new knowledge of the moon outshone scientists’ disappointment over the cancellation of the asteroid flyby. Clementine’s lunar map, which characterizes the topography, surface composition and even some of the interior geology of the moon, drew rave reviews.
“It’s going to allow us to make the next major step in our understanding of the geology and the evolution of the moon,” said Shoemaker, a planetary scientist at the Lowell Observatory in Flagstaff, Ariz.
One of the most surprising findings of the Clementine mission is that the moon has topographic relief equal to Earth’s, said Maria Zuber of Johns Hopkins University in Baltimore and NASA’s Goddard Space Flight Center in Greenbelt, Md. The spacecraft also showed that faint, ancient craters on the moon, obscured by subsequent bombardment, are still significant topographic features.
Clementine’s survey also discovered unknown topographic features, especially near the moon’s poles. Some of those features, in particular a deep crater at the moon’s south pole, may never see the sun’s light. Such craters may contain ice that could prove useful to lunar pioneers.
Beneath the moon’s surface, Clementine detected a curious pattern in the geology of lunar craters. Craters on the near side of the moon, which always faces Earth, seem to have denser material beneath them than craters on the moon’s far side. Even more curious is the discovery that the moon, long thought to be a dead planet, may have small amounts of liquid magma lurking inside.
“I think people are realizing that we don’t understand the moon,” Zuber said.
When they analyzed Clementine’s observations, scientists were shocked to find that the moon’s terrain is far more rugged than had been thought. Clementine’s terrain map, the first complete topographic map of the moon ever made, showed that the distance from the moon’s deepest crater to its highest point is more than 12 miles. For comparison, the vertical distance from the lowest point on Earth, the Marianas Trench in the South Pacific, to the top of Mt. Everest is about 10.5 miles.
On Earth, plate tectonic motions create topography by pushing up mountains and shoving ocean crust into the planet’s interior. But the moon has no plate tectonics. Its topography came from bombardment by comets and asteroids early in its history.
Its surprisingly prominent topography suggests that the moon was colder in its early stages than was thought, Zuber said. The warmer rocks are, the faster they’ll sag back into a flat surface after being uplifted. So the more topography the moon has retained from those early bombardments, the colder it must have been in the past.
“Clearly we don’t understand the thermal history of the moon as well as we thought we did,” Zuber said.
In addition to the lunar topographic map, Clementine produced a map of subtle variations in the moon’s gravitational pull. Zuber and Dave Smith of NASA’s Goddard center drew the gravity map by tracking minute changes in Clementine’s position and velocity as it orbited the moon. The spacecraft flew faster and lower as it passed over areas with more gravitational pull. Because gravity is related to mass, such areas are thought to have slightly denser subsurface rocks.
“Geophysicists use gravity as a means of looking into the interior of the planet,” Zuber said.
For some reason, the craters on the near side of the moon seemed to tug at Clementine more than the craters on the moon’s far side. That suggests that the craters on the near side filled with dense magma soon after they were created, while craters on the far side remained unfilled. Planetary geologists can’t imagine how, or even why, such a thing would happen, Zuber said.
Even stranger, Clementine’s attraction to the moon changed periodically during the 10-week mission. The patterns of that attraction matched the fluctuations of tidal forces on the moon, as Earth’s gravity pulled on one side of its satellite and then the other. If Earth’s pull on the moon was affecting the moon’s pull on Clementine, then something inside the moon may be sloshing around in response to tidal forces. That something could be a small amount of molten material, Zuber said.
In addition to challenging planetary scientists with new mysteries, Clementine confirmed some of their earlier observations. During the Apollo program, lunar orbiters detected giant circular features on the moon that had been nearly obliterated by impact craters. Planetary geologists theorized that those features were craters themselves, formed during the last stages of the solar system’s formation. That theory was generally accepted, but never backed up with topographical information showing that the features are actually indentations in the moon’s surface.
Clementine’s topographic map shows that the circular features are in fact depressed basins, and enormous ones at that. One of them, the South Pole-Aitken basin, may be the largest and deepest impact crater in the solar system. Clementine found that the basin, which is 1,600 miles across, is 7.5 miles deep.
For the most part, Clementine provided a better look at places already known. But the extreme northern and southern parts of the moon have never been mapped. Clementine’s observations of those regions show tantalizing evidence that like Earth, the moon may have ice at its poles.
Actually, any ice on the moon would be more like ice on Mercury than the glaciers of Antarctica. Observations made over the last few years show that deep craters near Mercury’s poles harbor ice deposits. Ice can survive in the polar craters because the sun never shines there.
Clementine may have found craters with similar properties on the moon. A composite photo of the moon’s southern latitudes made with Clementine images shows a large, dark crater at the south pole. That crater could harbor tons of ice, carried to the moon by comets. Theoretical work performed in the 1960s showed that molecules of water brought to the moon by comets might bounce around the planet’s surface until they settled somewhere cold enough to freeze them.
An experiment on Clementine tried to settle the question by directly observing ice in the south pole crater. The results of that experiment, which Shoemaker called a “long shot,” aren’t in yet.
Many of the mission’s discoveries have yet to be made, scientists said. Researchers have only just begun to analyze the 3 million images Clementine collected during its 71 days in lunar orbit.
Said Paul Spudis, a planetary scientist at NASA’s Lunar and Planetary Institute in Houston, “No other object in the solar system has been mapped like this, including Earth.”
