Probe Finds That the Moon Hasn’t Been a Quiet Satellite

Ever since the Apollo astronauts left their final footprints in the moondust 25 years ago, Earth’s nearest neighbor has been largely neglected for the more exotic lure of planets and star systems far, far away. Now, new findings beamed back from a pint-size military probe named Clementine have served up a full plate of lunar surprises.

The results have yielded the first complete global portrait of the Earth’s orbiting partner. No longer just a pockmarked face, the moon seems to have had a far more interesting history than previously imagined.

Among the more interesting findings are:

* Volcanic activity as recent as a billion years ago.

* A wildly variable crust.

* The possibility of ice lurking in the shadow of the south pole.

* Confirmation of a crater large enough to span the continental United States from the East Coast to the Rocky Mountains.

* A fresh-looking crater that may have been created in an impact recorded by sharp-eyed 12th-Century monks.

“We had thought that nothing much happened on the moon in the last 3 billion years,” said geophysicist Maria Zuber of Johns Hopkins University and the Goddard Spaceflight Center. “Now we know that we don’t understand the moon as well as we thought we did.”

The findings were discussed at the recent American Geophysical Union meeting and will be published in Friday’s issue of the journal Science.

There is no question that the fate of the Earth and the moon have always been linked, and that our permanent companion cannot be taken for granted as a mere prop for poets or backdrop for lovers.

“We wouldn’t be here if it weren’t for the moon,” said Carle Pieters of Brown University. She explained that scientists now believe that the moon was condensed from the debris of a collision between Earth and a Mars-sized object that stripped off a good part of the Earth’s outer mantel and crust.

After a billion years or so of volcanic activity fed by the heat of radioactive decay, the moon was supposed to have cooled down rather uniformly, leaving a surface “as smooth as a billiard ball,” Zuber said.

Instead, Clementine’s 71-day rendezvous with the moon early this year revealed a global topography marked by steep peaks rising 10 miles higher than the lowest valleys. These unexpectedly craggy features may have frozen into place as parts of the moon’s surface cooled rapidly while others remained hot enough to melt.

Bouncing light off the moon in 11 different visible and infrared wavelengths, the spacecraft produced more than a million images revealing extreme variations in both the thickness of the crust and the rigidity of the surface. The deepest valley--the floor of the continent-size South Pole Aitkin Basin--is rimmed by the moon’s highest peaks, suggesting that an enormous object may have crash-landed at an usually low angle, plowing up mountains in its path.

“It might be serendipity,” Zuber said. “But it leads us to believe that impacts might have played a far greater role in the moon’s evolution than we thought.” The basin dominates the dark side of the moon that always has its back turned to Earth.

Nowhere is the fire-and-ice scenario more evident than in the newly mapped South Pole region. Evidence of relatively recent lava flows in the Schroedinger Basin suggest that the moon was erupting with volcanoes perhaps 2 billion years after it was supposed to have settled into staid middle age.

Closer to the pole, a large depression rotates about the moon’s axis shadowed from sunlight, making it a promising site for ice deposits. Ever since scientists found convincing signs of ice at the poles of hell-like Mercury--”the last place in the solar system you’d look for water,” according to Pieters--speculation about ice on the moon has been rampant, especially given its practical appeal as a water source for future lunar missions.

Even though liquid water does not exist on the moon, ice could accumulate, molecule by molecule, in the cold, shadowed regions at the poles. “It would be like a rock,” said planetary geologist Eugene Shoemaker, whose name became a household word when a comet named for his astronomer wife, himself and a colleague produced fireworks on Jupiter earlier this year. “It’s too early to say whether it’s really ice, but the data is tantalizing.”

Equally intriguing is the new light shed on a medieval sighting of fireworks on the moon recorded by eight Canterbury monks. On July 18, 1178, they wrote (in Latin, of course), “The upper horn of the new moon seemed to split in two and a flame shot from it . . . (after which) the moon . . . writhed . . . and throbbed like a wounded snake.” In 1976, space scientist J.B. Hartung suggested that this description was exactly what would have been seen had the monks chanced to witness a large impact on the lunar surface.

Now Clementine has come a few steps closer to confirming the monks’ observations. The crater in question--called Giordana Bruno--”is very fresh,” Pieters said. Its soils are just beginning to take on the reddish hue that accumulates after years of space weathering. The question is, how many years?

“The Clementine data gives new information, but we still don’t know how long it takes material in the lunar environment to show these signs of age,” Pieters said. If the answer is millions of years, then the crater could be older than it looks; but if it’s hundreds of years--as many scientists speculate--it could be the very impact the monks recorded. Said Pieters: “Either it’s very young, or we don’t understand weathering.”

The little craft that produced all this new information was supposed to be lost and gone forever by now--true to its namesake.

Developed to test Star Wars technology, Clementine was supposed to orbit the moon almost as an afterthought; its primary mission was to home in on an asteroid and track it “while flying by really fast,” on its way out of the solar system, Pieters said.

But somewhere a failure occurred and it never kept its rendezvous with the asteroid. Clementine, Pieters suspects, is still hanging around Earth orbit. With a price tag of only $100 million and an inception-to-launch time of less than two years, it proved to be exactly the kind of fast, cheap and productive mission that NASA has been hankering for.

As for future lunar rendezvous, it is unclear whether another Clementine-type project--or any number of other possible visits to the moon--will be approved anytime soon. Said Pieters, who is optimistic that one of four lunar missions now awaiting approval might be launched within three years: “We had fun while it lasted.”