For 40 years, the rocks hauled back from the moon have been changing the history of the Earth as we knew it.

The lunar rocks suggest that a smaller planet slammed into ours 4.5 billion years ago, creating the moon and enlarging the Earth. They tell the tale of a storm of space debris violently pounding both the Earth and moon, perhaps triggered by a dramatic reshuffling of the entire solar system.

Scientists today are still studying those rocks, hoping to decipher whether life had already emerged before the near-apocalyptic pummeling 3.9 billion years ago - and, incredibly, survived.

"Nobody in their wildest dreams thought the program would be that successful," said Gary Lofgren, a NASA geologist who helped teach the Apollo astronauts how to spot the most interesting rocks.

They ended up collecting 842 pounds of rock and soil, and Lofgren, who is now NASA's curator of lunar samples, sends several hundred pieces a year to scientists probing for information.

If anything, the study of the moon rocks is picking up today, said David Kring, a researcher at the Lunar and Planetary Institute in Houston. "It may be that our best hope of understanding the first half-billion years of Earth's history will be found on the lunar surface," Kring said.

Some scientists suspect that life originated during that period. So reconstructing the Earth's early history could tell scientists something about the conditions under which life might arise elsewhere in the universe.

Lessons from the Apollo samples are filtering through other fields as well. In the 1970s, for example, scientists learned from the rocks that asteroid impacts leave iridium and other trace metals. In 1980, the discovery of iridium on Earth was used to substantiate the theory that an asteroid killed off the dinosaurs.

This year happens to be the 400th anniversary of Galileo's observation that the moon was solid, its surface a landscape of mountains and valleys. For centuries, the prevailing view had been of the moon as a perfect orb, made from some sort of heavenly ether.

Even Galileo's new telescope could not solve the mystery of the moon's origin, however. By the early 1960s, Lofgren said, there were a couple of theories.

Some scientists proposed that the moon was an asteroid that had wandered too close to Earth and was captured in its gravitational field. Others thought it was formed simultaneously with the Earth.

Many of the moon rocks that the astronauts brought home did look earthly and ordinary, Lofgren said - crystallized lavas called basalts, as well as granite-like anorthosites. There were also glassy rocks that looked like they'd been formed in asteroid impacts.

But the samples didn't lend much support to either major theory of the moon's formation. Analysis of the rocks showed that the moon wasn't a stray asteroid. And while its composition was similar to the Earth's, some key elements - iron and other heavy metals - were lacking.

"It looks like you could make the moon if you skimmed off a piece of the Earth's mantle and crust and took away all the water," said Steve Mojzsis, a geologist at the University of Colorado, Boulder.

That, in a nutshell, is what scientists proposed as the new theory of the moon's creation. Soon after the Earth formed, they reasoned, it was slammed by a smaller planet about the size of Mars. From the wreckage emerged a bigger Earth - and a huge ring of orbiting debris.

The debris coalesced to form the moon.

While this picture of the moon's origin isn't proven, it is the leading theory today.

The rocks yielded more surprises when scientists began to decipher their ages. Using a technique that relies on the steady decay of uranium into lead, they found that most of the samples from the lunar surface appeared to have crystallized from molten rock 3.9 billion years ago - a long 600 million years after its likely birth.

Something must have caused much of the moon to melt, said planetary scientist Bill Bottke, who has studied moon rocks at Southwest Research Institute's Boulder, Colo., office. Most likely, he said, was a pummeling by meteors that punched out most of the moon's craters.

The Earth, in the same vicinity, experienced similar pounding, Bottke said. But much of the evidence was covered up as plate tectonics and erosion recycled the planet's crust.

Indeed, the findings from the Apollo missions suggest that the Earth around this time would have been hit by dozens of asteroids or comets more than 100 miles in diameter, each one punching a hole in the crust, melting much of the planet's surface, and kicking up an atmosphere of vaporized rock.

What triggered all this punishment?

In 2005, scientists in Nice, France, devised an explanation. The chaos of the developing solar system had started to calm around 4.4 billion years ago, although the planets were in different orbits than they are today. Jupiter was farther out; Saturn, Uranus, and Neptune were closer.

Calmer, perhaps, but not quite stable, said Bottke. Giant Jupiter's gravity tugged on giant Saturn, distorting both orbits.

"That caused the planets to go berserk," Bottke said. In an abrupt shift, Jupiter moved in; Saturn, Uranus, and Neptune moved out.

The tumult disturbed nearby belts of comets and asteroids, sending some careening past - and slamming into - Earth, Mars, and the moon.

Most scientists have long assumed that even if primitive life forms had emerged that far back, they could not have survived such an attack.

But recent discoveries involving isotopes of carbon and other elements give strong evidence that life existed at the tail end of the debris storm, around 3.83 billion years ago, said Mojzsis, the Colorado geologist.

Mojzsis has found evidence of liquid water - through the presence of a type of mineral that requires water to form - in rocks that are far older.

"Our work with zircons shows the Earth was an eminently habitable place back to 4.4 billon years ago," he said. The Earth is about 4.5 billion years old.

Mojzsis and colleague Oleg Abramov recently modeled a worst-case scenario for the heavy pounding on Earth. If life had existed at the time, they found, a few organisms might have pulled through.

While the near-apocalyptic bombardment would have vaporized anything on the surface, said Abramov, no single impact would have melted all of the Earth's crust at once.

Bacteria today live in underwater heat vents that are close to boiling; the team calculated that similar organisms might have thrived back then. Perhaps all the extra energy even helped spark the origin of life.

"Maybe this was not an Armageddon but a Genesis," Abramov said.

With such big questions still open, Lofgren expects to be lending moon samples from his collection at NASA's Johnson Space Center in Houston to researchers around the world for some time.

The scientists all say they hope to get new rocks eventually - perhaps in 2020, theoretically the date of the next moon mission, and possibly, some time, from Mars.

"There's an amazing amount of information you can pack into a rock," said Bottke.

Contact staff writer Faye Flam
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