Jim Brook didn't actually see the fireball himself. Nor did he hear the thunderclap. On the morning of Jan. 18, 2000, he happened to be running an errand in Whitehorse, a town in the northeast corner of British Columbia about 10 miles from his home on Tagish Lake, where he and his elderly mother are the only year-round residents. He wasn't too far away, however, to see the curlicue trails of dust left in the sky. Being a meteorite buff, he knew precisely what had happened. "Frankly, I had kind of given up ever finding any trace of a meteorite," he says, "because they have to be so big to make it through the atmosphere. I thought, 'This must be a fairly large piece coming down somewhere'."
The next day, Brook got into his plane and began scouring the countryside for traces. Luckily, snow hadn't fallen for two weeks, and if any blackened fragments had landed on one of the area's many lakes, whose surfaces were frozen as smooth as windowpanes, he just might spot them. He found nothing that day, or for the next week. Then one afternoon as he was driving his truck across the ice of Tagish Lake (a common practice in this remote area 20 miles from the nearest road), he found them: dozens of fragments, none bigger than a potato. He wrapped them in plastic, took them home and put them in his freezer.
Brook wasn't the only one tracking the meteorite. The North American Aerospace Defense Command, the folks who watch the sky for missiles and suspicious satellites, quickly relayed data about the meteorite's trajectory to Peter Brown, an astronomer at the University of Western Ontario in London. He and Brook hooked up and arranged to have the samples flown to NASA's Johnson Space Center in Houston, Texas, keeping them frozen all the while.
There was a reason for the delicate handling. In a paper published in last week's journal Science, Brown and his co-authors present what may be the most important meteorite find of this century and the last. The Tagish Lake meteorite, he says, is most likely the purest sample of the early solar system anybody has ever seen. To get a feeling for how extraordinary a find it is, consider the odds. Most meteorites vaporize from frictional heat long before they reach the ground. This one made it through because, to begin with, it was exceptionally large--200 tons, of which 199 were consumed in the fireball and explosion.
Tagish Lake is also a particularly rare type of meteorite. Most meteors formed about 4.5 billion years ago, when our solar system was still a large cloud of hot gas collapsing in on itself, turning faster and faster and eventually flattening out into a disk, which later cooled into the sun and planets. Most meteorites were formed in this presolar gas, but 98 percent of them have been melted and refrozen so many times that they bear little resemblance to what they originally were. So-called carbonaceous chondrites, on the other hand, are pristine. They retain the carbon, water and other light minerals and gases of the early solar system. That is why they are so valuable.
Even among these rare meteorites, however, Tagish Lake is a still greater rarity. Scientists have found hundreds of carbonaceous chondrites in the last 150 years, but only five of them were the purest of the pure. Tagish Lake, by all measures, is purer still--so pure that scientists believe it should occupy a category of its own. "It is the most original snapshot of the material of the early solar system we have," says Brown. "It is unique."
The meteorite came down from the northwest, streaked over the Yukon and exploded about 40 miles north of Whitehorse, sending fragments another 15 miles or so to Tagish Lake. Had it missed the lake it might have been lost in the mountains. Had it come in the spring, it would have sunk into the water. Snow would have hidden the fragments. Rain would have turned them to mud and washed them away. Had Brook not known to keep the samples frozen, gases trapped in the rocks since the dawn of the solar system would have evaporated, diminishing their value. "It's almost as though somebody were pointing and saying, 'You really want to find these'," says Brown, "almost like predetermination."
What exactly do scientists hope to find in the Tagish Lake meteorite? Knowing more about the composition of the early solar system will help them piece together Earth's history, which wind, water and shifting tectonic plates have conspired to erase. Scientists believe that the moon was formed when a planet smacked into the young Earth and vaporized it, turning it into a lopsided cloud that cooled into the moon and the present-day Earth. New meteorites, much like Tagish Lake, would have had to replenish the water and other volatile chemicals that the collision stripped away.
Why, too, did the solar system form the way it did, with nine planets and a sun? One theory holds that when the solar system was still a cloud of gas, a nearby supernova (exploding star) blew it this way and that. Tagish Lake, in fact, contains interstellar dust, diamond grains that might have been formed in the shock wave of a supernova.
And then there is the ultimate question: where did life originate, and how? If it started on Earth, knowing the precise mix of organic materials that meteorites were depositing would come in handy. If life began elsewhere, where better to look than in a meteorite, such as Tagish Lake, that contains an accurate sample of the original brew? "You have a house standing there," says Michael Zolensky, a cosmic minerologist at NASA's Johnson Space Center in Houston, Texas, "but you don't have the blueprints. You don't even know what materials the builder started with." Tagish Lake is the most accurate sample so far of those materials.
It will take scientists many years, perhaps decades, merely to figure out what Tagish Lake is made of. Along the way they will no doubt answer many of the small questions only a scientist could love. The big answers will come, as they always do, in their own time.