Rocks on the ice.

Tramping through the woods on a dark, frigid morning in the Yukon last Jan. 18, geologist Charles F. Roots saw something brilliant flashing overhead: a flickering yellow, green, and white fireball. Then, he heard a crashing sound. Others, as far away as British Columbia and Alaska, noted a series of sonic booms, and several smelled burning sulfur. Minutes later, a long smoky trail appeared, illuminated by the rising sun. Another visitor from space had passed through Earth's atmosphere.

In several respects, this rock--fragments of which a resident of the southern Yukon Territory recovered just a few days later--represents a rare find. The meteorite belongs to a class known as carbonaceous chondrites, which make up only 2 percent of the rocks that fall to Earth and rank among the most primitive bodies in the solar system.

Moreover, the fragments fell on frozen, snow-covered terrain. Thanks to the foresight of the resident who collected about a kilogram of the material in clean plastic bags, the fragments remained frozen--boosting the odds that the rock retains whatever supply of fragile, organic compounds and water it might have carried. Resembling partially burned charcoal briquettes, the Yukon pieces are the only meteorite fragments ever recovered and transferred to a laboratory without thawing.

For these reasons, the meteorite "offers us a snapshot of the original composition of the entire solar system before the planets formed," says Michael E. Zolensky of NASA's Johnson Space Center in Houston. It's been 31 years, he notes, since the recovery of the Murchison meteorite--the last time that anyone found a carbonaceous chondrite soon after it fell to Earth.

Zolensky and his colleagues have begun analyzing samples of the Yukon meteorite. Preliminary results could be available as early as May, Zolensky says. In the meantime, residents of the southern Yukon haven't stopped talking about the spectacle. "The winters here are pretty cold and dreary," notes Roots, who works at the Geological Survey of Canada in Whitehorse, Yukon. The meteorite, he adds, was undoubtedly the highlight.