A visitor from the early solar system
On January 18, 2000, at 8:43 AM local time, a fireball brighter than the dawn sun exploded over northwestern Canada. The object, a meteoroid estimated to be four meters in diameter and weighing 56 tonnes, detonated in the upper atmosphere with the force of about 1.7 kilotons of TNT. This explosion scattered thousands of dark, charcoal-like fragments over a vast area, with many landing on the frozen surface of Tagish Lake in British Columbia.
Just one week later, a local resident named Jim Brook discovered the first fragments on the ice. His quick thinking to collect the pieces without bare hands and keep them frozen was immensely fortunate for science. Because the fragments were recovered from the ice within days and remained frozen, they are considered among the most pristine meteorite samples ever collected. This rapid, sterile collection prevented significant terrestrial contamination and preserved the volatile organic compounds and water ice from space.
In total, over 500 fragments were located across a strewnfield measuring at least 16 by 3 kilometers. Despite this large number, only about 10 kilograms (22 pounds) of the estimated 1.3 tonnes that survived atmospheric entry were ever recovered. The rest of the original 56-tonne object vaporized, creating noctilucent clouds visible hours later.
The record of prebiotic chemistry
The Tagish Lake meteorite is classified as a C2 ungrouped carbonaceous chondrite, a rare type that makes up a small fraction of all meteorite finds. Its age is estimated at 4.5 billion years, dating back to the formation of the solar system. The rock is exceptionally dark and has a low density, appearing similar to a charcoal briquette. It is composed of two different rock types, one rich in carbonates and one poor in them.
Analysis of the meteorite's composition shows the early solar system. It contains presolar grains—dust particles that predate our own sun—and organic material. These organics include soluble compounds like carboxylic acids and pyridine carboxylic acids, but surprisingly few amino acids compared to other carbonaceous chondrites like the Murchison meteorite. This chemical signature suggests a different path of chemical evolution in the part of the solar system where its parent body formed.
The meteorite's parent body is thought to have been a D-type asteroid, possibly 773 Irmintraud, from the outer reaches of the asteroid belt. Some evidence even suggests an origin farther out, perhaps in the Kuiper Belt, the region where many comets originate. This is supported by the meteorite's high ratio of carbon dioxide to water, which is more similar to comets than to other known carbonaceous chondrites. The water it contains is isotopically different from Earth's water, confirming its extraterrestrial origin.