Rare Canadian Meteorite Tagged as First of Its Kind Found on Earth

The Tagish Lake meteorite on its discovery. Initial analysis showed showed the meteorite was a type of carbonaceous chondrite – a rare, organically rich, charcoal-like class of meteorites. Credit: UWO / University of Calgary

Geology is easy in one sense. You know where the rocks in question came from because you (or someone paying attention) banged them off some Earthly outcrop with a hammer.

Space geology is harder. The rocks fall to Earth. Even if you see them fall, you don’t know exactly where they came from. So planetary scientists who study meteorites spend a lot of time figuring out what those rocks are made of and where they came from.

In a triumph for space geology, a team of scientists said this week they found that not only did one space apple fall far from its tree, it’s the first of its kind to be found on Earth — a dark and rare type called a D asteroid (before it hit our atmosphere). The team’s results were published in this week’s issue of the journal Science.

The meteorite, which fell at frozen Tagish Lake in British Columbia in January 2000, belongs to a broad and common class of space rocks called carbonaceous chondrites, carbon-rich fine-grained matrixes dotted with melted-down blobs. But this is no regular carbonaceous chondrite.

"This is completely different from other types of carbonaceous chondrites," said Takahiro Hiroi of Brown University, who headed up the research. "It is completely darker and completely redder. I immediately knew this is not the same type of meteorite. It is a carbonaceous chondrite but it is a new type of carbonaceous chondrite."

This D asteroid probably originated in the mid-to-far end of the Main Belt between Mars and Jupiter, and it is thought to contain the oldest raw ingredients that came to comprise the solar system. The Tagish Lake meteorite and other asteroids are leftovers from the formation of the solar system 4.6 billion years ago so scientists want to learn their composition to better understand its genesis.

The Tagish Lake meteorite’s unique composition made it hard to classify in any of the known meteorite classes, so its possible asteroidal origin has been a mystery until now.

Reflective reasoning

ice from Tagish Lake
Porous ice makes the Tagish Lake meteors among the most fragile ever found.
Credit: University of Calgary

When the so-called Tagish Lake meteorite fell, it blazed through the sky as a fireball, sparking on-lookers to photograph the event. Hundreds of fragments were collected from frozen Tagish Lake. From observations and satellite data, researchers were able to estimate the rock’s orbit.

Scientists quickly started analyzing samples from the rock, as well, and announced last year that it was a carbonaceous chondrite from the Main Belt.

But it was Hiroi and his colleagues Carle Pieters, also of Brown University, and Michael Zolensky of NASA’s Johnson Space Center in Houston who nailed that the Tagish Lake meteorite came from a D asteroid.

D asteroids, like some of the other carbonaceous chondrites, usually reflect about only 4 percent of the sunlight that hits them. They have been identified in space but no one has ever had a sample to work with on Earth.

Once Hiroi and his colleagues got some pieces of the rock, they measured the rock’s reflectivity over a wide range of wavelengths of light from visible to near-infrared to get an optical "fingerprint" that would reveal the chemical composition of the Tagish Lake meteorite.

They found its reflectivity was actually lower than that of most carbonaceous chondrites — around 2 percent. Hiroi also found that it reflected infrared light better than visible light. It was that finding which distinguished the rock from other carbonaceous chondrites.

A close neighbor?

animation of Tagish meteorite smoke trail
Animated gif showing Tagish meteorite smoke trail recorded over 14 minutes on January 18, 2000.
Credit: Ewald Lemke (Atlin Realty, Atlin, British Columbia)

Hiroi then compared the Tagish Lake meteorite’s reflectivity with a few asteroids in the Main Belt and found one that was a close match located close to something called the Kirkwood Gap. Such asteroids orbit the Sun two or three times exactly for every single orbit of Jupiter — in other words they are in a resonance orbit with Jupiter. That orbit brings the rock periodically closer to Jupiter, which can gravitationally throw the rock’s orbit off permanently, flinging it toward the Sun or even out of the solar system.

Hiroi identified a D asteroid presently orbiting quite close to the Kirkwood Gap — at .03 astronomical units or AUs (an AU is the distance from Earth to the Sun — 93 million miles) — as a possible source for the Tagish Lake meteorite. An impact to the asteroid could have dislodged the Tagish Lake meteorite, Hiroi said.

The D asteroid Hiroi found in space and the Tagish Lake meteorite share another feature besides reflectivity. They both are very fragile and porous compared to most asteroids. Most asteroids with such a low density would be destroyed either upon entering Earth’s atmosphere or upon striking the planet.

Hiroi thinks the Tagish Lake meteorite survived in such large fragments because its fall was cushioned by ice and snow.

Asteroids are classified by content into other types assigned letters of the alphabet like C, G and E. Now that Tagish Lake has been identified as a D asteroid, there remains just one type of carbonaceous chondrite which scientists have yet to find fallen to Earth and study — a P asteroid.