The Tagish Lake meteorite may be one of the most primitive solar system materials yet studied.

On the morning of 18 January 2000, a small asteroid slammed into the Earth’s atmosphere over northwestern North America. Hundreds of thousands of people witnessed its last moments – as did American military satellites. When correlated together, these observations made it easy to pinpoint a debris zone.
As the object approached the Earth’s surface the shock of entering the atmosphere caused it to shatter into many pieces. A sizeable portion of the meteorite’s fragments landed on an ice-covered lake in British Columbia – Lake Tagish – and were quickly encased in ice.

Despite a rather clear idea where pieces would have hit the Earth, it was several months before fragments were located and identified. Eventually, more than 500 fragments were recovered – many from the ice covering Lake Tagish. Not only was the entrapment in ice fortuitous for meteorite hunters who would arrive to locate the pieces months later, it was also a boon to those who would later study the fragments. A dedicated amateur had the foresight to exercise extreme care in collecting the meteorite samples – including keeping them encased in the ice within which they had come to rest. His actions were to allow others to open a door into an early portion of our solar system’s history.

The Lake Tagish meteorite, as it was later to be named, is a carbonaceous chondrite, a comparatively rare type of meteorite rich in organic compounds. By virtue of having been encased in ice almost instantly upon arrival on Earth, the chance of terrestrial contamination was dramatically reduced. Moreover, despite its blazing descent into our atmosphere, some of the materials within the meteorite may never have been appreciably warmed from their long term refrigeration in interplanetary space. As such, their potentially pristine condition held out the promise of an unusual glimpse into the structure of its parent body – and that of the early solar system.

After months of study in laboratories around the world, many of the hopes pinned on the examination of this unusual specimen have come to fruition.
Last month, scientists at Purdue University have presented a paper at the Meteoritical Society’s annual meeting in Chicago detailing the results of their analysis of fragments of the Lake Tagish meteorite.

This week, the research team published their findings in the journal Science. According to Peter Brown, a professor in Western’s Department of Physics and
Astronomy, “We can now say that this may be the ‘crown jewel’ of meteorite finds. This discovery will aid scientists in the reconstruction of the early solar system.” According to Neil MacRae, earth sciences professor at Western
and a co-author on the Science paper: “The standard composition of the solar system is partly defined by the most primitive meteorite in existence. If our results are proven correct, this new discovery will ultimately change that definition.”

The Tagish meteorite is 4.5 billion years old – dating from the time when the Earth was forming. While it was readily categorized as a carbonaceous chondrites, detailed analysis showed it to have unusual chemical properties – properties that suggest that it originated in a portion of our solar system heretofore unexamined.

According to the article in Science “the preatmospheric mass of the Tagish Lake meteoroid was about 200,000 kilograms. Its calculated orbit indicates affinity to the Apollo asteroids with a semimajor axis in the middle of the asteroid belt, consistent with a linkage to low-albedo C, D, and P type asteroids. The mineralogy, oxygen isotope, and bulk chemical composition of recovered samples of the Tagish Lake meteorite are intermediate between CM and CI meteorites. These data suggest that the Tagish Lake
meteorite may be one of the most primitive solar system materials yet studied.”

According to a Purdue University press release in September 2000, Purdue Chemistry Professor Michael Lipschutz said “this meteorite is unique, and represents a new kind of planetary material. From the chemical composition, it’s reasonable to assume it came from parts of the solar system different than the ones that produced other carbonaceous chondrites previously studied.” Lipschutz says the oxygen isotopes in the sample place the Tagish Lake meteorite somewhere between the subtypes called CI and thermally metamorphosed CM, putting the sample in a class by itself. “Thermally metamorphosed meteorites came from parts of their parent bodies that went through some type of major heating experience that caused some volatile elements to vaporize,” he said. “The fact that it’s not thermally metamorphosed means that this meteorite is much more closely related to the CI meteorites than to any other kind of meteorite.”

According to Lipschutz “CI meteorites are considered a “measuring stick” of sorts in cosmochemistry because they contain a chemical composition similar to the outside surface of the Sun. The Tagish Lake meteorite is, in fact, a sample of the pre-solar nebula, out of which the planets formed. We have never before had a sample of this material.”

The significance of the Tagish Lake Meteorite discovery? According to this week’s issue of Science: “Primitive meteorites provide a glimpse into the early history of our solar system, but some of the most primitive meteorites are also rarely found on Earth. Well-preserved organic matter in the Tagish Lake meteorite provides a unique opportunity to study the nature and origin of organic matter that may have accreted on early Earth and played a role in the origin of life.”

Ancient samples from our solar system’s youth seem to be appearing all over the place recently.

Results from the NEAR Shoemaker mission to asteroid 433 Eros show that the asteroid is very ancient. So ancient, in fact, that it may have witnessed the earliest days of our solar system’s history. Last month, Dr. Andrew F. Cheng of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which manages the NEAR mission said “we can now say that Eros is an undifferentiated asteroid with homogeneous structure, that never separated into a distinct crust, mantle and core. We have definitive mass and density measurements plus spectacular images and movies showing ridges, pits, troughs and grooves that provide fascinating clues about its history.”

In June, scientists at the University of Manchester and the Natural History Museum in London announced that they had found salt crystals
containing brine water inside a meteorite Radioisotope dating shows that the crystals formed within 2 million years of the birth of our solar system. The implications of this discovery are rather profound. First, it would indicate that the dust and gas from which our solar system coalesced began to clump together much sooner than was previously thought. Secondly, it would seem
that the conditions (or at least the prime ingredients) required for the origin of life may have existed at a very early period of solar system formation.

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