Fragment of ancient asteroid falls to Earth
[23 Aug 2001] A meteorite that plunged to Earth in January 2000 may be the first fragment ever found of a primitive kind of asteroid. The characteristics of the Tagish Lake meteorite closely match the 'D-type' asteroids common in the outer solar system, according to Takahiro Hiroi of Brown University, US, and colleagues. Meanwhile, Sandra Pizzarello of Arizona State University and team analysed the carbon-rich meteorite to shed light on the chemistry of the early solar system (T Hiroi et al; S Pizzarello et al 2001 Science to appear).
Asteroids are classified by their optical properties and their dynamics. Most meteorites are thought to be fragments of asteroids, so the spectrum and brightness of the light reflected from a meteorite helps astronomers to identify which type of asteroid it came from. The arrival time of the meteorite on Earth is also a crucial factor in matching it to the motion of a certain asteroid.
Hiroi and colleagues found that the Tagish Lake meteorite reflected just 3% of the light incident upon it, a similar value to D-type asteroids. The spectrum of this reflected light also very closely matched that of a particular D-type asteroid called 368 Haidea. But the team found that another candidate - asteroid 773 Irmintraud - was more likely to have lost fragments at the right time because it was more strongly distorted by Jupiter's gravity.
Hiroi and colleagues hope that better observations of D-type asteroids and more detailed studies of their dynamics will allow them to identify the parent asteroid of the Tagish Lake meteorite with greater certainty. They warn, however, that it is difficult to match meteorites with asteroids: those meteorites that reach Earth are not a representative sample because many are too fragile to survive the journey through our atmosphere.
Pizzarello and co-workers believe that the Tagish Lake meteorite - preserved in freezing conditions - is one of the most pristine meteorites to be studied. Although rich in carbon, the carbon was present in a narrow range of rather light compounds - including carboxylic acids, aromatic hydrocarbons and fullerenes - compared with meteorites studied previously. This implies that many distinct chemical processes took place in the early solar system, according to the researchers.
The team believes this discovery is consistent with a parent asteroid that initially contained little carbon. The abundance of chain-like carboxylic acids would have formed as it acquired carbon and water from interstellar dust, before it ejected the meteorite.