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Astronomy and space

Astronomy and space

Rock blasts in from the past

01 Dec 2000

Fragments of a meteorite recovered from a frozen lake in Canada earlier this year may provide a unique insight into the evolution of the solar system and life on Earth.

The Tagish Lake meteorite

 

Rocks from space have had a bad press recently. The action movies Armageddon and Deep Impact explored what would happen if the Earth was threatened by a huge asteroid impact. Somewhat more down to Earth, so to speak, was a recent UK government report suggesting that we really should take the threat of an impact seriously. The authors of the report recommended that we increase our efforts to monitor near-Earth asteroids that may endanger our civilization, or at least wipe out a city, if these rocky masses were to hit.

Now Peter Brown from the University of Western Ontario in Canada and co-workers from Canada, the US and the UK have shown that space rocks are not all doom and gloom (Science 2000 290 320).

Chip off an old block

On 18 January this year, a six-metre-wide chip of an asteroid weighing 100 tonnes hurtled through space and smashed into the Earth’s atmosphere at 16 km s-1. Over 70 people were lucky enough to see the resulting fireball travel across northern Canada. The rock fragmented in the atmosphere, scattering pieces across an area 16 km long and 2 km wide. Far from wiping out civilization, this impact missed everyone, and may now help us to figure out the origins of the Earth and of the life on it.

Once any extraterrestrial material lands on Earth, it is called a meteorite. This latest example fell near Carcross in the Yukon, and the recovered fragments were found on the frozen surface of nearby Tagish Lake. A few days later, local man Jim Brook went out onto the lake and picked up some of the frozen pieces of the meteorite. Being a keen amateur astronomer, he knew to collect the fragments without touching them, a point that proved crucial when the importance of this rock to astrobiology was later established. The Tagish Lake meteorite turned out to be unique. It appears to be one of the most pristine examples of early solar-system material that meteorists have ever studied.

Like about 4% of all meteorites, Tagish Lake – as the rock itself is called – is one of the carbonaceous chondrites, a group of meteorites that are relics from pre-planetary times in the solar system. But when viewed in more detail, it differs from all other known meteorites.

Tagish Lake has a similar chemical composition to meteorites known as CI1s, which are made from the same elements as the Sun minus the hydrogen and helium. Chemically speaking, CI1 meteorites have changed the least since they accreted from solar material at the beginning of the solar system, just under 5000 million years ago. But CI1s are not completely unscathed by their long history. They contained a lot of water when they formed, which usually completely permeates and destroys their original features and textures.

In contrast, Tagish Lake has not been so severely affected by water damage. It is more like another meteorite type, called CM2, than CI1 in appearance. CM2 meteorites have preserved much of their original textures. For example, they contain rounded fragments called chondrules, which are thought to be among the oldest solid particles to form around the Sun. Brown and co-workers have found that Tagish Lake also contains chondrules.

In other words, Tagish Lake uniquely contains both the primitive textures of CM2s and the primitive chemical composition of CI1s. It may be the first hybrid of these two meteorite groups, and Brown and colleagues have suggested it should be classified as a CI2. That means this precious sample may be the most unaltered sample of early solar-system material we have ever been able to hold.

Studying the age, chemical and isotopic composition, and the mineralogy of the meteorite will show us – perhaps better than any other rock we have in the world’s museum collections – how the early solar system evolved from a young star with a dusty disk to the Sun and planets we know today.

The meteorite is also unusual in its carbon content. It contains 5.4% carbon, more than any other known meteorite. This carbon is mainly in the form of carbonate minerals and organic compounds. Meteorites like these may have been critical to the evolution of life on Earth, bringing the basic building blocks of life to the surface of our planet billions of years ago. Studying this meteorite may provide us with vital clues to how life formed here, and perhaps how it may form in other places in the galaxy as well.

A dream come true

The meteorite could not have fallen at a better time. Over the last few years interest in the new field of astrobiology has soared. In 1996 NASA researchers suggested that a meteorite from Mars, ALH 84001, contained evidence for ancient bacteria.

Years of controversy have followed this allegation. Some scientists believe that ALH 84001 contains terrestrial organic impurities, rather than indigenous bacteria. Further work showed, in fact, that all meteorites become easily contaminated once they fall to Earth. Many researchers believe that contamination is almost inevitable, especially for meteorites that are particularly rich in carbonaceous material that provides a good source of food for bugs. The cause and effects of terrestrial contamination on extraterrestrial material is now being studied extensively, not just for meteorites but also for future sample-return space missions.

The Tagish Lake meteorite was like a dream come true for astrobiologists, as it fell in a frozen, almost-sterile area, and was quickly and expertly collected. The meteorite has since been kept frozen under controlled conditions to ensure that the carbonaceous material is altered as little as possible before analysis. Although the organic characterization is not yet complete, it is likely to show us exactly what the original building blocks of life were.

Tagish Lake serves as a reminder that rocks from space are not all bad. Although they can – and will – wipe out entire species (just ask the dinosaurs), they may also have given life to our planet in the first place. Perhaps it is time we celebrate these celestial gifts, as well as fear them.

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