Older than the sun, the meteorite scientists call 'the real time machine'

Ian Sample, science correspondent
Friday December 1, 2006
The Guardian

As lumps of rock go it looks much like any other, unexceptional despite the deep red of its cool, smooth surface. The pieces range in size from pea-sized lumps to larger fist-sized chunks. But today, scientists will announce this is no ordinary stone. Prised from a frozen lake in northern Canada, it has become a prime candidate for the oldest known object on Earth.

The chunk came from a meteorite that scored an arc of fire across the skies before slamming into Lake Tagish in British Columbia in 2000. It has been pored over by scientists ever since, and is today revealed to contain particles that predate the birth of our nearest star, the sun.

The Tagish Lake meteorite was already regarded as exceptional because its mineral composition linked it to the earliest days of the formation of the solar system, more than 4.5bn years ago. The fragments of meteorite that still exist are among the most pristine in the world, as they were protected from contamination when they became wedged in blocks of lake ice.

The latest research shows that peppered throughout the meteorite are grains that formed even earlier, in a frigid cloud of molecules, possibly at the edge of the swirling disc of dust that ultimately collapsed to form the sun and all the planets of the solar system.

The discovery suggests that while the first light from the sun fell on the fledgling Earth, as the dinosaurs rose and died out and humans gained dominance, the meteorite was hurtling around the heavens on a billions-of-years-long journey destined to terminate with a thud in Yukon territory.

Researchers at Nasa's Johnson Space Centre in Houston examined a two gram fragment of the meteorite and focused on tiny, hollow, carbon spheres embedded within it. Each "globule" measured just a few thousandths of a millimetre across.

Using electron microscopy and isotope tests, the scientists looked at the chemical make-up of the grains and discovered they had unusual ratios of different forms of nitrogen and hydrogen. Ratios of the isotope nitrogen-15 to nitrogen-14 were nearly twice those on Earth, while the ratio of deuterium, a heavy form of hydrogen, to normal hydrogen, was between 2.5 and nine times higher than usual.

Reporting in the journal Science today, a team lead by Keiko Nakamura-Messenger and Michael Zolensky show the levels of the isotopes in the meteorite could only arise from chemical reactions taking place in an extremely cold climate, where temperatures were as low as -260C. Those conditions would only be found in remote molecular clouds before the formation of the solar system, or at the very edge of what is known as the protosolar disc that was later to coalesce into the celestial bodies of the solar system. "These little particles within the meteorite seem to predate everything else. We don't know exactly how old they are, but they could be billions of years older than the rest of the meteorite," said Dr Zolensky.

Between 40,000 and 60,000 tonnes of meteorite matter is believed to land on Earth every year, and around 90% of this rains down steadily as fine particles that are rarely even identified.

Much of the material immediately disappears beneath the waves, and significant amounts are lost in the world's deserts and forests. Only a few tens of kilograms, in larger chunks, are usually recovered from any year's fallout.

Fragments of the Tagish Lake meteorite were recovered after locals spotted the fireball it created as it tore through the atmosphere at 20 miles per second. Large clumps of the meteorite were collected from the surface of the frozen lake, but other chunks were removed later embedded in blocks of ice, and transported to research labs. Around one tonne of fragments from the meteorite is now held in the Natural History Museum in London and at other sites in the US, Canada and Germany.

"These are the real time machines, the material that goes back to the earliest formation of the solar system," said Caroline Smith, meteorite curator at the Natural History Museum.

The meteorite is known as a carbonaceous chondrite and contains what many scientists regard as the building blocks for life: carbon, myriad clay minerals and even amino acids. Scientists say the clay layers, principally silicates, can form protective pockets around the organic chemicals and act as reaction chambers where more complex molecules can form. The possible role of these pockets in the ultimate emergence of life has lead some scientists to refer to them as "wombs".

"These things tell us what kind of chemicals are out there in interstellar space. They could have been the original seeds for life to get started," said Dr Zolensky.