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Meteorite hints at life’s origins

As debate continues to swirl around arsenic-loving bacteria, a space rock yields new astrobiological clues.

Jun 9, 2011
Tia Ghose

Artist's depiction of early earthNASA/JPL-CALTECH

Organic compounds from a meteorite may hold clues to the origin of life on Earth, according to a study published today (June 9) in Science. Water on the asteroid reacted with the rock to form organic compounds—including many scientists believe are the crucial ingredients that sparked life in Earth’s primordial oceans about 4 billion years ago.

“It’s real evidence of hydro-synthesis occurring in asteroids and creating compounds that might be biologically useful,” said Mark Sephton, a geochemist at Imperial College London, who was not involved in the study.

The space rock was discovered in 2000, after a meteoroid blazed through the atmosphere and fell in pieces to the frozen surface of Tagish Lake, in Northern British Columbia. A local man gathered nearly two pounds of fragments; to avoid contamination and to preserve them, he didn’t touch them and kept them frozen for years. In 2008, a consortium of Canadian research institutions purchased them for $850,000.

Christopher Herd, a University of Alberta planetary geologist, and his colleagues suspected the pristine, carbon-rich meteorite might hold clues to how Earth life began billions of years ago, when the planet was bombarded by space debris. Many scientists believe those meteor showers provided the quantities of carboxylic acids, amino acids, and amines necessary to create life in the primordial soup of Earth’s ancient seas.

While photographing the sample, the team identified four different types of rock, with some salt and pepper portions and others that appeared sooty, Herd said. They used techniques such as mass spectrometry to describe each rock type’s chemical composition.

With that data, the group was able to reconstruct the rock’s history. Ice combined with stellar dust as the asteroid formed in space, and radioactivity heated some of that ice, causing water to seep through the asteroid. The organic compounds were created in the process.

The analysis also suggested that the amino acids formed directly on the asteroid, rather than existing in space before the asteroid’s formation, Herd said. Fragments that were almost untouched by water had lower concentrations of amino acids than those with slightly more exposure. Sooty portions of the asteroid that were highly exposed to water showed barely any traces of amino acids, he said.

“You need a little bit of water, but not too much. There’s kind of a Goldilocks zone in there, a sweet spot.”

An alternative interpretation of the data is that the compounds formed when a life form from outer space was degraded, said Chandra Wickramasinghe, an astrobiologist at the Cardiff Center for Astrobiology, who was not involved in the study.

But both Herd and Sephton disagreed with that interpretation.

The meteorite doesn’t have the patterns that you see in biological material, even degraded life forms, Sephton said.

C.D.K Herd, et al., "Origin and evolution of prebiotic organic matter as inferred from the Tagish Lake meteorite," Science, 332:1304-07, 2011.

 

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