By KENNETH CHANG
Published: August 18, 2009
For the first time, a building block of proteins — and hence of life as we know it — has been found in a comet.That adds to the prevailing notion that many of the ingredients for the origin of life showered down on the early Earth when asteroids (interplanetary rocks orbiting the inner solar system) and comets (dirty ice balls that generally congregate in the outer solar system beyond Neptune) made impact with the planet.
In the new research, scientists at the Goddard Space Flight Center, in Greenbelt, Md., detected the amino acid glycine in comet bits brought back in 2006 by the NASA space probe Stardust.
“It tells us more about the inventory of organics in the early solar system,” said Jamie Elsila, an astrochemist at Goddard who led the research.
Amino acids are small molecules that, when strung together into chains, form a diversity of proteins. For four decades, scientists have found a multitude of amino acids in some meteorites, the bits of asteroids that land on Earth. More recently, astronomers reported that amino acids might float throughout the cosmos, a belief resulting from their detection of the color signatures of glycine, the simplest of the amino acids, in distant interstellar gas clouds.
Some doubts remain about that claim, but if it is true, it would then not be surprising that when the clouds condense into stars and planets, the building blocks of life might be readily available there.
As for our solar system, meteorite data show that amino acids are present in its inner neighborhood, where asteroids orbit, but until now nothing has been known for certain about what might have formed farther out, where comets gather.
But on Jan. 2, 2004, the Stardust spacecraft flew through the tail of dust and gas of the comet Wild 2 (pronounced vilt two). Two years later, the probe returned to Earth, sending collected samples to the ground by parachute for scientists to analyze. Comets are thought to preserve material of the early solar system, largely unchanged for the last 4.5 billion years.
Within a few months, the Goddard scientists found glycine embedded in aluminum foil of the collecting apparatus. They had spent the time since then confirming that the glycine indeed came from the comet and not from contamination.
“It’s not necessarily particularly surprising,” Dr. Elsila said of her extraterrestrial glycine in a phone conversation Tuesday. “I would have been surprised if it wasn’t there.”
Dr. Elsila and her colleagues were able to show that the glycine from the comet had heavier quantities of the isotope carbon 13 than what occurs on Earth. They also detected a second amino acid, beta-alanine, but the quantities were too minuscule to confirm.
The findings were presented Sunday at a Washington meeting of the American Chemical Society and will be published in the journal Meteoritics & Planetary Science.
Donald E. Brownlee, a professor of astronomy at the University of Washington and principal investigator of the Stardust mission, said the discovery indicated that the chemical reactions that produce glycine, and presumably other amino acids, occurred throughout the early solar system.
“That means production of amino acids is fairly common,” Dr. Brownlee said.
That had not been a foregone conclusion, he said. Some scientists had suggested that the chemical reactions might have required warm and wet conditions that existed in early asteroids but not comets.
