January 19, 2011

Earth’s Amino Acids Could Have Come From Asteroids

According to new NASA research, a wider range of asteroids were capable of creating the kind of amino acids used by life on Earth.

Researchers at NASA's Goddard Space Flight Center reported in 2009 that they discovered an excess of the left-handed form of the amino acid isovaline in samples of meteorites that came from carbon-rich asteroids. 

NASA said that this discovery suggests that left-handed life got its start in space, where conditions in asteroids favored the creation of left-handed amino acids.

Amino acids come in two varieties that are mirror images of each other.  Earth uses the left-handed kind exclusively, but scientists say that right-handed amino acids would work fine.

"This tells us our initial discovery wasn't a fluke; that there really was something going on in the asteroids where these meteorites came from that favors the creation of left-handed amino acids," Dr. Daniel Glavin of NASA Goddard said in a statement.  Glavin is lead author of a paper about this research published online in Meteoritics and Planetary Science January 17.

"This research builds on over a decade of work on excesses of left-handed isovaline in carbon-rich meteorites," Dr. Jason Dworkin of NASA Goddard, a co-author on the paper, said in a statement.

"Initially, John Cronin and Sandra Pizzarello of Arizona State University showed a small but significant excess of L-isovaline in two CM2 meteorites. Last year we showed that L-isovaline excesses appear to track with the history of hot water on the asteroid from which the meteorites came. In this work we have studied some exceptionally rare meteorites which witnessed large amounts of water on the asteroid. We were gratified that the meteorites in this study corroborate our hypothesis," explained Dworkin.

According to Glavin, L-isovaline excesses in these water-altered type 1 meteorites suggest that extra left-handed amino acids in water-altered meteorites are more common than previously thought.

The researchers said that there are several processes to create extra left-handed amino acids, and it will take more research to identify the specific reaction to them.

However, "liquid water seems to be the key," Glavin wrote. "We can tell how much these asteroids were altered by liquid water by analyzing the minerals their meteorites contain. The more these asteroids were altered, the greater the excess L-isovaline we found. This indicates some process involving liquid water favors the creation of left-handed amino acids."

Another clue comes from the total amount of isovaline found in each meteorite.

"In the meteorites with the largest left-handed excess, we find about 1,000 times less isovaline than in meteorites with a small or non-detectable left-handed excess. This tells us that to get the excess, you need to use up or destroy the amino acid, so the process is a double-edged sword," says Glavin.

He said that the water-alteration process only amplifies a small existing left-handed excess, it does not create the bias. 

Something in the pre-solar nebula created a small bias toward L-isovaline and presumably many other left-handed amino acids as well.


Image 1: This artist's concept uses hands to illustrate the left and right-handed versions of the amino acid isovaline. Credit: NASA/Mary Pat Hrybyk-Keith

Image 2: This is a photo of a carbon-rich meteorite analyzed in the study. Credit: Antarctic Meteorite Laboratory/NASA Johnson Space Center


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