Many types of asteroid could have created the kind of amino acids used by life on Earth to build proteins and regulate chemical reactions, according to new NASA research.
Amino acids come in two varieties that are mirror images of each other and functionally identical - but life on Earth uses the left-handed kind exclusively. It's been widely speculated that this 'monochirality' could show that life on Earth could have been triggered by asteroids.
In March 2009, NASA reported the discovery of an excess of the left-handed form of the amino acid isovaline in samples of meteorites that came from carbon-rich asteroids.
And now it says its found an excess of left-handed isovaline (L-isovaline) in a much wider variety of carbon-rich meteorites.
"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," says NASA's Dr Daniel Glavin.
Liquid water seems to be the key.
"We can tell how much these asteroids were altered by liquid water by analyzing the minerals their meteorites contain," says Glavin. "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," he says.
Whatever it may be, the water-alteration process only amplifies a small existing left-handed excess - it doesn't create the bias. Something in the pre-solar nebula seems to have created a small initial bias toward L-isovaline and presumably many other left-handed amino acids as well.
One possibility is radiation. And it's possible that other young solar systems encountered different radiation that favored right-handed amino acids - leading to life based on these instead.