A mysterious bias in the way the building blocks of proteins twist could be due to supernovas, researchers now suggest.
If correct, this could be evidence that the molecules of life weren't created on Earth, but came from elsewhere in the cosmos.
Organic molecules are often chiral, meaning they come in two versions that are mirror images of each other, much as right and left hands appear identical but possess reversed features.
Curiously, on Earth, the amino acids that form the proteins for life are virtually all "left-handed," even though it should be as easy to make one version as the other. Even more strangely, samples of certain amino acids obtained from the Murchison meteorite were mostly left-handed also, suggesting there could be a bias for left-handed amino acids throughout the rest of the cosmos.
Now researchers suggest that supernovas might be the culprits behind this mysterious effect. The key lies in the nitrogen atoms common to all amino acids, explained researcher Richard Boyd, a nuclear astrophysicist at Lawrence Livermore National Laboratory, and his colleagues.
As stars collapse right before they become supernovas, they generate an intense burst of electron antineutrinos that the researchers suggest would preferentially interact with nitrogen atoms in right-handed amino acids. All atoms possess "spin," and the handedness of an amino acid can influence how the spin of the nitrogen atoms within them align.
The antineutrinos, possessing a spin of their own, would prefer to interact with the way nitrogen atoms spin in right-handed amino acids rather than left-handed ones, since the spins of the antineutrinos and nitrogen atoms would align.
As a result, the antineutrinos would preferentially convert the nitrogen atoms in right-handed amino acids into carbon atoms. Boyd and his colleagues suggest this would result in the destruction of right-handed amino acids, leaving only the left-handed versions behind.
It might be possible to run experiments using intense neutrino sources, such as the Spallation Neutron Source at Oak Ridge National Laboratory, to test whether this effect actually occurs, Boyd added.
Supernovae also would generate electron neutrinos possessing opposite spin. This would have an effect on nitrogen atoms in left-handed amino acids, converting them into oxygen atoms. However, because this reaction requires more than four times more energy, it would occur to a much smaller degree than the antineutrinos'-right-handed amino acid reactions.
A Supernova standard
Supernovas are fairly common in the Milky Way galaxy. In a standard supernova, a star explodes after it uses up its nuclear fuel supply. These standard supernovas occur roughly once every 30 years in our galaxy.
A supernova only would destroy a very small portion of the right-handed amino acids in the neighboring molecular clouds. However, as the remaining left-handed molecules mixed throughout the galaxy, these molecules could be used in the formation of new amino acids. An initial imbalance of left-handed molecules as small as one part in 1 million or even less caused by supernovas could eventually lead to a dominance of left handed amino acids throughout space.
It is "the conspiracy of the very large, supernovae, with the very small, neutrinos, to impact something that exists on the human scale," Boyd said.
There remain a number of questions concerning this idea that Boyd and his colleagues are still investigating. For instance, after stars explode as supernovas, the remnants can form neutron stars. The powerful magnetic fields of these neutron stars could affect the molecular structures of the amino acids or their precursors, which in turn might have an impact on which handedness dominates.
If this idea proves true, the fact that virtually all the amino acids used by life on Earth are left-handed might suggest that the molecules of life were not created on this planet. Instead, they might have been born in our galaxy's molecular clouds and subsequently delivered via meteorites or included in the mixture that formed the Earth when the planets were created.
"I find it really mind-boggling that the same constraints that exist on our chemicals of life might also exist for every other entity in the universe," Boyd said. "If other entities are out there, the constraints on their chemistry appear to be sufficiently similar to ours that we may have lots of things in common with them."
Boyd and his colleagues Toshitaka Kajino and Takashi Onaka detailed their findings in the June issue of the journal Astrobiology.