January 26, 2011

Life On Earth May Have Come From Asteroids

Scientists have found more evidence that life on Earth may have been seeded by material from asteroids or comets.

Prior research has shown how amino acids could form elsewhere in the cosmos.

These molecules can form in two versions, but life on Earth exclusively uses just one of them.

New research shows how conditions around a far-flung star could favor the formation of one type over another.

Amino acids are corkscrew-shaped molecules that can form twisted to the left or right, and chemistry does not inherently favor one corkscrew direction over another.  However, life on Earth makes use of the left-handed version.

A famous experiment in 1952 showed how a spark across a soup of simple chemicals representing the primordial Earth could form amino acids and right-handed types.

The idea that amino acids might have been delivered to the early Earth by meteorites provided another route, and studies of meteorites have even shown excess of left-handed amino acids.

NASA astrobiologist Daniel Glavin and his colleagues said last week that their research showed that a wide variety of meteor types might play host to excesses of this sort.

Uwe Meierhenrich of University of Nice Sophia Antipolis and colleagues have found one way that this "symmetry breaking" may happen.

They started with chunks of icy material that included several simple molecules like water, methanol, and ammonia. 

The researchers then exposed the ices to ultraviolet light.  The researchers used circularly polarized light, rather than along a single direction, the polarization traces out a corkscrew shape.

Light in the regions around a forming star is known to become circularity polarized like this as it passes through vast clouds of dust grains that are aligned by magnetic fields.

The experiments showed the circularity polarized light led to the formation of both left- and right- handed amino acids.  However, there were slightly over a percent more of the left-handed version.

That is the level of excess that the researchers have found in meteorites found on Earth. 

"This excess is pretty cool," Dr Glavin told BBC News.

"You've got to break the symmetry somehow, this is critical. But how do you break it? That's one of the most important questions: did life just randomly choose one type over another? It's starting to look like Nature helped a bit."

However, Glavin said that these molecules can swap their forms, and that an unequal mixture of the two types will settle out to an equal mixture in time, which is a process known as racemization.

"These are exactly the kinds of experiments we need to be doing but we do need to keep the big picture in mind," he told BBC.

He said to further shore up the idea that life on Earth started with a delivery of extraterrestrial ingredients, it still remains to pin down the mechanism by which the unequal mixtures can be preserved for the long journey from far-flung stars.


Image Caption: This is a photo of a carbon-rich meteorite. Credit: Antarctic Meteorite Laboratory/NASA Johnson Space Center


On the Net: