Space.com (September 24, 2009)
Billions of years ago, comets may have ferried life-sustaining water to our planet's surface, but that may not be all that we should thank these dirty snowballs for. Researchers are simulating comet impacts to see if they might help proliferate the left-handedness in molecules that life on Earth depends upon."There is evidence from meteorite studies that amino acids may have been delivered to Earth from space.
" 'There is interest in how these building blocks came to be on primordial Earth,' says Jennifer Blank of the SETI Institute.
"She and her colleagues study comets as a second avenue for depositing these biological compounds on Earth. Their current work, which is supported by NASA's Exobiology and Evolutionary Biology Program, is looking at how the fire and brimstone of a comet impact may benefit the formation of complex molecules of a particular handedness...."
The proteins we're made of are built of 20 amino acids - which are 'left-handed' molecules. 'Right-handed' equivalents can exist - but they're not involved in our amino acids.
Why?
Good question.
We've gotten more accurate (or less-inaccurate) models of what Earth's atmosphere was like as the years go by: and this article discusses one of the more recent models.
The 'it came from outer space' explanation for how things are now is useful, and allows for more research to see how closely it matches verifiable realities - but then the question is 'why were the space chemicals left-handed?'
I ran into the possibility that circularly polarized ultraviolet light might be involved several decades back. That idea's still being discussed, it seems. But, although circularly polarized infrared light has been observed in the Orion molecular cloud, we haven't detected circularly polarized ultraviolet light in nature.
Yet.
There are some other possible explanations, too. ("Space Hand-Me-Downs," Google HTML version of http://www.astrobio.net/includes/html_to_doc_execute.php?id=3251&component=news, a Word document)
The article discusses why these researchers focused on comets, rather than meteors. Comets aren't as common, but they presumably don't do quite as much damage - or generate as much heat - when they hit, and they carry water with them.
All in all, a pretty good update on the state of knowledge - and informed speculation - about what happened in Earth's first billion years.
Related posts:
- "Left-Handed Life: Why are We So Sinister?"
(September 17, 2009) - More posts, at
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