Did comet crashes help spark Earth life?

By Matt Schirber

updated 6:38 p.m. ET Sept. 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.

Life on Earth uses 20 amino acids to build up the thousands upon thousands of different proteins that perform a myriad of cell functions. Astrobiologists often focus on the origins of amino acids in order to understand where life may have come from.

One of the first experiments aimed at reproducing the primordial Earth and its chemistry was undertaken by Stanley Miller in 1953. He was able to synthesize amino acids using lightning-like discharges in a reducing atmosphere of methane, ammonia and water — similar to what exists on Jupiter.

Since that pioneering work, researchers have come to believe that Earth's early atmosphere was in fact more oxidative, containing mostly nitrogen and carbon dioxide.

A ‘compact evolution kit’
"Without the reducing atmosphere, the Miller mechanism becomes much less efficient at producing amino acids," Blank says.

One way to get around this is to make the amino acids in space and have them come crashing down on-board meteorites and comets. There is ample evidence that meteorites carry amino acids. And just recently, an amino acid was discovered in comet material brought back by NASA's Stardust spacecraft.

Blank and her colleagues were curious as to what happens to these biomolecules when the "space capsule" they are riding in smacks into the Earth.

The team has focused their work on comets, rather than meteors. Although comets are less prevalent in the inner solar system, they have a few possible advantages over their dry rocky counterparts when it comes to delivering biologically relevant material to a planet's surface. 

First of all, a comet impact is thought to be less harsh than that of a meteorite because comets are less dense, which means their impact generates lower temperatures and pressures. Blank says that the blow would be further softened on a comet arriving at an oblique angle.

The second advantage of comets is that they carry water, which is key for the chemical reactions that beget life. When the comet lands, its ice melts, forming a little puddle near the crash site.

"Comets give you all the ingredients, like a compact evolution kit," Blank says.

Of course, the primordial Earth was stocked with its own water, but "if a comet or meteor were to land in the ocean, any interesting chemistry would quickly be diluted away," says co-investigator George Cooper of NASA Ames. A comet impact on dry land would give the organic molecules on board the chance to amplify their numbers in the localized puddle.

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