Comets May Give Life a Hand
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.
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.
"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.
Like Shooting Comets in a Barrel
To simulate a comet hitting pay dirt, Blank and her colleagues fire a bullet into a metal container the size of a can of beans. In this scenario, the container is the comet and the bullet is the hard ground. Inside the container is a small chamber about as big as a quarter, in which the scientists place a liquid sample of organic molecules.
"It's not super high-tech, but it is rather involved as far as the structural complexity is concerned," Blank explains.
In 2001, the team reported that amino acids placed in the comet simulator were still intact following the impact, which surprised other scientists.
"It's the coolest thing," Blank recounts. "People told us, 'Nothing is going to survive, so why should we fund you?'"
Normally, the 1,000-degree-temperatures inside the smashed "comet" would destroy any amino acids. But Blank believes the temperature rises and falls too fast for the molecules to react. There is also enormous pressure of 10,000 atmospheres that may be preventing the breakdown of compounds.
However, the amino acids did more than just survive the crash. They also started bonding together to form short chains up to 5-amino-acids long.
This comet-induced bonding may have played a role in the origin of life. Typically, there is an energy barrier that prevents amino acids from latching together. Indeed, organisms require enzymes to overcome this barrier when putting together their proteins. But enzymes themselves are proteins, so there is a bit of a chicken-and-egg problem: how do you build up proteins before you have proteins to help build them up?
It is perhaps conceivable that a comet impact fused together the first rudimentary protein pieces (called "peptides") and thereby got the whole ball rolling.
Blank's group is now running simulations to see if they can model how the energy barrier to amino acid bonding changes under the high temperature and pressure of a comet impact.
Molecular Crash-Test Dummies
In regard to the handedness, Blank thinks there might be a difference in how the amino acids hook up together during the impact. Left-handed amino acids may form chains more readily with other left-handed amino acids, rather than with right-handed ones.
Such a preference, if it exists, might be able to enhance a slight overabundance of one hand (a so-called enantiomeric excess) in the original comet material. This might explain why organisms only use left-handed amino acids to form proteins.
"It will be a great discovery if they can get definite evidence as to formation of sugars, peptides, or enantiomeric excess," says Yoshihiro Furukawa of Tohoku University in Japan, who was not involved with this work.
He says the one concern will be contamination of the sample with the left-handed biology we are already familiar with. He suggests using amino acids made with carbon-13, so that any subsequent contamination with normal carbon-12-based amino acids will be easy to detect.