Comets May Give Life a Hand
Most types of amino acids exist in ‘left-handed’ and ‘right-handed’ forms. Compounds with this property are known as chiral, and exist as mirror images of each other. Life as we know it makes and uses only ‘left-handed’ amino acids.
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 Miller-Urey experiment generated electric sparks – meant to model lightning – in a mixture of gases thought to resemble Earth’s early atmosphere.
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.
"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.
|This schematic diagram shows the basic apparatus used by Blank’s team to simulate a comet impact. A projectile is fired (red arrow) into a stationary target that contains an organic sample. The "comet" material is then extracted and analyzed to determine what chemical reactions have occurred.|
Credit: Jennifer Blank
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.
Comet Hale-Bopp. Comets may have played an important role in the origin of life on Earth.
Image Credit: Philipp Salzgeber
She and her colleagues take special care to ensure that the metal container doesn’t leak from the impact. Afterwards, they carefully drill down to the chamber and draw out their "shocked" liquid sample.
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
The Stardust spacecraft captured particles from comet Wild 2. This photo shows tracks left by two cometary particles trapped by aerogel in the Stardust collector grid as they entered from the right. The tracks are magnified several hundred times.
Image Credit: NASA/JPL–Caltech
The scientists are also planning to do more comet crash tests. They will be looking at sugars, which play an important part in the structure of DNA and RNA. And they will be looking at amino acids again, this time studying whether the handedness of comet passengers might be affected by the impact.
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.