Comets May Give Life a Hand
http://www.astrobio.net/exclusive/3257/comets-may-give-life-a-hand
Comets May Give Life a Hand
Michael Schirber
Astrobiology Magazine
September 24, 2009
Summary: A comet hitting Earth would seem to bring only death and
destruction, but one group is studying how such an impact could promote
certain necessary chemical steps in the origin of life. The researchers
are focusing on how comet collisions might have influenced the molecular
orientation, or handedness, of our planet's biology.
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.
Primordial Lab
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.
[Diagram[
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.
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 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.
Source: Ron Baalke, NASA
