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.
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.
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