A new experiment suggests that comet impacts
could have sowed the seeds of life on Earth billions of years ago.
Four billion years ago Earth was bombarded
by a hail of comets and asteroids. The shattering collisions rendered
our planet uninhabitable during a period scientists call the Late
It surely sounds like the LHB was an
awful time for the beleaguered young planet -- but perhaps the pelting
was a good thing after all, say researchers. Kamikaze comets could
have delivered important organic molecules to Earth -- sowing the
seeds for life.
Genesis by comets is a controversial
idea, but it has received an important boost with the knowledge
that a NASA supported experiment has revealed that complex molecules
hitchhiking aboard a comet could have survived an impact with Earth.
"Our results suggest that the notion
of organic compounds coming from outer space can't be ruled out
because of the severity of the impact event," says Jennifer Blank,
a geochemist at the University of California, Berkeley. Blank and
colleagues simulated a comet collision by shooting a soda-can sized
bullet into a metal target containing a teardrop of water mixed
with amino acids - the building blocks of proteins.
Not only did a good fraction of the
amino acids survive, but many polymerized into chains of two, three
and four amino acids, so-called peptides. Peptides with longer chains
are called polypeptides, while even longer ones are called proteins.
"The neat thing is that we got every
possible combination of dipeptide, many tripeptides and some tetrapeptides,"
said Blank. "We saw variations in the ratios of peptides produced
depending on the conditions of temperature, pressure and duration
of the impact. This is the beginning of a new field of science."
Freezing the target to mimic an icy
comet increased the survival rate of amino acids, she added.
Blank's ballistic test was designed
to simulate the sort of impact that would have been frequent in
Earth's earliest history when rocky, icy debris in our solar system
combined to form the planets. Much of the debris would have resembled
comets - dirty snowballs thought to be mostly slushy water surrounding
a rocky core - slamming into Earth at velocities greater than 16
miles per second (25 km/sec).
The severity of the laboratory impact
was akin to that of an oblique collision between the rocky surface
of Earth and a comet coming in at an angle of less than 25 degrees
from the horizon.
Benton Clark, chief scientist of Flight
Systems at Lockheed Martin Astronautics, proposed in 1988 that if
comets are slowed sufficiently -- by for example, drag from the
Earth's atmosphere, which would be greatest at low impact angles.
That some water and organic compounds might survive the collision.
"At very low angles, we think that
some water ice from the comet would remain intact as a liquid puddle
concentrated with organic molecules," ideal for the development
of life, Blank said. "This impact scenario provides the three ingredients
believed necessary for life: liquid water, organic material and
Though comet hunter Eugene Shoemaker
estimated that in Earth's early history only a few percent of comets
or asteroids arrived at low enough angles, the bombardment would
have been heavy enough to deliver a significant amount of intact
organic material and water, according to Blank's estimates.
One well-known model for the beginnings
of life on Earth posits that terrestrial life sprang from complex
molecules such as amino acids and sugars produced by electrical
discharges in a primeval atmosphere replete with gases such as methane,
hydrogen, ammonia and water. The famous Miller-Urey experiment in
1953, conducted by Stanley Miller and Harold Urey of the University
of Chicago, demonstrated that a lightening-like discharge in a test
tube filled with these molecules could produce amino acids.
Other scientists believe that the building
blocks of life on Earth arrived from space. Astronomers have detected
many kinds of organic molecules in space, floating in clouds of
gas or bound up in dust particles. They range from the simplest
- water, ammonia, methane, hydrogen cyanide and alcohols, including
ethyl alcohol - to more complex molecules.
Interestingly, of the more than 70
amino acids found in meteorites, only eight of them overlap with
the group of 20 which occur commonly as structural components of
proteins found in humans and all other life on Earth.
Credit Jennifer Blank, UC Berkeley.
schematic diagram of Blank et al.'s apparatus
the red arrow indicates the projectile, which is fired
from the breach toward the stationary target (inset).
The 3 smaller red triangles indicate transducer pins
that measure the velocity of the projectile as it passes.
"Upon impact, the sample container and a similarly-sized
metal plug fly backwards into the recovery area, where
they're trapped as gently as possible in layers of felt,"
To test whether water and organic compounds
could survive the high pressures and high temperatures of a collision,
Blank and her colleagues worked for three years to design a steel
capsule that would not rupture when hit with a mile-per-second (1.6
kilometre-per-second) bullet fired from an 80-mm bore cannon at
the University of Chicago and later at Los Alamos National Laboratory.
The target she and her team developed - a two-centimetre diameter
stainless steel disk about a half-centimetre thick - was able to
withstand about 200,000 times atmospheric pressure without bursting.
They filled the small cavity with water
saturated with five amino acids: three from the list of 20 that
comprise all proteins in humans (phenylalanine, proline and lysine)
and two varieties detected in the Murchison meteorite (aminobutyric
acid and nor-valine). That meteorite plummeted to the ground in
1969 in Australia and is thought to be from the core of a comet.
In a related
project conducted by members of NASA's Astrobiology Institute,
scientists have created primitive organic cell-like structures.
They did it in their laboratory by duplicating the harsh conditions
of cold interstellar space! Did comets carry such protocells
The liquid contents were analysed
afterwards at Argonne using liquid chromatography and mass spectroscopy
to determine the species and concentrations of molecules present.
The survival of a large fraction of
the amino acids and their polymerization during the collision makes
the idea of an extraterrestrial origin of organic compounds a strong
contender against Miller-Urey style theories, Blank said.
"About one comet per year arriving
in a low-angle impact would bring in the equivalent of all the organics
produced in a year in an oxidizing atmosphere by the Miller-Urey
electric discharge mechanism," Blank estimated. "An advantage is
you get all of it together in a puddle of water rather than diluted
in the oceans."
The next hitchhikers she plans to subject
to a shock test are bacterial spores, which some have proposed arrived
on Earth via comets to jump-start evolution.