Simulating a high-velocity comet collision with Earth, a team of
scientists has shown that organic molecules hitchhiking aboard a
comet could have survived such an impact and seeded life on this
planet.
The results give credence to the theory that the raw materials for
life came from space and were assembled on Earth into the
ancestors of proteins and DNA.
“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,” said research geologist Jennifer G. Blank of the
Department of Earth and Planetary Science in the College of
Letters & Science at the University of California, Berkeley.
Blank and her colleagues Randy Winans and Mike Ahrens of the
Chemistry Division of Argonne National Laboratory, and
engineer-mathematician Gregory Miller of the Applied Numerical
Algorithms Group of Lawrence Berkeley National Laboratory, will
report their preliminary findings on April 5 at the national
meeting of the American Chemical Society in San Diego, Calif. The
talk is part of an April 4-5 session on extraterrestrial organic
chemistry organized by Blank and colleague Max P. Bernstein, a
chemist in the Astrochemistry Laboratory at NASA Ames Research
Center in California.
Blank’s team shot a soda-can sized bullet into a nickel-sized
metal target containing a teardrop of water mixed with amino
acids, the building blocks of proteins. More than seventy
varieties of amino acids have been found in meteorites – many the
suspected cores of comets that smashed to earth – and are presumed
to exist in interstellar dust clouds.
Not only did a good fraction of the amino acids survive the
simulated comet collision, 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, a
geochemist. “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.
The ballistic test was designed to simulate the type of impact
that would have been frequent in Earth’s early history, some four
billion years ago, when rocky, icy debris in our solar system
accreted to form the planets in what must have been spectacular
collisions. 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 an oblique
collision with the rocky surface of the Earth – a comet coming in
at an angle of less than 25 degrees from the horizon, rather than
head on perpendicular to the Earth’s surface.
“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 energy.”
Benton C. Clark, chief scientist of Flight Systems at Lockheed
Martin Astronautics, proposed in 1988 that if comets are slowed
sufficiently, for example by drag from the Earth’s atmosphere,
some water and organic compounds might survive the collision. They
would collect in what he called a “comet pond” of concentrated
organic material where life could develop.
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.
The best known theory of the origin of life on Earth is that it
derived from complex molecules such as amino acids and sugars
produced early in the planet’s history by electrical discharges in
an 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, however, have proposed that the building blocks
of life 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, including chains of up to
eight carbon atoms.
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.
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
kilometer-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-centimeter diameter stainless steel disk about a
half-centimeter 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 the core of a comet.
The liquid contents were analyzed 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 make the idea of an
extraterrestrial origin of organic compounds a strong contender
against the Miller-Urey theory, 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
comet to jump-start evolution.
###
The work was sponsored by the National Science Foundation, NASA
and the Department of Energy.
NOTE: Jennifer Blank returns from San Diego on April 9, but can be
reached via email during that time at
jenblank@seismo.berkeley.edu. Her office phone is 510-643-0540.
