Recreating
how the seeds of life might have survived aboard an ancient meteorite that
crashed to Earth is no small feat, but scientists have begun doing just that in
a recent lab experiment. The project could help indicate whether life on Earth
got its start from alien organic material that hitched a ride aboard space
rocks.
Perhaps
one of the likeliest building blocks of primordial life on Earth came in the
form of amino
acids, which are the basic components of proteins. And so a team of U.S.
and European researchers focused on trying to replicate how well amino acids would
fare when a meteorite slams into the ground.
"This
study is the first which tested amino acid quantities similar to those found in
real meteorites," said Marylene Bertrand, a biophysicist funded by the
National Center for Scientific Research (CNRS) in France and lead author of the
work published in the December issue of the journal Astrobiology.
More
than 70 different amino acids have been found in meteorites
that fell to Earth. Past studies have tested the survivability of many amino
acids, but did not try to replicate the concentrations of organic molecules
found in actual meteorites.
Bertrand's
group also took the new step of testing the amino acids embedded inside
saponite, a clay material found in carbonaceous chondrite meteorites that
represents a possible signature of water.
How
to recreate a falling meteorite
Simulating
a meteorite impact involved firing cylindrical plugs from a 20 mm gun at a
target holding the amino acid-saponite samples in place. Researchers fired the
gun inside a vacuum chamber to allow the projectiles to strike the samples at
the highest possible speed, and tested a range of low to high impact pressures.
"It
is easier to launch a projectile than to launch the samples," Bertrand
told Astrobiology Magazine, and noted that the resulting shock from hitting
the samples with a projectile was the same as if the team had shot the samples
out of the gun.
The
toughest amino acid survivors, including the smallest amino acid glycine,
turned out to have a molecular alkyl side chain. A second group of amino acids
with structures that include a functional side chain and dipeptide proved less
resistant, and failed to survive the highest impact pressures.
A
failure of many amino acids in the experiment to resist higher shocks seems
puzzling, because many of the same amino acids appear to have survived in
meteorites. Bertrand's group suggested that the samples containing the amino
acids could not expand inside the metal target containers, causing them to endure
higher pressures and temperatures than what they would have experienced inside falling
meteorites.
Also,
amino acids may have been better able to survive the shock of impact if they
arrived on Earth billions of years ago, when the atmosphere was more dense and
was mainly comprised of nitrogen, carbon dioxide, and methane.
"The
atmosphere was very different in primitive Earth and the conditions and effects
of the impacts on organic matter could have been very different than now,"
Bertrand explained.
Amino
acids also might evaporate upon impact and then eventually condense once more
inside meteorites, the team noted. Or larger organic compounds could have been
destroyed by the impact shock and broke apart into amino acids.
Stirring
the primordial soup
Such
studies raise plenty of new questions to join the unknowns already swirling
around the puzzle of how life
on Earth first arose. For instance, scientists still do not know the rate
of meteorites that fell at low or medium velocities, which would have made
survival more likely for amino acids and other organic molecules.
Still,
they know that roughly 20,000 tons of meteorites and other space particles fall
to Earth each year. Any organic-bearing rocks could have made a big difference
during the first 500 million years of the Earth's existence as a planet.
"I
think that all organic matter present on Earth in living organisms could come
from the meteorites, micrometeorites or [interplanetary dust particles],"
Bertrand said. "The major question now is how the organic matter could
have been processed and organized to lead to living organisms."
Scientists
agree that life somehow emerged from Earth's water around four billion years
ago, and also agree that such organisms mainly consisted of liquid water and
organic molecules. But they still debate whether the organic building blocks
arose from the Sun's radiation, providing energy for the early ocean "soup,"
or whether undersea hydrothermal
vents provided the necessary biochemistry touch.
If
meteorites provided a third way by bringing life's ingredients to Earth, then
their precious organic cargo must have somehow survived an even more daunting
prospect than impact — the long journey through space.
Bertrand's
group also wants to check out the effects of space conditions on organic
molecules. They have already conducted experiments that exposed amino acids to
ultraviolet light in the harsh vacuum outside the International Space Station,
as well as in lab settings on Earth.
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