From: Andy Soos, ENN
Published June 6, 2013 09:23 AM
Unworldly Life Source
Nowadays Earth is perfectly lovely but once it was a barren rock. So
how did life arise on such an unpromising property? In fact, new
research shows that life on Earth may have come from out of this world.
Lawrence Livermore scientist Nir Goldman and University of Ontario
Institute of Technology colleague Isaac Tamblyn (a former LLNL postdoc)
found that icy comets that crashed into Earth billions of years ago
could have produced life building organic compounds, including the
building blocks of proteins and nucleobases pairs of DNA and RNA. Comets
contain a variety of simple molecules, such as water, ammonia, methanol
and carbon dioxide, and an impact event with a planetary surface would
provide an abundant supply of energy to drive chemical reactions.
The origin of life is a scientific problem which is not yet solved.
There are plenty of ideas, but few established facts.
It is generally agreed by scientists that all life today evolved by
common descent from a single primitive life form. It is not known how
this early form came about, but scientists think it was a natural
process which took place perhaps 3,900 million years ago.
Scientific hypotheses about the origins of life may be divided into
several categories. Most approaches investigate how self-replicating
molecules or their components came into existence. For example, the
Miller—Urey experiment and similar experiments demonstrated that most
amino acids, often called the building blocks of life, can be
synthesized in conditions thought to be similar to those of the early
Earth. Several mechanisms have been investigated, including lightning
and radiation. Other approaches ("metabolism first" hypotheses) focus on
understanding how catalysis in chemical systems in the early Earth might
have provided the precursor molecules necessary for self-replication.
"The flux of organic matter to Earth via comets and asteroids during
periods of heavy bombardment may have been as high as 10 trillion
kilograms per year, delivering up to several orders of magnitude greater
mass of organics than what likely pre-existed on the planet," Goldman
said.
Goldman's earlier work is based on computationally intensive models,
which, in the past, could only capture 10-30 picoseconds of a comet
impact event. However new simulations, developed on LLNL's
supercomputers Rzcereal and Aztec, Goldman used much more
computationally efficient models and was able to capture hundreds of
picoseconds of the impacts -- much closer to chemical equilibrium.
"As a result, we now observe very different and a wider array of
hydrocarbon chemical products that, upon impact, could have created
organic material that eventually led to life," Goldman said.
Comets can range in size from 1.6 kilometers up to 56 kilometers. Comets
passing through the Earth's atmosphere are heated externally but remain
cool internally. Upon impact with the planetary surface, a shock wave is
generated due to the sudden compression. Shock waves can create sudden,
intense pressures and temperatures, which could affect chemical
reactions within a comet before it interacts with the ambient planetary
environment. An oblique collision where an extraterrestrial icy body
impacts a planetary atmosphere with a glancing blow could generate
thermodynamic conditions conducive to organic synthesis. These processes
could result in significant concentrations of organic compounds being
delivered to Earth.
The team found that moderate shock pressures and temperatures
(approximately 360,000 atmospheres of pressure and 4,600 degrees
Fahrenheit) in a carbon-dioxide-rich ice mixture produced a number of
nitrogen-containing heterocycles, which dissociate to form aromatic
hydrocarbons upon expansion and cooling. These are thought to be
prebiotic precursors to DNA and RNA base pairs.
In contrast, higher shock conditions (about 480,000 to 600,000
atmospheres of pressure and 6,200-8,180 degrees Fahrenheit) resulted in
the synthesis of methane and formaldehyde, as well as some long-chain
carbon molecules. These compounds are known to act as precursors to
amino acids and complex organic synthesis. All shock compression
simulations at these conditions have produced significant quantities of
new, simple carbon-nitrogen bonded compounds upon expansion and cooling,
which are known prebiotic precursors.
"Cometary impacts could result in the synthesis of prebiotic molecules
without the need for other special conditions, such as the presence of
catalysts, UV radiation, or special pre-existing conditions on a
planet," Goldman said. "This data is critical in understanding the role
of impact events in the formation of life-building compounds both on
early Earth and on other planets and in guiding future experimentation
in these areas."
For further information see
Early Life.
Stromatolites image via Wikipedia.
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