Early Life on Earth and Amino Acids
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. Occasionally, they will collide with planets such as the Earth. New research from Lawrence Livermore National Laboratory scientists shows that comets that crashed into Earth millions of years ago could have produced amino acids — the building blocks of life. Amino acids are critical to life and serve as the building blocks of proteins, which are linear chains of amino acids.
Comets are often described as "dirty snowballs". Comets also contain a variety of organic compounds; these may include methanol, hydrogen cyanide, formaldehyde, ethanol and ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.
Amino acids are molecules containing an amine group, a carboxylic acid group and a side chain that varies between different amino acids. These molecules contain the key elements of carbon, hydrogen, oxygen, and nitrogen.
Amino acids are critical to life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of proteins, which are just linear chains of amino acids. Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked together in varying sequences to form a vast variety of proteins. Due to their central role in biochemistry, amino acids are very important in nutrition and are commonly used in food technology and industry.
Most amino acids can form via natural chemical reactions unrelated to life, as demonstrated in the Miller—Urey experiment and similar experiments, which involved simulating some of the conditions of the early Earth, in a scientific laboratory. In all living things, these amino acids are organized into proteins, and the construction of these proteins is mediated by nucleic acids. Which of these organic molecules first arose and how they formed the first life is the focus of origin of life debates.
In the September 12 online edition of the journal Nature Chemistry, Livermore's Nir Goldman and colleagues found that simple molecules found within comets (such as water, ammonia, methylene and carbon dioxide) just might have been instigators of life on Earth. His team discovered that the sudden compression and heating of cometary ices crashing into Earth can produce complexes resembling the amino acid, glycine.
Comet computer simulations show that long chains containing carbon-nitrogen bonds can form during shock compression of a cometary ice. Upon expansion, the long chains break apart to form complexes containing the protein building amino acid glycine.
"There’s a possibility that the production or delivery of prebiotic molecules came from extraterrestrial sources," Goldman said. "On early Earth, we know that there was a heavy bombardment of comets and asteroids delivering up to several orders of magnitude greater mass of organics than what likely was already here."
Comets range in size from 1.6 kilometers up to 56 kilometers. Comets of these sizes 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. The previous general consensus was that the delivery or production of amino acids from these impact events was improbable because the extensive heating from the impact would destroy any potential life-building molecules.
However, Goldman and his colleagues studied how a collision, where an extraterrestrial ice impacts a planet with a glancing blow, could generate much lower temperatures.
"Under this situation, organic materials could potentially be synthesized within the comet’s interior during shock compression and survive the high pressures and temperatures," Goldman said.
For further information: https://publicaffairs.llnl.gov/news/news_releases/2010/NR-10-09-03.html