Could life on Earth as we know it have come from outer space? 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.
“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.
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 as the sudden compression and heating of cometary ices crashing into the planet can produce complexes resembling the amino acid, glycine.
Origins of life research initially focused on the production of amino acids from organic materials already present on the planet. Further research showed that Earth’s atmospheric conditions consisted mainly of carbon dioxide, nitrogen and water. Shock-heating experiments and calculations eventually proved that synthesis of organic molecules necessary for amino acid production will not occur in this type of environment.”
Comets ranging in size from 1.6 kilometers up to 56 kilometers that pass 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 (1000s of Kelvin degrees) from the impact would destroy any potential life-building molecules. (One Kelvin equals 457 degrees Fahrenheit).
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. “Once the compressed material expands, stable amino acids could survive interactions with the planet’s atmosphere or ocean. These processes could result in concentrations of prebiotic organic species ’shock-synthesized’ on Earth from materials that originated in the outer regions of space.”
Using molecular dynamic simulations, the team studied shock compression in a prototypical astrophysical ice mixture (similar to a comet crashing into Earth) to extremely high pressures and temperatures. They found that as the material decompresses, protein-building amino acids are likely to form.
The fact that life appeared soon after the termination of the heavy bombardment about 3.8 billion years ago suggests that it seems more reasonable that the incoming comets and asteroids delivered the compounds essential for life.
The 2005 Deep Impact mission to Comet Tempel 1 discovered a mixture of organic and clay particles inside the comet that show it is overwhelmingly likely that life began in space, according to a paper by Cardiff University scientists, professor Chandra Wickramasinghe and colleagues at the university’s Center for Astrobiology.
One theory for the origins of life proposes that clay particles acted as a catalyst, converting simple organic molecules into more complex structures. The 2004 Stardust Mission to Comet Wild 2 found a range of complex hydrocarbon molecules - potential building blocks for life.
The Cardiff team proposed that the controversial theory that radioactive elements can keep water in liquid form in comet interiors for millions of years, making them potentially ideal “incubators” for early life. They also point out that the billions of comets in our solar system and across the galaxy contain far more clay than the early Earth did. The researchers calculate the odds of life starting on Earth rather than inside a comet at one trillion trillion (10 to the power of 24) to one against.
Professor Wickramasinghe said: “The findings of the comet missions, which surprised many, strengthen the argument for panspermia. We now have a mechanism for how it could have happened. All the necessary elements - clay, organic molecules and water - are there. The longer time scale and the greater mass of comets make it overwhelmingly more likely that life began in space than on earth.”
Jason McManus via Lawrence Livermore National Laboratory