Earlier this year, the work of Nir Goldman and colleagues was noted (here). Using sophisticated computer modeling tools, it was concluded that cometary impacts could generate C-N bonded oligomers that subsequently break apart to form a glycine-containing complex. This research has now been published in Nature Chemistry, resulting in a new flurry of discussion about the shock synthesis of life.
Topographic map of the Moon based on measurements from the Lunar Orbiter Laser Altimeter, showing the boundary between Oceanus Procellarum, a smooth, relatively young mare region on the western nearside (upper right), and the older, more heavily cratered highlands (center and lower left). Colors indicate increasing elevation from blue to red. Both Earth and Moon experienced the effects of impactors. (Source here)
It is known from Stanley Miller's experiments that amino acids can be synthesized in a reducing atmosphere. However, the evidence for such an atmosphere has become less convincing with time - and even a neutral atmosphere means the Miller route for generating amino acids is unproductive. Cometary impacts, however, can make this point irrelevant, as is explained by John Timmer here.
"One of the problems facing origin-of-life research is that building complex organic chemicals requires a reducing environment, but the early Earth's atmosphere is now thought to have been weakly oxidizing. None of this matters as the comet hits. A typical shockwave quickly reaches conditions where the simple compounds break down, liberating hydrogen ions. These create local reducing environments no matter what the atmosphere looks like."
We should note the nature of the computing challenge for the research team. They started with a mixture of water, methanol, ammonia, carbon monoxide and carbon dioxide. Then, as explained by Timmer:
"they ran molecular dynamics simulations of what might happen to a typical cometary mixture as a blazing hot shockwave passed through, and was followed by a rapid decompression. These were pretty elaborate calculations, with femtosecond time resolution, and molecular interactions that considered quantum effects. Simply modeling the decompression that followed a shockwave for 50 picoseconds involved about 80,000 CPU hours. They also reran the model to simulate different speeds and angles of impact, which produce different pressure/temperature combinations within the shockwave that passes through the comet."
Somehow, the leap from glycine (and amino acids in general) to life has become instinctive rather than reasoned. Nature carried a short report with the title: "Origins of life: Shock synthesis". The research however reported a route to synthesise glycine, not life! Chemistry World was overconfident in its headline: "Comet shockwaves helped stimulate life on Earth", but more nuanced with the byline: "Comet strikes could have delivered the necessary ingredients and conditions to stimulate life on Earth". The reputation of science journalism is not helped by these headlines: the idea that it is a small step from amino acids to life is fantasy! We have had many decades of serious research by very dedicated people, and this has revealed an enormous gulf between organic molecules and organic life. For more on this, go here.
Timmer's assessment of the research is probably the best that can be said:
"Right now, most scientists think that life originated in an RNA world, where proteins didn't exist, and amino acids simply acted as co-factors for some key chemical reactions. So this doesn't necessarily help us understand how life first got started. It may, however, provide some insight into how life started using amino acids in the first place, starting it on the road towards the production of proteins. If basic amino acids were plentiful, then evolution might have simply worked with what was already around."
Everyone acknowledges that cometary impacts have more potential to destroy life than to promote it, so it is worth drawing attention to the most recent study of the lunar impact craters greater than 20 km. This research drew on data gathered by the Lunar Orbiter Laser Altimeter, an instrument on board the Lunar Reconnaissance Orbiter spacecraft. The researchers explain: "These data provide a view of the global distribution of impact craters without the observational uncertainties that arose from measurement of craters on images of heterogeneous illumination condition and uneven coverage and quality." Their findings validate the hypothesis that there has been an early and a later impactor population inside the asteroid belt. The authors write: "Furthermore, it places the transition between these two populations at about the time of Orientale Basin, the last large multi-ringed basin thought to have formed ~3.8 billion years ago." The significance of this for abiogenesis advocates is that their thinking about the origin of life must be temporally constrained. They cannot reasonably postulate an origin prior to 3.9 Ga. Writing in The Daily Telegraph, Matthew Moore points out:
"Any life which may have existed on Earth 3.9 billion years ago would have been wiped out in a devastating asteroid strike, new analysis of Moon craters indicates." [. . .] "Earth and its satellite were bombarded with large asteroids during the solar system's "turbulent youth", striking new topographical maps show. The impacts would have been powerful enough to evaporate any water on our planet and destroy any early organisms."
Compare this with some reports of photosynthetic life at 3.8 Ga and with the general acceptance of life by 3.5 Ga. Life was on Earth in the Early Archaean. The resultant time constraints undermine all chance-based scenarios of abiogenesis. This leaves us with law-based explanations (which are totally unable to account for biological information) or design-based explanations. The latter option is the direction where science is leading us.
Synthesis of glycine-containing complexes in impacts of comets on early Earth
Nir Goldman, Evan J. Reed, Laurence E. Fried, I.-F. William Kuo & Amitesh Maiti.
Nature Chemistry, (September 2010) | doi:10.1038/nchem.827
Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlikely mechanism for the origins of life because of unfavourable chemical conditions on the planet and the high heat from impact. In contrast, we find that impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce complexes that resemble the amino acid glycine. Our ab initio molecular dynamics simulations show that shock waves drive the synthesis of transient C-N bonded oligomers at extreme pressures and temperatures. On post impact quenching to lower pressures, the oligomers break apart to form a metastable glycine-containing complex. We show that impact from cometary ice could possibly yield amino acids by a synthetic route independent of the pre-existing atmospheric conditions and materials on the planet.
Mitchinson, A. Origins of life: Shock synthesis, Nature, 467, 281, (16 September 2010) | doi:10.1038/467281a
Head, J.W. et al., Global Distribution of Large Lunar Craters: Implications for Resurfacing and Impactor Populations, Science, 329, 17 September 2010: 1504-1507 | DOI: 10.1126/science.1195050
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