Science Literature

OOL researchers need chemical building blocks with which to work, and amino acids are the most basic pre-biotic molecules. The Miller-Urey route for synthesising these molecules in the primordial Earth gained instant popularity for kicking off a self-assembly process. That mechanism has been eclipsed in recent years, not least because the necessary reducing atmosphere was perceived to be unrealistic by researchers. Consequently, interest in other ways of generating amino acids is high and there has been a steady stream of publications that address the issues. One of these mechanisms is concerned with the chemistry of comet and meteorite collisions with planet Earth.

Organic molecules are found in space, but do they have anything to do with biology? (Image credit Mario Iliev, source here)

A previous blog noted Japanese research simulating chondritic meteorite impacts, producing various organic chemicals including one amino acid (glycine). A US group led by Nir Goldman, based at the Lawrence Livermore National Laboratory, has considered cometary impacts. The initial conditions were chosen to match known compositions: a mixture of water, methanol, ammonia, carbon monoxide and carbon dioxide. The mechanism under investigation is shock compression.

"When a comet strikes a planet, a shock wave travels through it as it comes to a sudden halt. This, Goldman explains, compresses the comet, and the compression wave travels through the comet faster than the speed of sound. As a result, the molecules inside deform and bonds break."

The research makes use of validated theory and extensive simulation modeling. The findings are the result of "around one million computer hours on the powerful Atlas computer cluster at Lawrence Livermore". They chose a side-on impact because a head-on collision was considered likely to destroy rather than fabricate.

"For shock compressions lasting about 20 picoseconds at temperatures up to 4,000 K and pressures about 60 gigapascals, the researchers observed formation of chains of carbon and nitrogen atoms, some parts of which were akin to chains of amino acids. When they modeled an expansion period of 50 picoseconds, the longer chains broke up into smaller components, including a glycine-CO2 complex. The overall mixture is acidic, so the glycine-CO2 complex could react with H3O+ to produce glycine and CO2 in an exothermic reaction, Goldman said."

The implications of the research are more controversial. Do the findings help or constrain speculations about panspermia or the origin of life? The answers appear to be both yes and no.

Yes, those advocates of panspermia that point to amino acids in meteorites or comets as indicating a biotic source or as evidence of a directed or undirected colonization of the galaxy need to rethink their arguments. These organic molecules appear to be products of shock chemistry, UV radiation or some other process yet to be discovered. The mere fact of their existence does not provide any basis for inferring extraterrestrial biology. Although the enantiomeric composition of some of these molecules has been deemed to point to a biotic source, this conclusion may be wrong. Mechanisms for producing left-handed molecules in extraterrestrial environments are being examined (example here). So, this casts doubt on the recent claim of Chandra Wickramasinghe that "it seems likely that interstellar organics in large measure [. . .] derive from biology". Consequently, panspermia needs to be defended on other grounds (For example, go here).

No, speculations about the origin of life are not constrained. This is because there is an unwritten rule that all prerequisites for life count as evidence for abiogenesis. Finding a planet in the habitable zone; detecting the presence of water; recovering amino acids from space - all fuel the appetites of those who think that if life can self-assemble on Earth, it can self-assemble anywhere if conditions permit! Thus, the article from Nature News refers to crashing comets creating the "potential for life" and amino acids are described as "markers of potential life".

However, the finding of science emerging from OOL research is that an availability of traces of amino acids does not suggest that life can emerge via either Law or Chance processes. It is significant that people have become familiar with the 1953 experiments of Stanley Miller, but few ever refer to his subsequent work. He spent his career trying to achieve something more significant than amino acid residues but, although other organic compounds were obtained, he repeatedly faced dead ends. "Making the amino acids made it seem like the rest of the steps would be very easy," he said in a 1996 interview with Reuters. "It's turned out that it's more difficult than I thought it would be. It's a series of little tricks. Once you learn the trick, it's very easy. The problem is learning the trick." Miller never learned the trick.

No self-assembly pathway has ever been identified and the fate of all these molecules appears to be degradation. Few in the field have even begun to grapple with the challenge of producing biologically-meaningful information. Amino acids do not create the potential for life: only intelligence deserves to be described like this.

Production of pre-biotic molecules from extraterrestrial sources
Nir Goldman, Evan J. Reed, Laurence E. Fried, I-Feng William Kuo
ACS National Meeting, San Francisco, March 21-25, 2010, Paper 281.

Abstract: It has been proposed that impacts of extraterrestrial ices on early Earth could have been partially responsible for the creation of amino acids on the planet. We present ab initio molecular dynamics simulations of shock compressed aqueous mixtures representative of astrochemical ices found on dust grains and within other celestial bodies. We discover that high shock velocities drive the synthesis of a number of transient, exotic C-N bonded species at significantly higher pressures and temperatures than previously studied. Upon quenching to lower pressure conditions we observe a simple mechanism for the formation of the alpha amino acid glycine, an important component of protein synthesis. We find that shock compression of astrophysical ices followed by rapid expansion is a viable pathway for amino acid formation on a primitive planet, that can be verified by future experimentation.

Comet crash creates potential for life
Katharine Sanderson
Nature News, 26 March 2010 | doi:10.1038/news.2010.152

Abstract: Striking a glancing blow to a planet could create the perfect conditions in a comet's icy core to create amino acids - molecules that are vital to forming life on Earth.

See also:

Deyes, R. Improbabilists, Inevitabilists and the Astonishing Mystery of Life, ARN blog (2 July 2008)

Kemsley, J. Prebiotic Comet Collision Chemistry, Chemical & Engineering News (24 March 2010)

Tyler, D. Did meteorite impacts help to spawn life? ARN Literature blog (12 December 2008)