ASTROBIOLOGY:

How did the chemical units essential for life on modern Earth appear on pre-life Earth? The leading hypothesis is that RNA and DNA developed from protein-like precursors, but scientists disagree on the origin of these precursors.

One possibility is that lightning may have catalyzed amino acid (protein subunit) production in Earth's early atmosphere. Another is that comets delivered amino acids and/or catalyzed their production.

The comet hypothesis is generally disfavored, due to decomposition of carbon-based materials at the expected high temperatures and pressures induced by the impact. However, new computer simulations by Nir Goldman (Lawrence Livermore National Laboratory, United States) and coworkers greatly add weight to this scenario.

The simulations suggest that a comet impact can indeed catalyze the production of glycine and other amino acids, independent of Earth's atmospheric conditions and chemical composition. This research is complimentary to that which reported the presence of amino acetonitrile in the Sagittarius B2 stellar nursery, and the detection of glycine in the comet Wild 2.

What favors and disfavors the comet hypothesis?

In addition to frozen water, comets also possess molecules relevant to early-Earth chemistry. These include ammonia, carbon dioxide, and other simple molecules.

Based solely on this information, comets are a likely origin of useful molecules to early Earth. However, large changes in pressure and temperature accompany a comet impact.

Although such conditions enabling a diverse range of chemistry to occur, possibly including the production of amino acids, the temperatures are thought to be high enough (thousands of degrees Celsius) to decompose carbon-based molecules. What happens if a comet hits the Earth at a small enough angle?

In this case, the temperatures are likely to be much lower. The lower temperatures may enable chemical synthesis in the comet interior as well as molecular survival after impact.

Some basics of the scientists' model.

The scientists developed the "multiscale shock-compression simulation technique" to simulate the thermodynamics of a comet impact on early Earth. They presumed a cometary ice composition of twenty water molecules and ten each of ammonia, carbon dioxide, carbon monoxide, and methanol, at a density of 1 gram per milliliter.

They considered two atoms to be bonded if they were close enough to each other to form a bond for over 50 femtoseconds. Incidentally, this is long enough to be observable with modern instrumentation.

The key point was to simulate carbon-nitrogen bond formation (relevant to peptide bonds, i.e. the fundamental chemical linkages between amino acids) and its capacity to withstand the conditions of an impact. As an example, a comet impacting at a velocity of 29 kilometers per second would have to hit at an angle of 24° or less to be relevant to the scientists' simulations; the probability of this angle is estimated to be 17% (i.e. is likely).

They found that a constant pressure was attained within 3 picoseconds after impact. Material in a comet of 1.61 kilometer (1 mile) radius would be held in this state for roughly 0.4 seconds, and their model studied what happened up to 11 picoseconds after impact.

Amino acid formation from a comet impact.

The complexity of the carbon-nitrogen bonds formed after impact, and its prevalence for the duration of the simulation, varied tremendously with pressure (velocity of cometary impact). Conditions were found which yielded carbamate, the simplest molecule which possesses both chemical units (carboxylate and amine) essential to peptide bond formation.

Other molecules were found as well, e.g. hydrogen cyanide, isocyanic acid, and more complex molecules, depending on the particular experimental conditions. Importantly, a significant quantity of protons (hydrogen atoms with a positive charge) were produced.

This generates a reducing chemical environment. Put simply, this enables chemical reactivity even in an oxidizing atmosphere, i.e. independent of the somewhat unknown chemical composition of the atmosphere on pre-life Earth.

The scientists focused their study on an impact of 47 gigapascals, i.e. an impact of 9 kilometers per second and a temperature of 2,868°C. In these conditions, 16% of the starting material remained, 24% of the molecular bonds were carbon-nitrogen bonds, and the percent proton compositon was 35%.

Carbon-carbon and carbon-nitrogen chains were consistently observed. Upon their decomposition with a reduction in pressure and temperature, these chains could easily turn into molecules resembling glycine or other amino acids.

For example, chain degradation to glycine-CO₂ requires only one further deprotonation step to yield glycine. Remember that the comet impact produces a highly protonated environment.

Glycine synthesis under these conditons, via either hydronium ion or ammonium ion, is thermodynamically favorable (a negative change in Gibbs free energy), according to the scientists' calculations. It's therefore reasonable to speculate that comet impacts catalyzed the formation of amino acids on pre-life Earth, and/or could have delivered them from extraplanetary sources.

Implications.

Unless someone develops a time machine, or we meet aliens who witnessed these prebiotic events themselves, there's no way to know for sure whether comet impacts helped to produce amino acids in pre-life Earth. Simulations such as these help us learn if the possibility is reasonable (an actual experiment isn't possible).

This research doesn't eliminate the possiblity that lightning was another catalyst for generating life-essential molecules to Earth. What this research shows is that the cometary impact hypothesis is reasonable after all.

NOTE: The scientists' research was funded directly by the Laboratory Directed Research and Development Program at Lawrence Berkeley National Laboratory, and indirectly by the United States Department of Energy.

for more information:
Goldman, N., Reed, E. J., Fried, L. E., William Kuo, I.-F., & Maiti, A. (2010). Synthesis of glycine-containing complexes in impacts of comets on early Earth Nature Chemistry DOI: 10.1038/nchem.827