This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Wednesday, June 5, 2013

A "shocking" start for life on Earth?

A schematic of the synthesis of nonbiological hydrocarbons
during impact of simple icy comets on the early Earth
Image from DOE/Lawrence Livermore National Laboratory
as published in ScienceDaily.com 
ScienceDaily.com has reported on a paper in press for the  January 20, 2013 Journal of Physical Chemistry A. The authors are from Lawrence Livermore Berkeley Laboratory (LLBL) (Nir Goldman) and the University of Ontario Institute of Technology (Isaac Tamblyn, who is a former LLNL postdoc). The exciting conclusion of the work is that it is possible that during the short duration of a comet impact, shock waves could have provided the necessary conditions (pressure and temperature) and energy (heat) to allow chemical reactions that would have produced prebiotic hydrocarbons such as nitrogen-containing heterocycles. These are believed to be the prebiotic precursors to DNA and RNA base pairs.
The structure of pyridine, C5H5N
Wiki image by Calvero
       What are "nitrogen-containing heterocycles"? A heterocyclic compound is a ring-structured compound, with the "hetero-" meaning that there are atoms of at least two different elements in the ring (the opposite of a homocyclic compound that has only one element in the ring). They can be inorganic, but the ones of interest contain at least one carbon. In the case here, the second element is nitrogen.
     The conclusions result from a computationally intensive calculation of conditions produced in shock waves produced by a comet impact on the earth. The comet is described as a CO2-rich icy body; the source of nitrogen isn't specified in the ScienceDaily article, but it implies that the nitrogen is contained within the comet and was not from our own atmosphere. Comets typically contain ammonia (NH3), carbon dioxide and monoxide, and methane, so perhaps the nitrogen is from the ammonia.
    I've posted a bit about impacts on the Moon, and meteor over Russia here. Here's a bit more on what happens. A typical comet impact velocity is about 50 kilometers a second (112,000 miles per hour!). Comets have a fairly low density (somewhere between porous ice and rocky ice), and they react ("burn") as they descend into our nitrogen-rich atmosphere.  They generate a strong shock wave in the atmosphere, creating a high-temperature, high-pressure blanket ahead of them as they soar to earth.
     Upon crashing to earth, two shock waves are generated. One spreads out into the ground telling the earth, so to speak, that the comet has arrived. The other spreads back into the comet, telling it, so to speak, that it has just run into an obstacle (the comet was already alerted that something had changed by shock waves sent into it during its descent through the atmosphere). A set of conservation equations, known as the Rankine-Hugoniot equations, govern the mass, momentum and energy conditions during the impact. When these shock waves arrive at free surfaces (the rear-end and sides of the comet, the surface of the earth), they reflect as expansion ("rarefaction") waves and these waves eventually reduce the pressures back to ambient.  However, energy is irreversibly deposited in the ground in the form of heat. To first order, the comet is entirely vaporized in the event.
     Goldman and Tamblyn used huge supercomputers at LLBL to solve not only the equations of motion, but equations for chemical reactions during the shock process, to arrive at their conclusions.  These computers calculate reactions at time steps of picoseconds.  Previously computers could only capture 10-30 picoseconds of reactions, not enough time for equilbrium, but with more efficient codes, they were able to run the computations for hundreds of picoseconds, time enough to allow approach to chemical equilibrium. Comets impacting rock at these velocities typically generate peak pressures in the megabar (Mb) range, millions of times our own atmospheric pressure. At these peak pressures, everything vaporizes.
     Of more interest are the chemical reactions in the range of 480-600 kilobars (480,000-600,000 times atmospheric pressure). At these pressures temperatures reach 6,200-8,180 F, and methane and formaldehyde are synthesized. These are known precursors of amino acids and complex organic compounds. As pressure decays in the rarefaction wave, significan quantities of simple, carbon-nitrogen bonded compunds that are known prebiotic precursors are produced.
     At lower pressures, in the range of 360 kb and 4,600 F, nitrogen-containing heterocycles, which can dissociate to form "functionalized aromatic hydrocarbons" in the rarefaction are produced. These are thought to be prebiotic precursors to DNA and RNA base pairs.
     The bottom line is that the "special conditions," such as UV radiation or the presence of catalysts, in many existing models for the origin of life on earth are not needed in this model. It's a simple physical model of chemistry during the impact process.  Some aspects of this may be amenable to experimental testing, and it will be interesting to see the full paper in a few weeks.

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