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.
   

The Physics of Making "Smoothies"

Frame from a video on the referenced article. Note the
cloudy region in the bottom of the liquid near the blades.
This is caused by minute bubbles created by cavitation at
the tips of the blades. The brown layer on top of the water
is oil.

Thanks to a colleague for pointing out an interesting article on shock waves in food blenders! Mathematician-turned-chef Chris Young and colleagues have collaborated in a series of articles to explain the science behind various cooking techniques. In this article, Young explains how the blades in the blender cut food up into small pieces, but then that the real work is done by small bubbles created by the blades through a process known as "cavitation."

Cavitation occurs when the pressure in a liquid drops below the equilibrium vapor pressure. It is a common occurrence in industrial settings, such as around the tips of rotating blades. Pressure drops occur in a variety of settings ranging from the flow of rivers around and over objects to flow in nozzles, to the wiggling of the tails and fins of swimming animals, and cavitation is a possibility in any of these settings. A bubble formed by cavitation is unstable. When such bubbles collapse, often asymmetrically, a tiny jet is formed. When these jets impinge on either the opposite sides of the bubble walls or on an adjacent surface, very high pressures, thousands of times atmospheric pressure, can occur. These jets erode the adjacent surfaces, causing structural damage.  Cavitation around the tips of dolphin or tuna tails may limit the speed with which these animals can swim! Cavitation is also a major problem at spillways from dams, and played a role in the near-failure of the bypass tubes during a major flood crisis at Glen Canyon Dam in 1983.

Rocks Flying out of Volcanoes at Breakneck Speed...or do they?

A meter sized bubble bursting
above the vent of Stromboli (top)
and a jet (well-collimated) from a
typical explosion of such a bubble
Figure 1 in the Taddeucci et al paper

I recently came across a year-old Science News article called "Volcanic Rush," by Alexandra Witze. This article summarized the results of two interesting papers (referenced below) on high-speed volcanic ejecta documented in the field, and simulated in the lab.
      The Taddeucci et al. paper reports on direct measurements of pyroclasts ejected from Stromboli at velocities up to 405 m/s, during eruptions from the south-west vent area in 2009. The velocities were obtained from high-speed videos in which centimeter-sized fragments were tracked for 5-10 frames. The velocities cited are believed to be conservative for several reasons.
      This velocity would correspond to a Mach number of 1.2 if the sound speed of air is taken as 330 m/s, and the authors point out that these velocities are "supersonic in ambient air."  It isn't clear whether or not these pyroclasts were actually traveling through ambient air where measured because shock waves which would compress and heat the atmosphere were often observed, and it may also be possible that the blocks were in a steam cloud, in which case their Mach numbers might have been subsonic because the sound speed is higher in steam (e.g., 850 m/s at 1000 C). The authors do not discuss these possibilities.
      The general picture that emerges for the eruption process is of the explosion of a large steam bubble that produces initially the high-velocity burst of small pyroclasts (cm -sized). Within tenths of a second, the velocities decrease and it is only later in the event that larger blocks and bombs are ejected. Only with the use of the high-speed cameras has there been resolution to detect the high-velocity phase of the eruption because lower-speed cameras only capture data later in the event when drag has already acted to slow down the small pyroclasts. The new measured values are a factor of four higher than previously measured. The authors were able to work out from the initial ejection pulses that the distance from the base of the pressurized gas pocket to the camera viewpoint at one of the vents (SW1) was 3-6 m, which corresponds to the size of the bursting bubbles driving this phase of the eruption. In contrast, application of the same analysis to the dominant pulses at the second vent, SW2, gave estimates of bursting length (similar to shock tube dimensions) of 100 m.
      In many older models of reservoirs and the pressures that drive eruptions, including work of this blogger, reservoir pressures are taken as ambient (lithostatic or hydrostatic) pressures, and the pressure difference between the reservoir and atmosphere is assumed to drive the eruption. Alatorre-Ibarguengoitia et al. (2011 and 2010 below) have developed a conceptual model that says that the fragmentation of the magma itself may consume considerable energy, and that the kinetic energy available for driving pyroclasts is significantly less than from these older gas models. In their 2011 paper they put natural volcanic samples of differing porosities into a 1-D shock tube and measure both the fragmentation speed inside the shock tube and ejection velocities. They relate the results using 1-D conservation laws.
     The lab experiments are instructive. In these experiments, a volcanic pumice is capped by a small volume of gas which, in turn, underlies a diaphragm. The duration of the shock-tube experiment is about 300-400 ms after a diaphragm separating the reservoir from the atmosphere is burst. The fragmentation threshold pressures were significant: about 4 Mpa (40 bars) for pumice; 8 Mpa (80 bars) for a denser sample. Below these pressures, the sample either does not fragment, or only partially fragments. From the experiments, the authors developed an empirical relation between the reservoir pressure and the fragmentation velocity (U is a logarithmic function of the ratio of the reservoir pressure to the fragmentation threshold pressure). Using this empirical relation in conjunction with the conservation laws for a pseudogas, they derive the theoretical velocity of eruption of the gas-particle mixture, and found it to be in excellent agreement with the lab measurements. This provides a way, then, to use measured or inferred field velocities to calculate reservoir pressures.
    They then outline some conditions under which the experiments can be applied to volcanic eruptions, one of which is that it is restricted to magmatic eruptions, and produce the following graphs of ejection velocity versus initial reservoir pressure:

Ejection velocity as a function of initial reservoir pressure for two different porosities, and four different values of f,
the fraction of particles in thermal equilbrium with the gas phase. f=100% implies a fine-grained ash eruption; f=0%
would apply to an eruption of scoria or bombs.

There are some interesting implications given the observations of  Taddeucci summarized above (these are my thoughts, not the authors, so they shouldn't be blamed if I'm off track). Ejection velocities on the order of 400 m/s can only be obtained from porous magma initially at high pressure having high heat transfer--this is consistent with the high-velocity ejecta observed being small (1-cm pyroclasts, or less?).  Denser magma can generate only significantly less velocity even with efficient heat transfer, and only low velocities if there are big particles (scoria, bombs) with little possibility of heat transfer.

M.A. Alatorre-Ibargüengoitia, B. Scheu and D.B. Dingwell. Influence of the fragmentation process on the dynamics of Vulcanian eruptions: an experimental approach. Earth and Planetary Science Letters. Vol. 302, February 1, 2011, p. 51. 

J. Taddeucci et al. High-speed imaging of Strombolian explosions: The ejection velocity of pyroclasts. Geophysical Research Letters. Vol. 39, published January 18, 2012, L02301. doi:10.1029/2011GL050404. 

Alatorre-Ibargüengoitia, M.A., Scheu, B., Dingwell, D.B., Delgado-Granados, H., Taddeucci, J., 2010. Energy consumption by magmatic fragmentation and pyroclast ejection during Vulcanian eruptions. Earth Planet. Sci. Lett. 291, 60–69. doi:10.1016/j.epsl.2009.12.051.