Survivable shocks/impacts

These anthracene simulations indicate that PAHs of moderate size may survive and even undergo polymerization reactions under shock impact conditions to be expected in cometary impacts on planetary atmospheres. Preliminary simulations of shock-induced chemistry in naphthalene [51] suggest a similar reaction threshold for the smallest PAH as well. 

Survivable Shocks and Impacts. Estimates for survivable exposures to single impacts and shocks are given for both headward acceleration and spineward deceleration in Table 10.2. For headward acceleration, experience with nonfatal ejections from militaiy aircraft suggests that a DRI of 18 is associated with a 5 to 10 percent probability of spinal injury among healthy, young males who are restrained in their seats. For spineward deceleration, the estimate is based on the head injury criterion (HIC), which was developed from the severity index (see Sec. 10.1.1). The metric is defined as ... 

The question of the stability of the biomolecules is a vital one. Could they really have survived the tremendous energies which would have been set free (in the form of shock waves and/or heat) on the impact of a meteorite Blank et al. (2000) developed a special technique to try and answer this question. They used an 80-mm cannon to produce the shock waves the shocked solution contained the two amino acids lysine and norvaline, which had been found in the Murchison meteorite. Small amounts of the amino acids survived the bombardment , lysine seeming to be a little more robust. In other experiments, the amino acids aminobutyric acid, proline and phenylalanine were subjected to shock waves the first of the three was most stable, the last the most reactive. The products included amino acid dimers as well as cyclic diketopiperazine. The kinetic behaviour of the amino acids differs pressure seems to have a greater effect on the reaction pathway than temperature. As had been recognized earlier, the effect of pressure would have slowed down certain decomposition reactions, such as pyrolysis and decarboxylation (Blank et al., 2001). 

Besides being desiccated and irradiated, microorganisms traveling in space will be exposed to space vacuum that can reach 10-14 pascal (a unit of pressure—100 Pa = 1 mbar).57 The result is extreme dehydration, and naked spores can survive for only days if exposed to space vacuum. Survival of spores is increased if they are associated with various chemicals such as sugars, or are embedded in salt crystals. Nicholson et al. (2000) discuss the various stresses that a microbial cell or spore would have to endure to survive interplanetary travel.58 They include the process that transports them out of Earth s atmosphere, such as volcanic eruptions and bolide impacts, long periods of transit in the cold of space, and atmospheric entry into a new planetary home. Spores have been shown to survive the shock conditions of a meteorite impact and the ultraviolet radiation and low temperature of space.59 It is clear that panspermia is possible and even probable if bacterial spores become embedded in rocks that are ejected from one planet and eventually enter the atmosphere of another. Bacterial... 

Opportunities for such secondary reactions certainly existed in the history of meteorites. Temperatures in the nebula (360-400 K, Table 1) may alone have been high enough for secondary reactions in the time available, 10 -10 yr. Kinetic studies of a similar reaction (formation of benzene from alcohols, amines, or fatty acids on Fe Oj or iron-rich peat catalysts Galwey, 1972) indicate a benzene formation rate of 5 x 10 molecules g yr at 360 K. At this rate it would take only 5000 years to transform all the meteoritic carbon to benzene. Further opportunities were provided by brief thermal pulses during chondrule formation, impact, or transient shocks. Of couree, any high-temperature episodes must have happened early or on a local scale, to permit survival of other, more temperature-sensitive compounds. 

Q-Quartz, which has a trigonal crystal structure, undergoes a rapid, reversible transition to hexagonal /J-quartz at 573 °C and then slowly changes to hexagonal /3-tridymite at about 870 °C tridymite in turn goes over slowly to cubic /3-cristobalite at 1470 °C, and this melts at 1713 °C. The reversion of cristobalite and tridymite to quartz is slow, so that these forms can exist at room temperature (as a-modifications). In addition, dense modifications with six-coordinate Si are found in shocked rocks associated with meteorite impact craters coesite forms only above 450 °C and 3.8 GPa, and stishovite requires over 1200 °C and 13 GPa. Survival of those metastable polymorphs on the geological timescale is evidence of an extremely slow recrystallization rate. 

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