180 SHOCK METAMORPHISM As noted by Melosh in his book on impact cratering (see Further Reading), relationships between crater dimensions and impact parameters are poorly constrained There are a variety of empirical scaling relations extrapolated from small-scale laboratory impact experiments, high explosive and nuclear experiments, and large-scale computer calculations Here, we apply the observation that, for a variety of impact conditions, many scaling relations predict D/d, the ratio of the crater diameter to the impactor diameter, to be in the range of 10–20 Small objects with a mass in kilograms are slowed by atmospheric drag to terminal velocities in the region below about 200 m s The resultant impact pressure of about GPa for an impact on rock is too low to produce shock metamorphic effects other than fracture Thus, 20 GPa shock metamorphic effects found in some small meteorites may be interpreted as the result of impacts on a meteorite parent body The exception to this general rule is when there is evidence that a small meteorite is a fragment of a much larger body that impacted the Earth at high velocity Iron meteorites found in the vicinity of the Meteor Crater (northern Arizona, USA) (1.3 km in diameter) are interpreted as fragments of the rear surface of a 100 000 ton (approximately 30 m in diameter) iron meteorite that is estimated to have impacted the Earth at approximately 20 km s The bulk of this meteorite was melted or vaporized as a result of very high shock pressures Intuitively, it might be expected that the entire meteorite would be exposed to the same peak pressure, as predicted by some low-resolution calculations However, the most recent high-resolution calculations predict that rarefactions originating at free surfaces (the meteorite–air interface) will interact to create low-pressure regions near the rear surface of the meteorite At least 20 tons of meteorite fragments have been recovered from the vicinity of Meteor Crater Some of these fragments have shock metamorphic features indicative of peak pressures of less than 10 GPa Other fragments have shock metamorphic features, including shock-synthesized diamond, indicative of pressures in excess of 100 GPa Shock Waves and Large Impacts Pressure scale definitions: the modern unit of pressure, the pascal, is defined as N m Atmospheric pressure on the Earth at sea-level is approximately 105 Pa (100 000 Pa); shock pressures are usually stated as gigapascals (GPa), 109 Pa Earlier literature may refer to bars, kilobars (kb or kbar), atmospheres (atm), dynes per square centimetre (dyn cm 2), kilograms per square centimetre (kg cm 2), and pounds per square inch (psi) bar ¼ 105 Pa ¼ 106 dyn cm ¼ 0:9869 atm ¼ 1:0197 kg cm GPa ¼ 10 kbar ¼ 1010 dyn cm ¼ 14:504 psi ¼$ 145 000 psi A collision between two bodies produces a high pressure (shock wave) at the point of impact The shock wave propagates into both bodies and is attenuated by rarefaction waves originating at free surfaces The magnitude of the peak pressure depends on both the impact velocity and the relative stiffness of the impacting bodies, as shown in Table The pressure calculations are based on material properties extrapolated from much lower pressures Table Parameters of typical Asteriod Earth and Comet Earth Impacts Impactor target a Iron water (ice) Iron alluvium (1.5 g cm 3) Iron granite Iron peridotite Peridotite water (ice) Peridotite alluvium Peridotite alluvium over graniteb Peridotite granite Peridotite peridotite Snow (0.6 g cm 3) icec Ice alluviumc Ice ice a Velocity (km s 1) Peak pressure (GPa) 20 20 20 20 20 20 20 $360 $400 $750 $950 $280 $300 $300, then $400, shock reflection 550 650 $500 $600 $650 20 20 40 40 40 Fate of impactor Fate of target Completely molten Completely molten Partial vaporization Partial vaporization Partial vaporization Partial vaporization Partial vaporization Total vaporization of water or ice Total vaporization Total vaporization Total vaporization Total vaporization Total vaporization Total vaporization of alluvium Partial vaporization of granite Total vaporization Total vaporization Total vaporization Total vaporization Total vaporization Total Total Total Total Total vaporization vaporization vaporization vaporization vaporization At these very high pressures, the properties of ice and water are indistinguishable Shock interactions occur at interfaces between materials having different properties The 300 GPa shock in alluvium is reflected at the granite interface as a 400 GPa shock moving back into the alluvium A 400 GPa shock is transmitted into the granite c The properties of a comet are approximately bracketed by the properties of snow and ice b