Question

1. [12 points] Compelling evidence for the asteroid impact 65 million years ago which triggered the extinction of the dinosaurs was discovery of a thin Iridium-rich clay layer in sediments all around the world. A normal rock sample from the Earth's crust contains less than 1 nanogram per gram (or 1 part per billion) of Iridium, while the Iridium clay layer associated with the K-Pg impact contains about 3 parts per billion. (a) Assume the K-T impactor was a rocky asteroid roughly spherical in shape with a diameter of about 10 km. Mean densities of asteroids fall in the range 1.4-5.3 g/cm3. For this problem, assume the K-T impact asteroid had an average density of p 2 g/cm3. Estimate the mass of the asteroid and give your answer in kg. (b) If the asteroid struck Earth at a speed of 15 km/s, estimate the energy of the impact in Joules. (c) Estimate the temperature (in °C) of the atmosphere assuming this energy was spread evenly over the whole Earth. Assume the average temperature was 15 °C prior to the impact. NOTE: The thermal energy of gas of temperature T (in K) containing N molecules is 3 thermal=-N kT where k Boltzmann gas constant 1.3806x10-23 m2 kg s2 K-1. The number of molecules in Earth's atmosphere is N 1044 (d) Assume the impact vaporized the asteroid and all the rock in the Earth's crust out to distance 1/4 of the radius of the current Chicxulub crater. Assume said rock had a density of 2 g/cm3 and that the diameter ofthe Chicxulub crater is 195 km. How many kg of rock were vapourized? (e) If this volume of vapourized rock was spread evenly around the world (cooling into dust, settling back to the surface after being carried around by high-altitude winds), approximately how thick would the resulting layer of clay be? Assume that dry clay squeezed beneath layers of rock will have about the same density (2 g/cm3). (f) Assume there was a concentration 200 nanograms of Iridium per gram of the asteroid. How many kg of Iridium would the clay layer contain? What would be the concentration of Iridium in that clay layer? Is this consistent with the Iridium concentration of the K-Pg clay layer noted above?

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