Homework 2: Crater formation and real-life collisions

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Homework 2: Crater formation and real-life collisions
(due at the start of class October 9, 2003)
1. Write a paragraph each for comets and asteroids, describing their distribution in
the solar system, composition (what are they made of), range of sizes, and
anything that you think is relevant.
2. We believe that Tunguska-like events occur somewhere on the Earth’s surface
approximately once per millennium (1000 yr). Using this fact, estimate how
many times humankind have witnessed such an event in the past 12000 years.
Be sure to explain your calculation carefully, listing any assumptions that you
make. You will need to know that approximately 70% of the Earth’s surface is
covered by water. You will also need to make an estimate for the fraction of the
land that has been inhabited by humans over this time interval. How would such
events have been interpreted by ancient societies? If such an event had
occurred over the USA or the Soviet Union in the 1960s, how might it have been
interpreted?
3. In your reading, the method of radiometric dating for determining the age of a
rock was mentioned, such as measuring the U-235:Pb-207 and U-238:Pb-206
ratios in zircon spherules. Using the web, a textbook, or some other resource,
describe the method of carbon dating. Be sure to specify the parent isotope and
stable daughter product used in this method. Why is carbon dating not useful
for determining the age of most impact structures on Earth? What is carbon
dating typically used for instead?