Learning Objectives
After reading and studying this chapter, students should:
• Know the difference between asteroids, meteoroids, and comets.
• Understand the physical processes associated with aerial bursts and impact craters.
• Understand the possible causes of mass extinction.
• Know the evidence for the impact hypothesis producing the late Cretaceous mass
extinction.
Chapter Summary
This chapter focuses on potential hazards associated with large extraterrestrial object impacts. The
chapter begins with a description of the 1908 Tunguska event in Siberia, pointing to the potential threat
caused by such an event. Subsequent sections discuss the nature of extraterrestrial bodies in the solar
system, the characteristics of aerial bursts and impacts, and the relationship of impact events with mass
extinctions in the geologic record. The chapter closes with a discussion of potential means of
minimizing the hazard, including the identification of threatening extraterrestrial bodies and strategies
for reducing the potential hazard.
Chapter Outline
I. Earth’s place in space
A. Earth’s history is a story of change, with periodic drama caused by impact of material from outer
space (see Case History: The Tunguska Event)
B. Asteroids, meteoroids, and comets
2. asteroids: large bodies of rock and/or metal orbiting in asteroid belt
a. many are on orbits intersecting Earth’s orbit
4. meteor: a meteoroid that has entered Earth’s atmosphere
6. asteroids and comets were important in the formation of the Earth
II. Aerial bursts and impacts
A. Asteroids, comets, and meteoroids enter Earth’s atmosphere at high velocities
1. composition of asteroids varies from metallic to stony (differentiated)
3. meteorite: object that strikes Earth surface
B. Impact craters
2. rim overlain by ejecta blanket
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Impact of Extraterrestrial Objects
Chapter 12 – Instructor’s Manual
4. rock shattered and deformed
5. process of impact differentiates impact craters from those produced by other processes
7. craters on Mars and Moon have been subject of detailed study
8. Shoemaker-Levy impact with Jupiter, 1994
III. Mass extinctions
A. Mass extinction characterized by sudden loss of large numbers of plants and animals relative to
number of new species added
2. many hypotheses for causes of mass extinctions
4. Other possible impact-related extinctions include the terminal Pleistocene megafauna
extinctions (see A Closer Look: Possible Extraterrestrial Impact 12,900 years ago)
B. Late Cretaceous: K-T boundary mass extinction
1. 10-kilometer diameter comet or asteroid impacted Yucatan Peninsula
3. history of K-T impact hypothesis is one of intrigue, suspense, rivalries, and cooperation
5. impact crater discovered through pattern of cenotes on Yucatan Peninsula
6. impact caused vaporization of limestone bedrock, creating acid rain, and ejected dust into
atmosphere, causing mass extinction
IV. Minimizing the impact hazard
A. Risk related to impacts
2. smaller objects (few tens of millions): regionally catastrophic if in populated area
a. called Tunguska-type events
4. if Tunguska event is typical, an urban area is likely to be destroyed every few tens of thousands
of years
5. tremendous statistical variability in predicting likelihood and type of future impacts
B. Minimizing the impact hazard
1. identification of near-Earth objects (see A Closer Look: Near-Earth Objects).
2. an estimated 20 million extraterrestrial objects are in near-Earth orbit
a. only 4% are likely to penetrate atmosphere and create a crater
b. 50% are structurally weak and prone to explode at high altitudes
3. options to avoid or minimize hazard are limited
a. escape from effects of large comet or asteroid impact with Earth is unlikely
b. blowing apart an identified object would likely increase the hazard
Impact of Extraterrestrial Objects
c. gentle diversion of object away from Earth is more likely to reduce hazard
2) small nuclear explosions near the object could nudge it into a non-threatening orbit
4. evacuation might be an option for smaller events, but difficult and perhaps impossible given
Answers to Review Questions and Critical Thinking Questions
Review Questions
1. An asteroid is a large body of rock and metal orbiting in or near the asteroid belt. A meteoroid is a
2. An impact crater is characterized by a bowl-shaped depression with a pronounced, raised rim
3. A complex crater may grow over a period of seconds to minutes, and experience more complete rim
collapse and central crater uplift than does a simple crater.
4. The K-T impact hypothesis is supported by an iridium-enriched clay layer at the K-T boundary in
5. The risk of impact is determined by dividing the number of likely deaths by the average length of
6. The Tunguska event is a wake-up call because it was a major aerial burst but with few deaths. If
Critical Thinking Question
1. A response to this question should first focus on the extreme impacts of a Tunguska-type event on a
populated area, including the total destruction of vegetation and structures over a large area and the