1. Natural Hazards and Disasters Impacts of Asteroids and Comets
Chapter 17
Impacts of Asteroids and Comets

2. Projectiles from Space
Asteroids, comets and meteors cross Earth’s path
Asteroids and comets are known as bolides

3. Asteroid Impact 65 million yrs ago
Asteroid km wide struck Yucatan
Formed Chicxulub crater km wide
Crater walls collapsed to form 195-km basin
Asteroid impact killed dinosaurs and majority of other species on Earth
Energy = 1 million 1980 eruptions of Mount St. Helens

5. Crater is not exposed on surface
Figure 17-1

6. Time Scale is based on extinctions
Geological time before impact is Cretaceous
Geological time after impact is Tertiary
K-T (Cretaceous-Tertiary) boundary defined by this extinction

7. Major extinction event: 85% of all species died.
End of the Mesozoic
Major extinction event: 85% of all species died.

8. Cretaceous-Tyrannosaurus Rex

9. Side Effects
Huge tsunami waves left debris m above sea level around Gulf of Mexico
Breakup of asteroid may have caused multiple impacts
Manson impact structure in central Iowa is also 65 million years old, but only 35 km in diameter – too small to be main impact site

10. Meteor Crater,
Arizona
A more recent impact

11. Asteroids
One planet of solar system is missing, where asteroid belt is located
Some asteroids are pulled out of normal orbits by asteroid collisions or gravitational influence of sun and planets
Majority are less than 3 km in diameter, most between 100 meters and 1 km diameter

12. The inner planets are small and rocky. Jupiter Saturn Uranus Neptune
Venus
Earth
Pluto
Sun
Mercury
Mars
Inner
planets
Asteroid
belt
Outer
planets
The inner planets
are small and rocky.

13. Comets Consist of ice and some rock: dirty snowballs
Come from Oort cloud – vast spherical region extending more than 100,000 times Earth’s distance from sun, containing billions of comets
Comets travel up km/s, making impacts catastrophic

14. Comets
Comets spray off water, dust and volatiles to form glowing tails when hit by solar wind (tail points away from sun)
Figure 17-3

15. Meteors and Meteorites
Meteors: objects that form light streak in sky as they pass through Earth’s atmosphere
Meteorites: same objects once they collide with Earth
Most come from asteroid belt
Small meteors burn up in upper atmosphere
Large meteors become incandescent on outside (fireball) but cores remain cool
Large rocks in atmosphere break up to form strewn field

16. Identification of Meteorites
Iron meteorites are 6% of all meteorites
Similar to Earth’s core

17. Identification of Meteorites
Chondrites are stony meteorites, 93% of all meteorites
similar to Earth’s mantle

18. Evidence of Past Impacts
Figure 17-7

24. (1) Why are there so few craters on Earth?
(2) Why do some areas of the moon have many craters, and other areas have so few?

25. Evidence of Past Impacts
Most impacts into oceans (2/3 of Earth’s surface) are undetected or destroyed by subduction
Continental impact sites are broadly distributed, but more have been found in populated or better exposed areas

26. Impact Energy Energy =mass * velocity2
Asteroids have slower speeds but higher mass
Comets have lower mass but higher speeds
Kinetic energy of incoming object is converted to heat and vaporization of asteroid and target
Melts rock, excavates crater, blasts out rock and molten glass
Huge fireball heats and melts rock, burns everything

27. Impact Craters
Figure 17-8

28. Fallout of Meteoric Dust
End-Cretaceous impact deposited thin, dark layer of clay which contains
Soot
Shocked quartz
Spherules
Anomalous amounts of iridium and other platinum-group elements: iridium anomaly

29. Multiple Impacts
Asteroid would be likely to break up in atmosphere, so should expect multiple impacts in sequence
Shoemaker-Levy 9 comet broke up into 21 fragments before impacting Jupiter in 1994
Fragments (less than 1 km in diameter) impacted one after another in arc across planet over six days

30. Consequences of Impacts with Earth
Impact of km in diameter asteroid might kill up to 25% of people
Would threaten civilization as we know it

31. Immediate Effects of Impact
fires
smoke would linger in atmosphere
ozone layer would be destroyed
nitric acid and acid rain
Dust in stratosphere would block sunlight and cause cooling, wiping out agriculture

32. Impacts as Triggers for Other Hazards
Earthquakes would be generated
Impact into ocean would form tsunami waves up to 200 m high, with 300 m runup
Impact might cause volcanic activity

33. Evaluating the Risk of Impact
Odds of huge asteroid impacting Earth are
Consequences of impact would be truly catastrophic, could wipe out civilization
Small impacts are common, giant events are rare
There is a 1% chance of a 6-meter diameter bolide impacting Earth in any year

34. Evaluating the Risk of Impact
Figure 17-13
About 1,500 asteroids larger than 1 km across are known to be in Earth-crossing orbits
Most cross Earth’s orbit at long intervals, so chance of collision is small

35. Your Personal Chance of Being Hit by a Meteorite
Only well-documented case of person hit by meteorite:
In 1954, Alabama woman hit by 3.8-kilogram meteorite on hip – badly bruised but OK
2004: stony meteorite crashed through roof in Auckland, New Zealand
1997: 24-kilogram meteorite hit garden outside Moscow, Russia
1992: meteor shot across sky in fireball before hitting car in driveway in Peekskill, New York

36. Chances of a Significant Impact on Earth
Major impacts occur about every 33 million years
Major extinctions seem to occur about every million years

37. Chances of a Significant Impact on Earth
Hale-Bopp comet was seen by most people on Earth in early 1997
Passed 320 million km from Earth
Collision would have been s times larger than dinosaur-killing collision
Asteroid 1997 XF11 is 1.5 km in diameter
Will pass Earth at 2.5 times distance to Moon in 2028
Collision with Earth would expend energy of 2 million Hiroshima-size atomic bombs

38. What Could We Do about an Incoming Asteroid?
If very large asteroid was discovered on collision course with Earth:
Inside Moon’s orbit, would be three hours from impact
An hour from impact, would appear as bright as Venus
Fifteen minutes from impact, would appear as irregular mass
Would enter Earth’s atmosphere with blinding flash, then impact Earth three seconds later

39. What Could We Do about an Incoming Asteroid?
Suggestions for dealing with very large asteroid on collision course with Earth:
Blast asteroid into pieces with nuclear weapon
Might just pepper Earth with thousands of smaller pieces
Attach rocket to deflect its orbit so it misses Earth

40. What Could We Do about an Incoming Asteroid?
NASA catalogs near-Earth objects larger than 1 km in diameter
Sometimes there is not much warning before object comes close
There will eventually be an object to impact Earth – do not know when
No formal plan of action, national or international
No mechanism for implementing any action

41. Case In Point A Round Hole in the Desert: Meteor Crater, Arizona
Classic open-crater impact site
1.2 km across, 180 m deep, circular with raised rims
Figure 17-8a

42. Case In Point
Formed only 50,000 years ago by iron meteorite about 60 m across
Eugene Shoemaker studied crater as graduate student and found mineral evidence for impact, finally convincing scientific community

43. Case In Point A Close Grazing Encounter: Tunguska, Siberia
Asteroid 50 m diameter blew down and charred about 1,000 square km of forest in Siberia, but no crater was formed
Asteroid exploded with energy of 1,000 Hiroshima-sized atomic bombs, 8 km high in atmosphere
Huge fireball across sky was followed by bright flash, loud bangs, shaking ground, blasts of hot air
1921 expedition to area found trees blown radially outward from explosion site, but no meteorite fragments
Later microscope examination of soil discovered iron oxide meteoritic dust – object probably was stony meteorite