2. Learning Objectives
Differentiate between asteroids, meteoroids, and comets.
Describe the physical processes associated with airbursts and impact craters.
Suggest the possible causes of mass extinction, and provide examples.
Evaluate the evidence for the impact hypothesis that produced the mass extinction at the end of the Cretaceous Period.

3. Learning Objectives, cont.
List the physical, chemical, and biological consequences of impact from a large asteroid or comet.
Analyze the risk of impact or airburst of extraterrestrial objects, and suggest how that risk might be minimized.

4. Chelyabinsk Meteor, February 2013
Small asteroid traveling about 20 times faster than speeding bullet
Entered Earth’s atmosphere and exploded
People living in Chelyabinsk could feel the intense heat from the fireball as well as the shock wave from the explosion
Meteor did not impact Earth
Exploded in the stratosphere about 23 km (~15 mi) above the surface
Released energy equivalent to 20 to 30 World War II-era nuclear explosions
Largest known asteroid to enter Earth’s atmosphere since Tunguska airburst

5. Meteor Impact Locations

6. Chelyabinsk Meteor, February 2013, cont.
Before entry and explosion, no one knew it was coming
Meteor astrophysicists were looking the other way tracking a much larger and completely unrelated asteroid
Chelyabinsk meteor damage
More than 7000 buildings in six cities damaged
About 1500 people required medical attention
People were confused and frightened about what was happening
Mobile phone networks were overloaded with calls
Office buildings evacuated and schools closed
Broken windows in homes required immediate attention due to weather

7. 14.1 Earth’s Place in Space
Origins of universe begin with “Big Bang” 14 billion years ago
Explosion producing atomic particles
First stars probably formed 13 billion years ago
Lifetime of stars depends on mass
Large stars burn up more quickly ~100,000 years
Smaller stars, like our sun, ~10 billion years
Supernovas signal death of star
No longer capable of sustaining its mass and collapses inward
Explosion scatters mass into space creating a nebula
Nebula begins to collapse back inward on itself and new stars are born in a solar nebula

8. Earth History

9. 14.1 Earth’s Place in Space, cont.
Five billion years ago, supernova explosion triggered the formation of our sun
Sun grew by buildup of matter from solar nebula
Pancake of rotating hydrogen and helium dust
After formation of sun, other particles were trapped in rings
Particles in rings attracted other particles and collapsed into planets
Earth was hit by objects that added to its formation
Bombardment continues today at a lesser rate

10. Solar Nebula

11. Sizes in the Solar System

12. Asteroids, Meteoroids, and Comets
Particles in solar system are arranged by diameter and composition
Asteroids
Found in asteroid belt between Mars and Jupiter
Composed of rock, metallic, or combinations
Meteoroids are broken up asteroids
Meteors are meteoroids that enter Earth’s atmosphere
Burn and create “shooting stars”
Comets have glowing tails
Composed of rock surrounded by ice
Originated in Oort Cloud beyond the Kuiper Belt

13. Meteorites and Related Objects

14. Diagram of Our Planetary System Showing Asteroid Belt and Kuiper Belt

15. Comet Hale-Bopp

16. 14.2 Airbursts and Impacts
Objects enter Earth’s atmosphere at 12 to 72 km/s (~7 to 45 mi per second)
Metallic or stony
Heat up due to friction as they fall through atmosphere, produce bright light and undergo changes
Meteoroid will either
Explode into an airburst
Object explodes in atmosphere 12 to 50 km (~7 to 31 mi.)
Ex: Tunguska
Or collide with Earth as a meteorite
If the object strikes Earth
Concentrated in Antarctica

17. Meteoroid Entering Earth’s Atmosphere

18. Impact Craters Provide evidence of meteor impacts
Example: Barringer Crater in Arizona
Bowl shaped depressions with upraised rim
Rim is overlain by ejecta blanket, material blown out of the crater upon impact
Broken rocks cemented together into Breccia
Features of impact craters are unique from other craters
Involve high velocity, energy, pressure and temperature
Kinetic energy of impact produces shock wave into earth
Compresses, heats, melts, and excavates materials
Rocks become metamorphosed or melt with other materials

19. Extraterrestrial Impact Craters of the United States and Canada

20. Impact Craters, cont. Can be grouped into two types
Simple craters
Typically small < 6 km (4 mi.)
Ex. Barringer Crater
Complex impact craters
Larger in diameter > 6 km (4 mi.)
Rim collapses more completely
Center uplifts following impact
Ancient impact craters difficult to identify
Usually eroded or filled with sedimentary depostis
Examples: Chesapeake Bay in U.S. and Manicouagan Crater in Quebec

21. Simple Impact Crater in Arizona

22. Chesapeake Bay Impact Crater

23. Complex Impact Crater in Quebec

24. Notable Impacts and Airbursts of Extraterrestrial Objects

25. Impact Craters, cont. Craters are much more common on Moon
Most impact sites on Earth are in ocean where they are buried or destroyed
Impact craters on land are now generally subtle features because they have been eroded or buried by debris
Smaller meteoroids and comets tend to burn up and disintegrate in Earth’s atmosphere before impact

26. Uniformitarianism, Gradualism, and Catastrophism
Those studying the formation of mountains, large river valleys, and other features had a hard time understanding how they could be formed in 6000 years
Based on Archbishop Ussher’s “young Earth” belief, concluded that the processes were catastrophic in nature
Gradualism or Uniformitarianism
James Hutton introduced concept in 1785, popularized by Charles Hutton
Present geological processes may be studied to learn the history of the past
Argued Earth must be much older than 6000 years

27. Uniformitarianism, Gradualism, and Catastrophism, cont.
Uniformitarianism lasted into the twentieth century and culminated with plate tectonics
Occasionally scientists also discover evidence of catastrophic events
Impact craters
Rapid extinctions
Lead to a new concept
Punctuated uniformitarianism
Although uniformitarianism explains the long geologic record of gradual mountain building, canyon erosion, and landscape construction, periodic catastrophic events do occur and can cause mass extinctions

28. 14.3 Mass Extinctions
Sudden loss of large numbers of plants and animals relative to number of new species being added
Defines the boundaries of geologic periods or epochs
Usually involve rapid climate change, triggered by
Plate Tectonics
Slow process that moves habitats to different locations
Volcanic activity
Flood basalts produce large eruptions of CO2, warming Earth
Silica-rich explosions produce volcanic ash that reflects radiation, cooling Earth
Extraterrestrial impact or airburst

29. 14.3 Mass Extinctions, cont. Six major mass extinctions
Ordovician, 446 mya, continental glaciation in Southern Hemisphere
Permian, 250 mya, volcanoes causing global warming and cooling
Triassic-Jurassic boundary, 202 mya, volcanic activity associated with breakup of Pangaea
Cretaceous-Paleogene boundary (K-Pg boundary), 65 mya, Asteroid impact
Eocene period, 34 mya, plate tectonics
Pleistocene Epoch, initiated by airburst, continues today caused by human activity

30. Mass Extinction Events

31. K-Pg Boundary Mass Extinction
Dinosaurs disappeared with many plants and animals
70 percent of all genera died
Set the stage for evolution of mammals
First question, What does geologic history tell us about K-Pg Boundary?
Walter and Luis Alvarez decided to measure concentration of Iridium in clay layer at K-Pg boundary in Italy
Fossils found below layer were not found above
How long did it take to form the clay layer?
Iridium deposits say that layer formed quickly
Probably extinction caused by single asteroid impact

32. K-Pg Boundary Mass Extinction, cont.
Alvarez did not have a crater to prove the theory
Crater was identified in 1991 in Yucatan Peninsula
Diameter approx. 180 km (112 mi.)
Nearly circular
Semi-circular pattern of sinkholes, cenotes, on land defining edge
Possibly as deep as 30 to 40 km (18 to 25 mi.)
Slumps and slides filled crater
Drilling finds breccia under the surface
Glassy indicating intense heat

33. Large Impact Crater in Mexico

34. K-Pg Boundary Mass Extinction, cont.
Sequence of events
Asteroid moving at 30 km (19 mi.) per second
Asteroid impacts Earth produces crater 200 km (125 mi.) diameter, 40 km (25 mi.) deep
Shock waves crush, melt rocks, vaporized rocks on outer fringe

35. K-Pg Boundary Mass Extinction, cont.
Sequence of events, cont.
Seconds after impact
Ejecta blanket forms
Mushroom cloud of of dust and debris
Fireball sets off wildfires around the globe
Sulfuric acid enters atmosphere
Dust blocks sunlight
Tsunamis from impact reached over 300 m (1000 ft.)

36. K-Pg Boundary Mass Extinction, cont.
Sequence of events, cont.
Month later
No sunlight, no photosynthesis
Continued acid rain
Food chain stopped
Several months later
Sunlight returns
Acid rain stops
Ferns restored on burned landscape

37. K-Pg Boundary Mass Extinction, cont.
Impact caused massive extinction, but allowed for evolution of mammals
Another impact of this size would mean another mass extinction probably for humans and other large mammals
However, impacts of this size are very rare
Occur once ever 40 to 100 my
Smaller impacts are more probable and have their own dangers

38. 14.4 Linkages with Other Natural Hazards
Asteroid or comet impact or airburst direct cause for
Tsunamis
Most impacting objects land in the world’s oceans
Wildfires
Superheated clouds of gas and debris reach temperatures capable of drying out then igniting living vegetation
Earthquakes
Seismic waves created from impact
Mass Wasting
Earthquakes activate numerous landslides on land and under water
Climate Change
Inject large quantities of dust into atmosphere and causes cooling
Warming then follows from large amounts of greenhouse gases
Volcanic Eruptions
Impacts cause melting and instability in Earth’s mantle

39. 14.5 Minimizing the Impact Hazard: Risk Related to Impacts
Risk related to probability and consequences
Large events have consequences will be catastrophic
Worldwide effects
Potential for mass extinction
Return period of 10s to 100s of millions of years
Smaller events have regional catastrophe
Effects depends on site of event
Return period of 1000 years
Likelihood of an urban area hit every few 10,000s years

40. Energy from Impact

41. Risk Related to Impacts, cont.
Risk from impacts is relatively high
Probability that you will be killed by
Impact: 0.01 to 0.1 percent
Car accident: percent
Drowning: percent
However, that is AVERAGE probability over thousands of years
Events and deaths are very rare!

42. Minimizing the Impact Hazard
Identify nearby threatening objects
Spacewatch
Inventory of objects with diameter > 100 m in Earth crossing orbits
85,000 objects to date
Near-Earth Asteroid Tracking (NEAT) project
Identify objects diameter of 1 km
Use telescopes and digital imaging devices
Most objects threatening Earth will not collide form several 1000s of years from discovery

43. Asteroid 243 Ida

44. Minimizing the Impact Hazard, cont.
Options once a hazard is detected
Blowing it up in space
Small pieces could become radioactive and rain down on earth
Nudging it out of Earth’s orbit
Much more likely since we will have time to study object
Technology can change orbit of asteroid
Costly and need coordination of World military and space agencies
Evacuation
Possible if we can predict impact point
Could be impossible depending on how large an area would need to be evacuated

45. The Torino Impact Hazard Scale

46. Chelyabinsk Meteor, February 2013 – Applying the 5 Fundamental Concepts
Largest known asteroid to have entered Earth’s atmosphere since Tunguska event
Chelyabinsk meteor, fireball, and blast was video-recorded by numerous people and sensors monitored by the U.S government
Explosion of meteor
Compression of atmospheric gasses generated massive amounts of heat
Meteor erupted into a fireball
Then exploded and produced a tremendous shock wave

47. Damage from Chelyabinsk Airburst

48. Chelyabinsk Meteor, February 2013 – Applying the 5 Fundamental Concepts, cont.
Small meteorites collected from a hole found penetrating a frozen lake
Long meteorite fragment found at bottom of lake eight months later
Composition and density of the meteor was determined
Chondritic in compositions
Density about 3.6 g per cm (3.6 times that of water)
Total weight estimated to be 11,000 tons
Location of explosion was important
Catastrophic damage if closer to the ground
If over an urban area, millions of deaths could occur

49. Chelyabinsk Impact

50. Chelyabinsk Meteor, February 2013 – Applying the 5 Fundamental Concepts, cont.
How could the effects of an impact be minimized?
Predict when and where a strike may occur so there is enough time to evacuate
However, smaller extraterrestrial objects like the Chelyabinsk meteor are not easily identified
Scientists were tracking an unrelated larger meteor at the time of the Chelyabinsk meteor
Funds appropriated to identify 90 percent of the objects 150 m and larger are insufficient
Technology is available and as it improves, it could identify even smaller NEOs

51. Cosmic Coincidence

52. Chapter 14 Summary
The evolution of Earth most likely started about 14 billion years ago with the so-called explosion of the “Big Bang” that produced atomic particles that later formed galaxies, stars, and planets.
Asteroids, meteoroids, and comets are extraterrestrial objects that can intercept Earth’s orbit.
A meteoroid entering the atmosphere at about 85 km (~53 mi) above Earth’s surface at a velocity of km per second (~7 to 45 mi per second) will become a meteor and emit light.

53. Chapter 14 Summary, cont.
Objects that impact Earths surface produce both simple and complex craters.
Large objects can cause local to global catastrophic damage and mass extinction of life.
Recent studies suggest that an airburst may have been responsible for the extinction of many large Pleistocene mammals and the disappearance of the Clovis people at the start of the Younger Dryas period 12,800 years ago.

54. Chapter 14 Summary, cont.
Many linkages exist between impacts of comets and asteroids with Earth and other natural hazards.
Risk assessment is difficult for events with long return periods.
Sufficient studies of comets and asteroids and their interaction with planets enable us to make fairly straightforward predictions of the return time that an object of a particular size would impact Earth.

55. Chapter 14 Summary, cont.
Programs such as Spacewatch and NEAT (Near-Earth Asteroid Tracking) can, with high certainty, identify NEOs of diameters greater than a few hundreds meters at least 100 years before possible impact.
The human population of Earth today is about 7 billion, compared to about 1 billion in Thus, an event in 1830 that might have killed a million people would likely kill more than 10 million people today.
The most encouraging note is that a large comet or asteroid would probably be seen a hundred years or more before it is likely to strike Earth.