1. Impact Hazards
Run for your lives...!

2. Goals What is the evidence for impacts in the solar system?
What is the evidence for impacts on Earth?
What is the evidence for mass extinctions due to impacts?
What are the odds of future impacts?

3. Concept Test
The Moon shows far more craters on its surface than the Earth. We can conclude that:
The Moon has been hit by far more objects than the Earth.
The Earth’s magnetosphere protects us from the vast majority of impacts.
The Earth has encountered just as many potential impactors but most have burned up in our atmosphere.
The Earth has been hit just as often as the Moon, but geological processes have covered over or erased the majority of craters.
The Earth’s larger gravity acts as a slingshot and redirects most potential impactors away into the Oort Cloud.

4. Earth Clearwater Lakes Crater Mjolnir Crater Manicouagan Crater
Wolfe Creek Crater

5. Craters

6. SL9
Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen.
Tidal forces tore it apart during previous encounter with Jupiter

7. SL9 Geometry

9. IRTF

10. HST imaged impact plume rising high above Jupiter’s surface.

12. Galileo

13. Chain of craters on Callisto, of another comet torn apart by tidal forces from Jupiter.

14. Mass Extinctions
Large dips in total species diversity in the fossil record.
The most recent was 65 million years ago, ending the reign of the dinosaurs.
Impacts?

15. Evidence Iridium Shocked crystals Tektites Soot
Worldwide iridium layer laid
down 65 million years ago.
Very rare in Earth surface rocks but often found in meteorites.
Shocked crystals
Tektites
Soot

16. Comet or asteroid about 10km in diameter approaches Earth

21. show that a large impact occurred
An iridium-rich sediment layer and an impact crater on the Mexican coast
show that a large impact occurred
at the time the dinosaurs died out, 65 million years ago.
Map of variable gravity strength; white lines show the outlines of land masses
and the Mexican regions.

22. Other Impact-Extinctions?
Gravity Map
Credit – OhioState University
Other Impact-Extinctions?
Radar Map
Credit – Ohio State University
Permian-Triassic Exctinction.
250 million years ago.
Crater of similar age found in Antarctica.

23. Concept Test
Fossil evidence suggests a mass extinction occurring 65 million years ago. Which of the following is NOT a piece of evidence supporting the idea that an impact caused this mass extinction?
Unusually large abundances of iridium and other rare metals in a layer of clay that dates to 65 million years ago.
An impact crater along the coast of Mexico that dates to 65 million years ago.
Grains of quartz that must have formed under very high pressure are found in a layer of clay that dates to 65 million years ago.
Fossilized dinosaur bones that contain fragments of rock that must have been shot out by the impact.
A layer of 65 million year old soot at the K-T boundary.

24. Facts Asteroids and comets have hit the Earth.
A major impact is only a matter of time: not IF but WHEN.
Major impact are very rare.
Extinction level events ~ millions of years.
Major damage ~ tens-hundreds of years.

25. Tunguska, Siberia: June 30, 1908
The ~40 meter object disintegrated and exploded in the atmosphere
Several atomic bombs worth of energy.

26. Meteor Crater, Arizona: 50,000 years ago (50 meter object)
Crater is 1 km in diameter.
Impact was 20 megatons
Privately owned National Landmark.
Meteor Crater, Arizona: 50,000 years ago (50 meter object)

27. Concept Test
On average, how often should an asteroid or comet strike Earth with as much energy as that of a hydrogen bomb (like the Tunguska impact)?
Once every million years.
Once every few centuries.
Once every 100 million years.
Once every 10,000 years.
Once in the lifetime of the planet.

28. Chance of death by asteroid: About one in 100,000
From 7-April-2004 congressional testimony of Astronaut Ed Lu, B612 Foundation: 10% chance during our lifetime of a 70 m asteroid, impacting with energy of 10 megatons or 700 Hiroshima bombs. (Senate Subcommittee on Science, Technology, and Space.) Note that 50 meters was the size of the object that created Meteor Crater.
Impacts will certainly occur in the future, and while the chance of a major impact in our lifetimes is small, the effects could be devastating.

29. What Happens When an Impact Takes Place?
Bolides (up to 5 MT)
Great fireworks display, no damage
Tunguska-class (15 MT) impact
Damage similar to large nuclear bomb (city-killer)
Average interval for whole Earth: 100 yr.
Minor risk relative to other natural disasters (earthquakes, etc.)
Larger local or regional catastrophes (e.g.10,000 MT)
Destroys area equivalent to small country
Average interval for whole Earth: 100,000 yr.
Moderate risk relative to other natural disasters
Global catastrophe (> 1 million MT)
Global environmental damage, threatening civilization
Average interval for whole Earth: 1 million years
Major risk relative to other natural disasters

30. Hazards of Global Catastrophe
Kills more than 1.5 billion people
Energy threshold calculated to be near 1 million MT
Primary global effect is from stratospheric dust and smoke.
Average interval 500,000 to 1 million yrs.
Unique in capacity to destabilize civilization
Can be compared with global nuclear war
Only known natural hazard that can destroy civilization

31. Terrestrial Impact Frequency
year
Hiroshima
century
Tunguska
ten thousand yr.
million yr.
K/T
billion yr.
0.01 1
100
10,000
million
100 million
TNT equivalent yield (MT)

32. Terrestrial Impact Frequency
Hiroshima
year
Tunguska
century
Tsunami
danger
ten thousand yr.
Global
catastrophe
million yr.
K/T
billion yr.
0.01 1
100
10,000
million
100 million
TNT equivalent yield (MT)

33. Comparison with Other Risks
Statistical risk of death from impacts is about
1 in a million per year, or about 1:20,000 lifetime risk
Much less (in U.S.) than auto accidents, shootings
Comparable with other natural hazards (e.g. earthquakes, floods)
Near threshold for hazards most people are concerned about
Well above threshold for U.S. governmental or regulatory action
Severity of disasters (billions of people killed) is greater than any other known hazard we face
Apparently unique in its threat to civilization
Places this disaster in a class by itself
Average interval between major disasters (hundreds of millennia) is larger than for any other hazard we face
Causes some to question credibility of hazard

34. The asteroid with our name on it
We haven’t seen it yet (maybe).
Deflection is more probable with years of advance warning.
Control is critical: breaking a big asteroid into a bunch of little asteroids is unlikely to help.
We get less advance warning of a killer comet…

35. The asteroid with our name on it?
Asteroid Apophis: 320 meter diameter
Close approach: 2013, 2029
Possible collision: 2036
Potential significant local/regional damage
Plan of action
Use radar to get better orbit in 2013
Decide if we need a deflection mission by 2021, in time to do the mission in 2029.

37. Torino Scale

38. www.spaceweather.com Nov-Dec 2006 Earth-asteroid encounters
Potentially Hazardous Asteroids (PHAs) are space rocks larger than approximately 100m that can come closer to Earth than 0.05 AU. None of the known PHAs is on a collision course with our planet, although astronomers are finding new ones all the time.
On 28 Nov 2006 there were 834 known Potentially Hazardous Asteroids
Nov-Dec 2006 Earth-asteroid encounters
ASTEROID
 DATE (UT)
MISS DISTANCE
MAG.
 SIZE
2006 UQ216
Nov. 7
5.6 LD 21
~15 m
2006 WB
Dec. 4
6.9 LD 17
~130 m
Notes: LD is a "Lunar Distance." 1 LD = 384,401 km, the distance between Earth and the Moon. 1 LD also equals AU. MAG is the visual magnitude of the asteroid on the date of closest approach.

39. Concept Test
The greatest threat to life due to a major impact event is:
Being hit by the impactor itself.
Being within the formation of the impact crater.
Being within the initial fireball of the impact.
The resulting environmental and climactic effects of the impact.

40. Impact Models http://www.lpl.arizona.edu/impacteffects
Use asteroid Eros.
13 x 13 x 33 km (or 22 km diameter sphere)
Density 1.24 g/cm3
Impact:
LA (60 miles)
Washington DC (2,200 miles)

41. What are we doing about potential impacts?
Stay tuned to

42. Homework #21 Due Monday 24 November: Special Presentation,
Read