Natural Disasters Unit 2
Extraterrestrial ThreatsImpacts and Extinctions
Chapter 13
Learning Objectives Know the difference between asteroids,
meteoroids, and comets
Understand the physical processes associated with airbursts and impact craters
Understand the possible causes of mass extinction
Know the evidence for the impact hypothesis that produced the mass extinction at the end of the Cretaceous period
Learning Objectives, cont. Know the likely physical, chemical, and biological
consequences of impact from a large asteroid or comet
Understand the risk of impact or airburst of extraterrestrial objects and how that risk might be minimized
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 Releases energy and shock waves Click to see movie at right:
Earth’s Place in Space, cont. 5 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, adding to its formation
Bombardment continues today
Figure 13.2
Asteroids, Meteoroids, and Comets Particles in solar system are arranged by
diameter and composition.
Asteroids 10m (30ft)–1000 km(620 mi). 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.
We landed a probe on this asteroid: Eros
Comets have glowing tails composed of frozen water or carbon dioxide Originated in Oort cloud
Figure 13.3
Airbursts and Impacts Objects enter Earth’s atmosphere at 12–72 km/s
(27,000–161,000 mph) Metallic or stoney Heat up due to friction as they fall through atmosphere,
produce bright light
Meteorites If the object strikes Earth Concentrated in Antarctica
Airbursts Object explodes in atmosphere 12–5o km (7 –31 mi) Ex: Tunguska
Figure 13.5
Impact Crater Provide evidence of meteor impacts.
Bowl-shaped depressions with upraised rim Rim is overlain by ejecta blanket Broken rocks cemented together into breccia
Features of impact craters are unique from other craters. Impacts 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.
Figure 13.6
Simple Impact Craters Typically small < 6
km (4 mi)
Ex. Barringer Crater
Figure 13.7
Complex Impact Craters Larger in diameter >
6 km (4 mi)
Rim collapses more completely
Center uplifts following impact
Figure 13.9
Impact Crater, cont. Craters are much more common on Moon.
1. Most impacts are in ocean, buried, or destroyed.2. Impacts on land have been eroded or buried by
debris.3. Smaller objects burn up in Earth’s atmosphere
before impact.
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
Moves habitats to different locations Volcanic activity
Large eruptions release CO2, warming Earth Volcanic ash reflects radiation, Cooling Earth
Extraterrestrial impact
Six Major Mass Extinctions
1. Ordovician, 446 mya, continental glaciation in Southern Hemisphere
2. Permian, 250 mya, volcanoes causing global warming and cooling
3. Triassic–Jurassic boundary, 202 mya, volcanic activity associated with breakup of Pangaea
4. Cretaceous–Tertiary boundary (K-T boundary), 65 mya, asteroid impact
5. Eocene period, 34 mya, plate tectonics
6. Pleistocene epoch, initiated by airburst, continues today, caused by human activity
K-T Boundary Mass Extinction Dinosaurs disappeared with many plants and
animals. 70% of all genera died Set the stage for evolution of mammals
First question: What does geologic history tell us about K-T Boundary? Walter and Luis Alvarez decided to measure concentration
of Iridium in clay layer at K-T 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.
K-T 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 30–40 km (18–25 mi) Slumps and slides filled crater Drilling finds breccia under the surface
Glassy indicating intense heat
Figure 13.12
Sequence of Events
a) Asteroid moving at 30 km (19 mi) per second
b) 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
Figure 13.13b
Figure 13.13a
Sequence of Events, cont.
c) Seconds after impact:• Ejecta blanket forms• Mushroom cloud of dust
and debris• Fireball sets off wildfires
around the globe• Sulfuric acid enters
atmosphere• Dust blocks sunlight• Tsunamis from impact
reach over 300 m (1000 ft) Figure 13.13c
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
Figure 13.13d
Figure 13.13e
K-T Extinction, Final 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 40–100 my
Smaller impacts are more probable and have their own dangers.
Linkages with Other Natural Hazards Tsunamis
Wildfires
Earthquakes
Mass wasting
Climate change
Volcanic eruptions
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 10’s–100’s 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 10,000 years
Risk Related to Impacts, cont. Risk from impacts is relatively high.
Probability that you will be killed by Impact: 0.01%-0.1% Car accident: 0.008% Drowning: 0.001%
However, that is AVERAGE probability over thousands of years.
Events and deaths are very rare!
The Torino Impact Scale
What is it for? •A "Richter Scale" for newly discovered asteroids and comets.
•A communication tool for astronomers and the public.
Why is it needed? •Predictions for newly discovered NEOs are naturally uncertain.• •For most objects, even the initial calculations are sufficient to show that they will not make any close passes by the Earth within the next century.
•However, for some objects, 21st century close approaches and possible collisions with the Earth cannot be completely ruled out.
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 for several 1000’s of years from discovery.
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
EndNatural Disasters Unit 2
Extraterrestrial ThreatsImpacts and Extinctions
Chapter 13
