Final exam schedule at

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Final exam schedule at http//www.artsandscie
nce.utoronto.ca/current/exams (also linked
from course web site)
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AST 251 Life on Other Worlds Lecture 10
TODAY Great Extinctions and Hazards to Life
Coming up Lecture 11 Interstellar
Communication Exolinguistics
SETI programs Lecture 12 Interstellar Travel
Galactic Colonization Final
exam review sessions with the TAs

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Hazards to Civilizations and Life
Hazards from Without Comets
Asteroids Supernovae Gamma-Ray Bursts Molecular
Clouds Passing Stars Solar Evolution Hazards
from Within Epidemics War Volcanic
Eruptions Glaciations (or are they due to
external effects?)
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Extinctions
- 99.9 of all species have become extinct -
Without extinction, we would not be here - Mass
extinctions vs. normal extinction rate There
have been five major Mass extinctions over
time Cretaceous-Tertiary (KT) extinction (65
Myrs ago) dinosaurs, many marine
species Cambrian Extinction (500 Myrs ago) 50
of all animal families Devonian Extinction (345
Myrs ago) 30 of animal families Permian
Extinction (250 Myrs ago) 50 of all animal
families 95 of all marine species many trees
went extinct Triassic extinction (80 Myrs
ago) 35 of all animal families
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Extinctions
Impacts from large (gt 1km) comets or asteroids
can cause global environment and climatic
change Impacts likely played an important role
in the early evolution of life on Earth (and
more recent, too) An impact of a 10 km asteroid
is the theory that best explains the extinction
of the dinosaurs
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Late Heavy Bombardment The Earth suffered a
similar impact history as the Moon -- only worse
because the Earth is more massive (a bigger
target!!)
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Comet and Asteroid Impacts 1994 Jupiter Impact
of Comet Shoemaker-Levy 9
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30 million kg (30,000 tons) of meteorites fall to
Earth each year
shooting stars fireballs
dust sand
rock boulder
mountain
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Evidence of past impacts on Earth
Barringer Crater, Arizona 50,000 years ago 10m
diameter Fe meteorite 95 impactor vaporized
Tunguska event, 1908
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Canadian Impact Craters
Sask Deep Bay (100 Myr)
Ont Sudbury Crater (1.8 Gyr) Wanapitei Crater
(34 Myr)
Qbc Clearwater Lakes (290 Myr)
Nfld Mistastin Lake (38 Myr)
Qbc Manicouagan (212 Myr)
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Chicxulub impact crater
near Cancun, on the Yucatan Peninsula,
Mexico 180 km across 65 Myrs old likely
produced by the impact that killed off dinosaurs
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Evidence supporting the impact theory for
dinosaur extinction 1. Iridium in the KT
boundary layer sediments worldwide 2. Tektites -
glassy rock raindrops found in KT sediments 3.
Shocked quartz near impact site 4. An impact
crater (180 km diameter) of correct age 5. Ash
deposits in worldwide KT sediments (indicating
global wildfire) 6. Tidal waves near impact
site deposited boulders upstream -- now found in
KT sediments
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Evidence supporting the impact theory for
dinosaur extinction 1. Iridium in the KT
boundary layer sediments worldwide 2. Tektites -
glassy rock raindrops found in KT sediments 3.
Shocked quartz near impact site 4. An impact
crater (180 km diameter) of correct age 5. Ash
deposits in worldwide KT sediments (indicating
global wildfire) 6. Tidal waves near impact
site deposited boulders upstream -- now found in
KT sediments
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Major Impacts Economic and Archaeological
Implications
Melt Differentiation of ores (good for mining)
Shocked Fissures Oil Accumulates
Shocked Coal Beds Diamonds can form.
Raised Rim Protects from erosion by water
Interior Lake Good sediment history of climate,
evolution, etc
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Terrestrial Imacts Frequency
Impactor Radius
50 m 1km 3 km
10 km
Hiroshima
year
century
Tunguska
Visible in extinction record
kyr
Tsunami danger
10 kyr
Recurrence Interval
K/T (Dinosaur killer)
Myr
Global economic catastrophe
Gyr
108
106
10,000
100
1
0.01
TNT equivalent yield (MT)
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The Impact Hazard Today
There are about 1500 asteroids gt 1 km in
diameter whose orbits cross the
Earths Estimated 135,000 Earth-crossing
asteroids gt 100 m They sometimes approach
within a few times the Earth-Moon distance.
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• Statistical risk of death from impacts is about
• 1 in a million per year, or about 120,000
  lifetime risk
• Much less than auto accidents and (in U.S.)
  shootings
• Comparable with other natural hazards (e.g.
  earthquakes, floods)
• Near threshold for hazards most people are
  concerned about
• Well above threshold for governmental or
  regulatory action
• Severity of disasters (billions of people killed)
  is as bad as nuclear war, or worse.
• Average interval between major disasters
  (hundreds of millennia) is greater than for
  typical hazards

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AVERAGE ANNUAL RISK OF DEATH (IN PARTS PER
MILLION) 300 Accidents (not motor
vehicle) 200 Homicide suicide 160 Motor
vehicle accidents 10 Fire 5
Electrocution 1 Airplane accidents
0.5 TOTAL IMPACTS (global threshold 1
million Mt) 0.3 Storms and floods
(declining) 0.1 LOCAL / REGIONAL IMPACTS
0.1 Earthquakes (poor statistics) 0.01
TUNGUSKA-TYPE IMPACTS lt0.01 Nuclear accidents
(design goals)
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Protecting against Impacts
Sophistication of protection measure depends on
how early warning arrives.
Detection Monitoring
Intervention
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Protecting against Impacts
Sophistication of protection measure depends on
how early warning arrives.
Detection Monitoring
Intervention
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Protecting against Impacts
Q. What would you do to avoid an impending
impact 1. If you had 100 years 2. If you
had 3 years 3. If you had 6 months
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Protecting against Impacts
Q. What would you do to avoid an impending
impact 1. If you had 100 years A.
Paint-bomb the comet. A fraction of a millimeter
per second is enough to deflect it past the
Earth. 2. If you had 3 years A. Inflate a
huge balloon that is pushed with a rocket
thereby moving the comet gently (to cm/s)
without breaking it. 3. If you had 6 months
A. Not much choice try a nuclear bomb, and
hope for the best
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Another Hazard Supernova Explosions
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Early Phases of a Supernova Explosion
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Effect of Cosmic Rays on Life and Climate

• Cosmic rays from Solar Corona
• stop in upper atmosphere
• produce aurorae
• More energetic cosmic rays from
• supernova remnants
• Seed cloud formation, affecting global climate!
• Add to the mutation rate of organisms on the
  surface

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Protection from Cosmic Rays ?
Cosmic rays are charged particles (atomic nuclei)
that have been accelerated to nearly the speed of
light Charged particles get caught by
magnetic field lines, because the field makes
them spiral around itself
Path of charged particle
This gives the magnetized solar wind the capacity
to shield us from cosmic rays in the galaxy
Note the shielding effect is greater when
there are more sunspots (suns magnetic activity
is strongest)
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Protection from Cosmic Rays ?
cosmic rays
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How close does a supernova have to be to affect
global climate?
About 1 parsec Earth is exposed to expanding
stellar material. About 10 parsecs
Supernova is close enough to compress the
heliopause and expose Earth directly to cosmic
rays. About 100 parsecs A bright flash of
UV radiation, absorbed in the upper atmosphere. A
very bright star in the sky.
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How often does a supernova get that close?
There is about 1 star per cubic parsec. About
1 out of 200 stars is massive enough (gt8 solar
masses, remember?) to go supernova. Within 10
parsecs there should have been about 10
supernovae over the age of the Galaxy, or about
1 supernova over the last Gyr. This makes
supernovae a somewhat lower risk than giant
impacts and we can say for sure that there
isnt one happening soon!
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Other External Hazards

• Passing through a molecular cloud
• Wind of dense material crushes Suns
  magnetosphere, exposing
• us to cosmic rays. May contribute to
  correlation with spiral passages.
• 2. Gamma-Ray Bursts
• Very spectacular visible across
• Universe! But quite rare compared
• to supernovae only 1 is visible per
• Myr inside the Milky Way.
• Gamma-rays are stopped at the top of the
• atmosphere could temporarily destroy
• ozone layer expose surface to solar UV.
• 3. Soft Gamma Repeaters dimmer but more
  frequent explosions from neutron
• stars. Have had visible effect on our
  ionosphere also relatively harmless.

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Latest GRB 030518 Detected by SWIFT satellite
last Friday
Aleks Scholz (a postdoc working with me) took a
spectrum of its afterglow at Magellan Friday
night get a redshift for it
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A gamma-ray flare halfway across our
Galaxy Detected on 2004 December 27 A giant
flare from a highly magnetized neutron star
(a magnetar) Even from 50,000 lyrs
away, it disrupted the Earths ionosphere If it
had happened 10 lyrs from Earth, it would
have destroyed the ozone layer and made
life difficult!
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Hazards from Within War
How well do you trust your fellow
humans? Consider Identifiable governments and
ideologies rarely last more than 1-2 kyr. There
are a thousand kyr per Myr!
100 yr
1 kyr
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Hazards from Within Climate Change