24.Comets

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Natural Hazards and Disasters
Chapter 17
Impacts of Asteroids and Comets
Projectiles from Space
• Asteroids, comets and meteors cross
Earth’s path
• Asteroids and comets are known as bolides
Asteroid Impact 65 million yrs ago
•
•
•
•
Asteroid 10-15 km wide struck Yucatan
Formed Chicxulub crater 80-110 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
Crater is not exposed on surface
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
End of the Mesozoic
Major extinction event:
85% of all species died.
CretaceousTyrannosaurus
Rex
Side Effects
• Huge tsunami waves left debris 50-100 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
Meteor Crater,
Arizona
A more
recent
impact
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
Jupiter
Venus
Sun
Saturn
Neptune
Earth
Pluto
Mercury Mars
Inner Asteroid
planets
belt
The inner planets
are small and
rocky.
Uranus
Outer
planets
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 60-70 km/s, making impacts
catastrophic
Comets
• Comets spray off water, dust and volatiles to form
glowing tails when hit by solar wind (tail points away
from sun)
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
Identification of Meteorites
• Iron meteorites are 6% of all meteorites
– Similar to Earth’s core
Identification of Meteorites
• Chondrites are stony meteorites, 93% of
all meteorites
– similar to Earth’s mantle
Evidence of Past Impacts
(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?
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
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
Impact Craters
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
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
Consequences of Impacts with Earth
• Impact of 1.5-2 km in diameter asteroid
might kill up to 25% of people
• Would threaten civilization as we know it
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
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
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
Evaluating the Risk of Impact
• 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
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
Chances of a Significant Impact on Earth
• Major impacts occur about every 33
million years
– Major extinctions seem to occur about every
26-31 million years
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 10-100s 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
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
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
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
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
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
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
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