Hazards from Impacts and Historical Impacts

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Hazards from Impacts and Historical Impacts
Dallas Abbott Dec 1, 2005
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Collaborators

• Andrew Matzen
• Stephen Pekar
• Edward Bryant
• Mauri McSaveney
• Mohi Kumar
• Paul Krusic

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Goals of Talk

• Hazards from Impact from Historical Impacts
• A) Explosion
• B) Pressure Pulse
• C) Wildfires
• D) Tsunami (Oceanic Impacts only)
• Holocene Impact Rate Could Be High

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6 historical impacts

• Shoemaker-Levy
• Lunar Impact in Medieval Times
• Lunar Impact in 1953
• Tunguska 1908
• Brazilian Tunguska 1930
• Mahuika crater 1443 A.D.

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Typical Comet near Sun
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Shoemaker-Levy July 16-22, 1994string of pearls
before collision
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Impacts G and H (out of over 21)
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Infrared ImagesCometShoemaker-Levyhits Jupiter
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Lessons from Shoemaker-Levy

• Most comets hit Jupiter, not Earth
• Jupiter is our safety net
• Comets rock plus ice weak, friable
• Gravity of Jupiter breaks up comet
• Shoemaker Levy type event multiple impacts
  common from comets
• Impact produces explosions

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June, 1178 Gervase of Canterbury
On the evening of June 18, 1178 after sunset when
the moon had first become visible a marvelous
phenomenon was witnessed by some five or more
men...Now there was a bright new moon...its horns
were tilted toward the east and suddenly the
upper horn split in two. From the midpoint of the
division a flaming torch sprang up, spewing out,
over a considerable distance, fire, hot coals,
and sparks. Meanwhile the body of the moon which
was below writhed, as it were, in anxiety...the
moon throbbed like a wounded snake. Afterwards it
resumed its proper state. This phenomenon was
repeated a dozen times or more, the flame
assuming various twisting shapes at random...Then
after these transformations the moon from horn to
horn...took on a blackish appearance. The present
writer was given this report by men who saw it
with their own eyes, and are prepared to stake
their honor on it.
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Horns of the Moon
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Giordano Bruno crater (bright with rays)
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Giordano Bruno-astronomer
Seven year trial by the inquisition Burnt at the
stake 1600
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Impact on moon in 1178

• Minimum crater size 20 km to see with naked eye
• Conventional estimate one crater this size every
  311,000-332,000 years (on Earth)
• Moon is smaller than Earth/lower gravity
• Impact rate on Moon lower
• Terrestrial impact rate gt13.5 times higher
• Source crater Giordano Bruno crater (22 km)

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Photo of Lunar Impact-1953
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Impact on the moon in 1953

• An amateur astronomer photographed a flash on the
  moon in 1953
• Could not find a source crater (too small)
• Not a satellite (pre Sputnik)
• Bonnie Buratti found 1.5 km source crater in 2002

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Lessons from Lunar Impacts

• Large impacts have occurred on the Moon in the
  last 1000 years
• High probability of large impacts on the Earth
  during this time period (fragment shower in
  1178?)
• 71 of Earth covered by water
• If large terrestrial impacts occurred, most would
  produce megatsunamis

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Tunguska June 30, 1908
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Tunguska

• 2000 square km of forest leveled
• Fallen trees radiate from impact center
• Impactor exploded in the air
• Trees in very center-upright but burned
• Event during Russian revolution
• Nearest survivor knocked to ground, lost hearing

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Tunguska in ice core data
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Tunguska results60 meter impactor
(Manhattan18x4km)
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Brazilian Tunguska August 13, 1930
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Brazilian Tunguska

• Upper Amazon-Curuca river
• Sun becomes blood red, darkness falls
• Large cloud of red dust, than fine white ash
• Followed by whistling sounds, then 3 explosions
• Ground shakes
• Forest begins to burn, fires last for months
• Several hundred eyewitnesses
• Written accounts lost for years

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Lessons from Tunguska Type Events

• Impactors exploding in air-more dangerous
• Tunguska impactor-only 60 meters
• Effects of Tunguska-like event
• 1) Pressure wave-knocks over trees/people
• 2) Burning from explosion/ejecta fragments
• 3) Loud noise-deafening
• 4) Ground shaking/earthquake

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What We Know Happened 1400-1500 A.D.

• 1) Fireball seen-SE Australia
• 2) Large tsunami runups-NZ, Aus, Lord Howe (over
  40 meters in Australia)
• 3) Torrential rain-NZ, Aus
• 4) Wildfires all over New Zealand
• 5) Coast abandoned in New Zealand

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Aboriginal Legend
Moon rocks Stars fall See Fire Ball In the
morning Steaming pits New caves on coast Rains
and Rains Inference large impact in Tasman
Sea Produced crater.
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Widespread Fires 1400 A.D.
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Do These Events Have One Cause?
Crater
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Location Map-Mahuika Crater
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Mahuika crater

• Named for Maori goddess of fire
• 24 km in diameter
• Average crater formation rate 1 per 330,000
  years
• Highest rate 1 per 1100 years if comet cloud
  disturbed (Oort cloud)
• Mahuika crater formed at 1443 A.D.

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How Hot Does It Get?Mahuika tektite on
leftRocks Melt at 1200C
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Asteriod Impact Off New York City
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Oblique Impact-2.9 Seconds Later
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Oblique Impact-8.4 Seconds Later
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Impact Splash
All impacts into water -make water
vapor Temperatures VERY HIGH 5000-15,000
C Splash to side-all impacts are oblique
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Rocks Melt and Then Crystallize
Result Impact Spherules! Tektites!
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Impact Spherules on 1 mm squares
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Detailed Location Map
F80 has tektite D160 has tektite GreenRippled B
lackUndisturbed RedDisturbed
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Oceanic Impacts Produce Big Waves How Big?
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Impact Tsunami
Biggest Wave Height Water Depth Mahuika Water
Depth 300 meters Impactor 1 km to make 20 km
crater Run Up Height 3-10 times wave height
(sometimes more with edge waves)
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Change in Height of Tsunami over Distance
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Australian tsunami record-Eastern Australia 2
big events

• Youngest
• 1450 A.D.
• 130 m runup in Jervis Bay
• no obvious seismic source

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Effects on Australian Coast
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More Boulders-SE Australia
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SE Australia-Boulder w Rocks Beneath
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Interpretation
First Slide-Large Boulders on Cliff- Minimum
Tsunami Height 15 to 20 meters (I have seen
large boulders on top of 32 meter high cliff in
Australia-are from this event.) Third Slide-
Small Rocks Beneath Boulder indicate it settled
out Largest tsunami run up identified 130
meters at Steamers Beach-Jervis Bay- Tsunami
transported boulders.
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Social Effects of Mahuika Impact
Tsunami removes shellfish (mussels, clams) from
rocks Maori move inland
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Lessons from Mahuika Impact
Impacts into oceans produce large
waves-megatsunami Large oceanic impacts vaporize
tons of seawater Large impacts produce hot
ejecta-can ignite fires/burn trees
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Reassuring Lessons
Mahuika impact dated from ice core as 1443
A.D.- no big climate change at 1443 A.D. 24 km
crater-did not destroy civilization big local
effects but no global effects
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Close up of asteroidnot round-too small
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How to Move an Asteroid
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Conclusions
Large impacts dangerous but rare Tunguska size
event 1 or 2 per century-locally devastating-but
planet large Mahuika size event 1 per 1100
years? (This is 300 times average long term rate)