Transcript Impact Hazards - the University of Redlands
Impact Hazards
Run for your lives...!
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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?
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Concept Test
• a.
b.
c.
d.
e.
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.
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Mjolnir Crater
Earth
Clearwater Lakes Crater Manicouagan Crater Wolfe Creek Crater 16
Craters
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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 16
SL9 Geometry
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IRTF
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HST imaged impact plume rising high above Jupiter’s surface.
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Galileo 16
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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?
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Evidence
• Iridium – 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
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.
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Other Impact-Extinctions?
Radar Map Credit – Ohio State University • Permian-Triassic Exctinction.
• 250 million years ago.
• Crater of similar age found in Antarctica.
Gravity Map Credit – OhioState University 16
Concept Test
• a.
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? b.
c.
d.
e.
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.
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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.
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Tunguska, Siberia: June 30, 1908 The ~40 meter object disintegrated and exploded in the atmosphere 16
Meteor Crater, Arizona: 50,000 years ago (50 meter object) 16
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)? a. Once every million years.
b. Once every few centuries.
c. Once every 100 million years.
d. Once every 10,000 years. e. Once in the lifetime of the planet.
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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.
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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 16
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 other hazard we face
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 • Causes some to question credibility of hazard 16
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… 16
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.
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www.spaceweather.com
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 2006 UQ216 2006 WB DATE (UT) Nov. 7 Dec. 4 MISS DISTANC E 5.6 LD 6.9 LD MAG.
21 17 SIZE ~15 m ~130 m Notes:
LD
is a "Lunar Distance." 1 LD = 384,401 km, the distance between Earth and the Moon. 1 LD also equals 0.00256 AU.
MAG
is the visual magnitude of the asteroid on the date of closest approach.
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Concept Test
• The greatest threat to life due to a major impact event is: a. Being hit by the impactor itself.
b. Being within the formation of the impact crater.
c. Being within the initial fireball of the impact.
d. The resulting environmental and climactic effects of the impact.
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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/cm 3 – Impact: • LA (60 miles) • Washington DC (2,200 miles) 16
What are we doing about potential impacts?
• Stay tuned to http://neo.jpl.nasa.gov/programs/ 16
Homework #21
• • • Due Monday 24 November: Special Presentation, Read www.planetary.org/parks 16