1166

Impact HazardsImpact Hazards

Run for your lives...!

1616

GoalsGoals

• 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?

1616

Concept Test

• The Moon shows far more craters on its surface than the Earth. We can conclude that: a. The Moon has been hit by far more objects than the

Earth.b. The Earth’s magnetosphere protects us from the vast

majority of impacts.c. The Earth has encountered just as many potential

impactors but most have burned up in our atmosphere.

d. The Earth has been hit just as often as the Moon, but geological processes have covered over or erased the majority of craters.

e. The Earth’s larger gravity acts as a slingshot and redirects most potential impactors away into the Oort Cloud.

1616

Earth

Manicouagan Crater

Wolfe Creek Crater

Clearwater Lakes CraterMjolnir Crater

1616

Craters

1616

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

SL9

1616

SL9 Geometry

1616

1616

IRTF

1616

HST imaged impact plume rising high above Jupiter’s surface.

1616Galileo

1616

1616

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?

1616

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

1616

An iridium-rich sediment layer and an impact crater on the Mexican coastshow that a large impact occurredat the time the dinosaurs died out, 65 million years ago.

1616

Other Impact-Extinctions?

• Permian-Triassic Exctinction.

• 250 million years ago.

• Crater of similar age found in Antarctica.

Gravity Map Credit – OhioState University

Radar MapCredit – Ohio State University

1616

Concept Test• 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? a. Unusually large abundances of iridium and other rare

metals in a layer of clay that dates to 65 million years ago.

b. An impact crater along the coast of Mexico that dates to 65 million years ago.

c. 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.

d. Fossilized dinosaur bones that contain fragments of rock that must have been shot out by the impact.

e. A layer of 65 million year old soot at the K-T boundary.

1616

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.

1616

Tunguska, Siberia: June 30, 1908 The ~40 meter object disintegrated and exploded in the atmosphere

1616Meteor Crater, Arizona: 50,000 years ago (50 meter object)

1616

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.

1616

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.

1616

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

1616

Comparison with Other Risks

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 actionSeverity of disasters (billions of people killed) is

greater than any other known hazard we face• Apparently unique in its threat to civilization• Places this disaster in a class by itselfAverage interval between major disasters

(hundreds of millennia) is larger than for any other hazard we face

• Causes some to question credibility of hazard

1616

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…

1616

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.

1616

1616

Tori

no S

cale

1616

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 DATE

(UT)

MISS DISTANC

E MAG.

 SIZE

2006 UQ216 Nov. 7 5.6 LD 21 ~15 m

2006 WB Dec. 4 6.9 LD 17 ~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.

www.spaceweather.com

1616

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.

1616

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/cm3

– Impact:• LA (60 miles)• Washington DC (2,200 miles)

1616

What are we doing about potential impacts?

• Stay tuned to http://neo.jpl.nasa.gov/programs/

1616

Homework #21Homework #21

• Due Monday 24 November:• Special Presentation,• Read www.planetary.org/parks