Impact Hazards
Run for your lives...!
16
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?
16
Concept Test
•
The Moon shows far more craters on its surface than the
Earth. We can conclude that:
a.
b.
c.
d.
e.
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.
16
Clearwater Lakes Crater
Mjolnir Crater
Earth
Manicouagan Crater
Wolfe Creek Crater
16
Craters
16
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
16
16
IRTF
16
HST imaged impact
plume rising high
above Jupiter’s
surface.
16
Galileo
16
16
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?
16
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.
16
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
•
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.
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.
16
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.
16
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.
b.
c.
d.
e.
Once every million years.
Once every few centuries.
Once every 100 million years.
Once every 10,000 years.
Once in the lifetime of the planet.
16
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.
16
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 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
other hazard we face
• 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.
16
16
16
Torino Scale
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
DATE
(UT)
MISS
DISTANC
E
2006 UQ216
Nov. 7
5.6 LD
21
~15 m
2006 WB
Dec. 4
6.9 LD
17
~130 m
MAG.
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.
SIZE
16
Concept Test
•
The greatest threat to life due to a major impact
event is:
a.
b.
c.
d.
Being hit by the impactor itself.
Being within the formation of the impact crater.
Being within the initial fireball of the impact.
The resulting environmental and climactic effects of
the impact.
16
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)
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