Meteorites: Chemistry and

classification

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Outline of lecture

• Meteorites:
– How are meteorites found?
– Main types
– Where do they come from?
– Meteorites as time capsules

• Cosmic Collisions
– Role of cosmic collisions in evolution of Solar System
– History of collisions
– Effects of impacts
– Prospects for future giant collisions with Earth

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 The main points: Meteorites

• Each year Earth sweeps up ~80,000 tons of extraterrestrial

matter, from microscopic dust particles to large rocks

• Some are identifiable pieces of the Moon, Mars, or Vesta;

most are pieces of asteroids

• Meteorites were broken off their parent bodies 10’s to 100’s

of million years ago (recently compared to 4 Billion Years)

• Oldest meteorites (chondrites) contain bits of interstellar

dust, tiny diamonds made in supernova explosions, organic
molecules and amino acids (building blocks of life), tiny
spherules left over from the very early Solar System

• Direct insight into solar system formation

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Meteor showers

• Time
exposure
image,
tracking
stellar motion

• Stars stay
still,
meteorites
make trails

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Page 5
 Rocks Falling from the Sky

• Some vocabulary
Meteoroid: chunk of debris in the Solar System.
Meteor: The visible path of a meteoroid that enters Earth's (or
another body's) atmosphere.
Meteorite: A meteoroid that reaches the ground and survives
impact
Meteor Shower: Many meteors appearing seconds or minutes
apart.
Origin: Comes from Greek meteōros, meaning "high in the air”.

• How can you tell that you have a meteorite?

– Higher metal content than terrestrial rocks
– Contain Iridium and other isotopes not in terrestrial rocks

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Meteors & Meteor Showers

A meteoroid is a piece of rocky space debris orbiting

meteoroid the Sun at typically 20 - 40 km/s,
some of which are on a collision
course with the Earth.

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 A meteor is the fireball seen in the sky
when a meteoroid collides with the
Earth’s atmosphere.

A meteorite is
the remnant of
a meteoroid
which has
survived the
fireball to
impact
on the Earth’s
surface.

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 What are meteorites?

• Chunks of rock or iron-nickel that fall to Earth from space

• Pieces of asteroids, comets, Moon, Mars, interstellar dust

– Can weigh from < 1 ounce to a few tons (!)

• “The Poor Man’s Space Probe”

– From parts of the Solar System astronauts may never explore

• Usually named after the place where they fall

– Examples: Prairie Dog Creek (US), Zagora (Morocco), Campo del
Cielo (Argentina), Mundrabilla (Australia)

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What do meteorites look like?

Vesta
Meteorite
from Mars

Allen Hills
(Moon)

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 Variety of meteorite “falls”

• Tiny pieces of cosmic dust

– Collected by special airplanes, in clay under the
oceans, or in Antarctic ice

• Find single small chunks of rock

– Sometimes at random, sometimes by following
trajectory of a “fireball” or meteor trail

• A several-ton meteorite breaks up during

descent, falls as separate pieces
– Biggest pieces can make large craters if they hit land
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 Small particles: spherules

• Tiny droplets from space

• Formed by melting and re-solidification after impacts

Spherule from Moon Spherule

Collected by Apollo 11 astronauts from bottom of the Indian Ocean

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Small particles: cosmic dust

• Sometimes from comets, sometimes left over from the

cosmic dust cloud from which the Solar System formed

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Single small chunks of rock

Iron-nickel meteorite
A few inches across Allende
Carbonaceous chondrite

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Several-ton boulders

Hoba Meteorite, Namibia

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How dangerous are meteorites?

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Worldwide frequency of meteorites
as function of size

Impact Frequency
Size Frequency Destruction Area
Pea 10/ hour
Walnut 1/ hour
Grapefruit 1/ 10 hours
Basketball 1/ month
50 meters 1/ century New York City
1 kilometer 1/ 100,000 years Virginia
2 kilometers 1/ 500,000 years France
10 kilometers 1/ 100 million years World-Wide?

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 The Great Daylight Fireball of 1972

• Skipped thru Earth’s atmosphere at shallow

angle, then exited again into space

• About 10-m diameter, moving at 15 km/sec

(33,000 MPH).

• If it had hit the surface of the Earth, it would

have had H-bomb equivalent impact energy.

• http://www.youtube.com/watch?v=dKiwzLFzQfc&feature=related

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1908 Tonguska meteorite in Siberia
caused widespread devastation

• Fortunately it hit in an unpopulated area!

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How meteorites are found

• Random “finds” lying on ground

• Fragments around meteor craters

• Follow glowing trail of meteor or fireball

• Systematic searches in Antarctica

• Special high-flying airplanes (for dust)

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Random “finds”

• Rare: a big meteorite in desert of Oman

• Pretty rare: random “finds” of smaller chunks

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 Fragments around meteor craters

Barringer Crater, Arizona

• Very large meteorites vaporize when they hit ground,

form big craters

• Sometimes small pieces are found around crater

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The Peekskill (NY) Fireball

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Recent year in Sudan....

• Link to Scientific American article

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 University of Khartoum students
did systematic search
• 45 students and staff of the University of
Khartoum rode buses out to desert, searched
in long lines. Found more than 280 pieces.

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Macroscopic features of the Almahata Sitta meteorite.

P Jenniskens et al. Nature 458, 485-488 (2009)

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Systematic searches in Antarctica

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Systematic searches in Antarctica

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Searching for rare meteorites
amidst thousands of Earth-rocks

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Victory!

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Primitive vs. processed meteorites

Based on composition, meteorites fall into two basic categories:

• primitive
• about 4.6 billion years old
• accreted in the Solar
nebula

• processed
• younger than 4.6 billion
years
• matter has differentiated
• fragments of a larger object
which processed the
original Solar nebula
material

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 Origin of Meteorites

• Primitive meteorites condensed and accreted directly

from the Solar nebula.
• the stony ones formed closer than 3 AU from the Sun
• the Carbon-rich ones formed beyond 3 AU from the Sun, where
it was cold enough for Carbon compounds to condense

• Processed meteorites come from large objects in the

inner Solar System.
• the metallic ones are fragments of the cores of asteroids which
were shattered in collisions
• the rocky ones were chipped off the surfaces of asteroids, Mars,
and the Moon by impacts

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Main classes of meteorites

There are 3 main classes of meteorites: the irons, the stones, and the
stony-irons.

As their names imply, meteorites are classified by their

composition. The irons are composed of nearly pure metallic
iron-nickel; the stones are made of rocky mostly silicate material;
and the much rarer stony-irons are a mix of stone and iron.
The irons and stony-irons are more obviously extraterrestrial, as
‘pure’ iron is very rare on Earth (it is usually found in oxides).
The stones are the more common type, but are difficult to
recognise as they look much like terrestrial rocks.

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 Meteorite compositions
The iron meteorites are composed of
about 90% iron and 10% nickel. They
make up about 4% of the Earth’s
meteorites.

The stony-irons are composed

of about 50% silicates and 50%
iron, and make up only about 1% the Iron meteorite
of meteorite falls.

The stones make up the remaining 95% of meteorite falls, and

are divided into three types: chondrites, achondrites &
carbonaceous.

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Main types of meteorites

• Chondrites
– Carbonaceous
– Non-carbonaceous

• Achondrites

• Iron

• Stony-Iron

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Chondrites

• Rocky, inhomogeneous, contain round

“chondrules”

Microscope
image

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 Carbonaceous Chondrites contain
complex organic molecules

• Amino acids, fatty acids,

other so-called “building
blocks of life”

• Did building blocks of life

come to Earth from space?

• Did life itself come to Earth

from space?
– “Panspermia” theory

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Carbonaceous Chondrites: Insights
into Planet Formation?

• The oldest meteorites; quite rare

• Chondrules (round): primitive chunks of early
Solar System
• Calcium aluminum inclusions (CaI’s): isotope
ratios (26 Al and 26 Mg) suggest that a
supernova explosion went off right next to the
early Solar Nebula
– Did the supernova stimulate formation of our Solar
System?

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Some types of Chondrites were formed
all at once: from one asteroid breakup

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Iron meteorites

• Made of iron and nickel

• Pits made during atmospheric entry (hot!)

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Iron meteorites: from core of
differentiated asteroids

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The making of future meteorites!

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 Crystalization pattern of the iron is
unique

• Characteristic of very
slow cooling of iron
within an asteroid core

• Due to diffusion of
nickel atoms into solid
iron as core cools

• Says original asteroid

must have been large
enough to be
differentiated

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 Stony-Iron meteorites - the prettiest

• Crystals of olivene (a rock mineral) embedded in iron

• From boundary between core and mantle of large

asteroids?

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Achondrites: from Mars and Moon

• From Mars:
– Tiny inclusions have same elements and isotope
ratios as Martian atmosphere (measured by
spacecraft on Mars)

• From the Moon:

– Astronauts brought back rocks from several regions
on the Moon
– Some achondrites match these rock types exactly

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 Where do meteorites come from,
and how do we know?

• Spectra: reflection of sunlight as function of

wavelength of light
• Spectra of some meteorites identical to some asteroids
• Implies asteroid was parent body

Toro

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 The main points: Meteorites

• Each year the Earth sweeps up ~80,000 tons of extraterrestrial

matter

• Some are identifiable pieces of the Moon, Mars, or Vesta; most are
pieces of asteroids

• Meteorites were broken off their parent bodies 10’s to 100’s of

million years ago (recently compared to age of Solar System)

• Oldest meteorites (chondrites) contain interstellar dust, tiny

diamonds made in supernova explosions, organic molecules and
amino acids (building blocks of life)

• Direct insight into pre-solar system matter, solar system formation

Page 47
 The main points: Cosmic Collisions

• Cosmic collisions played major role in Solar System evolution

– Aggregation of planets from planetesimals
– Formation of Moon, tilt of Venus’ and Uranus’ rotation axes,
composition of Mercury

• Also played a major role in Earth’s evolution

– Tilt of axis
– Mass extinctions (dinosaurs, others)

• Collision history derived from crater patterns, isotope ratios

• Probability of global catastrophic impact event once every 100

million years

• Strong interest in tracking all Near-Earth Objects (NEO’s) that

might hit the Earth in the future
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 Role of cosmic collisions in
evolution of Solar System
• Early phase (4.5 billion yrs ago): planet formation
– Planetesimals collided or accreted to form larger pieces

• Formation of Moon by glancing collision with Earth

• Removal of most of Mercury’s crust by collision

• Collision made Venus rotate backwards

• Collision tipped Uranus onto its side (now rotates at 90 deg to

rotation axes of all other planets)

• “Late Heavy Bombardment” (~3.9 billion years ago) from Lunar

record
– First signs of life on Earth immediately followed “Late Heavy
Bombardment” period. Is there some sort of causal connection?
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Early phase (4.5 billion yrs ago):
planet formation relies on collisions

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 Evidence that Moon formed as
result of a collision

• Earth has large iron core, but the moon does not
– Earth's iron had already drained into the core by the time of the giant
impact that formed the moon

• Debris blown out of both Earth and the impactor came from their
iron-depleted, rocky mantles

• Explains why mean density of Moon (3.3 grams/cm3) is much less

than Earth (5.5 grams/cm3)

• Moon has same oxygen isotope composition as the Earth

– Mars and meteorites from outer Solar System have different oxygen
isotope compositions
– Moon formed form material formed in Earth's neighborhood.

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Formation of the Moon….

– Large planetesimal collides w/ Earth at glancing angle

– Removed material is from mantle of Earth
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 Uranus’ rotation axis lies in plane of
its orbit

• Unique in Solar System

• All other planets’ rotation axes point out of the plane of

their orbits

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Collision with a massive body is
best way to explain this

• Would have to have collided with a body at

least as big as the Earth

• Approached Uranus at a large angle to the

plane of the Solar System

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Theories suggest young outer solar
system was very unstable place

• Many tens of Uranus and Neptune-mass

planets initially

• Unstable orbits: most of them were ejected

from solar system

• Perhaps on the way out, one of them hit

Uranus

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 Venus rotates “backwards”
compared with all other planets

• Did two roughly equal-mass bodies merge to form Venus? Was

early Venus hit by another planetary object?
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Removal of most of Mercury’s crust
by collision
• Theory developed to explain why Mercury has
so little lithosphere compared with its core

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The Moon
“Late Heavy Bombardment” of
Moon

• Evidence from Moon suggests impact rate was

1000 times higher 4 billion years ago than 3.8
billion years ago

• Heavy bombardment of Moon slowed down

about 3.8 billion years ago

• Similar evidence from Mercury, Mars

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Evolution of the Moon’s Appearance

"Mare" are huge lava flows that came from fissures

in Moon’s crust 3.2-3.9 billion years ago. There are
similar flows on Earth (Siberia, India).

Even during heavy bombardment, a major impact only

occurred every few thousand years. Now they only
occur over tens or hundreds of millions of years (so the
lunar surface hasn’t changed too much). Page 60
Basins on Mercury, Moon, Mars

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How general was the "late heavy
bombardment" ?

• If Moon, Mars,
Mercury all were hit,
probably the Earth
was too

• Was it the “last

gasp” of planetary
accretion? Or a real
spike in impact
rate?

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 One theory: a real spike in impacts

• Initially Solar System had large population of icy objects beyond

Saturn

• In stable orbits around Sun for several hundred million years until
Neptune and Uranus began to form

• As these planets grew, their gravitational attraction began to

scatter the remaining planetesimals into the inner Solar System

• A small fraction crashed into the Moon and rocky planets, making
immense craters

• Calculations suggest that the bombardment would have lasted

less than 100 million years

• Consistent with ages of craters and impact basins in Lunar

highlands
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Earth experienced major collisions
as well

• But most craters got eroded away, subducted, or drowned

• A tour of craters on Earth:

Algeria Chad (Africa) from airplane

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 Earth’s craters

Clearwater, Canada Henbury, Australia

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Earth’s craters, continued

Tswaing, South Africa

New Quebec, Canada

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Arizona’s Meteor Crater, the most
famous example

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 Impact event created opening of
Chesapeake Bay

• 35 million yrs ago, 2 mi wide

• 56 mile-wide crater

• Drilling  mixed bits of

crystalline and melted rock
that can be dated, as well as
marine deposits, brine, etc

• Tidal waves 1000 ft high

Inundated area (in blue)

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Giant impact 64 million years ago:
best idea for dinosaur extinction

• Chicxulub crater
north of Yucatan
peninsula, Mexico

• 180 km wide

• Dated to same
period as
extinctions at
Cretacious-
Tertiary boundary

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Corroborating evidence: Iridium
layer
• Layer of enhanced
abundance of Iridium
found worldwide

• Dated to same time as

dinosaur impact

• Asteroids contain high

concentration of Iridium,
relative to Earth

• Ash on top of Iridium

(huge fires)

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 BBC News, 2002: Evidence for Late
Heavy Bombardment on Earth

OUR PLANET WAS BEATEN UP

• The first convincing evidence that the Earth was bombarded by a
devastating storm of meteoroids and asteroids four billion years ago
has been found in Earth's oldest rocks.

• Scientists have looked for clues in sedimentary rocks from Greenland

and Canada - the oldest on Earth - that date from the waning phases
of the Late Heavy Bombardment.

• Researchers from the University of Queensland, Australia, and the

University of Oxford, UK, say they have detected in these rocks the
chemical fingerprints of the meteorites left over from the Late Heavy
Bombardment - various types of tungsten atoms (tungsten isotopes)
that must be extraterrestrial.

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 Impact energies are very large!

1
Kinetic energy = MV 2 where V is velocity of impactor
2
V is very large (estimate orbital speed around Earth): 30 km/sec = 66,000 mph
gm
M  density volume  5  volume
cm 3
4 1
Volume of sphere  r 3  d 3 where d is diameter
3 6
Combine :
3 3
1  d   d 
2
Kinetic energy = MV =  10 gm cm /sec  250
19 2 2
tons of TNT
2 1 meter 1 meter

If diameter d = 200 meters, Kinetic Energy = 2 billion tons of TNT!

Note VERY strong dependence on size of impactor, d (Energy  d 3 )

Credit: Bob O’Connell, U Virginia Page 72

Collision of Comet Shoemaker-Levy 9 with
Jupiter, 1994

• Comet discovered March 1993, after it was captured into orbit

around Jupiter

• In 21 separate pieces! Broke up due to Jupiter’s tidal forces

• All 21 fragments hit Jupiter in one week in July 1994

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Tidal breakup of a comet when it
passes too close to Jupiter

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Worldwide network of astronomers
observed collisions over one week

• I was at Lick Observatory on Mt Hamilton

• As Earth turned, e-mails flew around the planet

to tell people what to look for
– As Jupiter was setting at one place on Earth,
scientists sent e-mails to places where Jupiter was
just rising

• Examples: “Impact B is a dud” “Impact G is

spectacular”

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Initial impact with atmosphere on
night side, seen by Galileo spacecraft
• Time sequence

• White dots are hot gases

exploding out of Jupiter’s
atmosphere on night side

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Hubble Space Telescope was next
to see impacts

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G impact spot as Jupiter rotated
(our group at Lick Observatory)

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Multiple fragments of Shoemaker-
Levy 9 hit Jupiter in sequence

Infrared image of Hubble Space Telescope

multiple impact points visible-light image
(Keck Telescope)
Page 79
 Lessons learned from Comet
Shoemaker-Levy 9
• Made us realize that “impacts happen” !

• Many comets must break up into pieces the way SL-9

did: linear crater patterns on Ganymede

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What if a Shoemaker-Levy 9 size
comet were to hit the Earth?

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 Drastic effects of impact on a
terrestrial planet

• At “ground zero” rock, water, biomass are vaporized or melted

• Deeper rock is shock recrystallized (ultra high pressures) and
fractured
• Series of deep fractures form, lava from the interior may erupt
• Shockwaves obliterate life just outside of “ground zero”
• Earthquakes (and impact itself, if in ocean) generate giant waves in
oceans, wipe out coastal areas
• Friction in atmospheric dust generates widespread lightening
• Thick dust in atmosphere blots out sun for months or years
• Aerosols caused by eruptions and vaporization remain in
atmosphere for decades

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Future extinctions might not be
limited to dinosaurs

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Near Earth Objects: will Earth have
another collision soon?

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There have been many impacts in
the past

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 What can be done?

1) Vigorous program to detect objects that are aiming

near Earth
• Several are under way; not as vigorous as they might be
• Also need better orbit prediction methods

2) Characterize mechanical properties of the main types

of asteroids, comets
• Are they solid? Rubble piles? Makes a difference.

3) Work on conceptual ways to divert an incoming object

• Gentle (ion thruster for 50 yrs)
• Not so gentle (e.g. nuclear blast, ….)
• Solar radiation pressure? (paint one side white!)

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There are several projects to find
near Earth asteroids and comets

• It is thought that there are about 1600 Earth

crossing asteroids larger than 1 km in
diameter.

• Only about 100 are known. Programs to find

most of them are under way.

• New survey telescopes (LSST, PanSTARRS)

will search more systematically.

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Question

• If one of the Near Earth Object programs finds

an incoming asteroid that will likely hit the
Earth, should they announce it to the public?

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• Low probability
of a rare but high-
consequence
event

• Difficult for
policy-makers
and public
opinion to deal
with

Page 90
 The main points

• Cosmic collisions played major role in Solar System evolution

– Aggregation of planets from planetesimals
– Formation of Moon, tilt of Uranus’ axis, composition of Mercury

• Also played a major role in Earth’s evolution

– Tilt of axis
– Mass extinctions (dinosaurs, others)

• Collision history derived from crater patterns, isotope ratios

• Probability of global catastrophic impact event once every 100

million years

• Recent advances in tracking all Near-Earth Objects (NEO’s)

– Very active field of research!
– Probability is 100% that a Near Earth Object will hit us. The big
questions are "how soon?" and "what can we do about it?"
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