Download Lecture 08

Where did asteroids and comets
come from?
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Asteroids and Comets
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Leftovers from the accretion process
Rocky asteroids inside frost line
Icy comets outside frost line
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Where are comets and asteroids today?
Most asteroids are in the asteroid belt between Mars and Jupiter.
Most comets are in the Kuiper Belt and the Oort Cloud.
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Heavy Bombardment
• Leftover
planetesimals
bombarded
other objects
in the late
stages of solar
system
formation.
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Origin of Earth’s Water
• Water may have
come to Earth
by way of icy
planetesimals
from the outer
solar system.
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How do we explain the existence
of our Moon and other exceptions
to the rules?
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Giant Impact
…then accreted into the Moon.
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Odd Rotation
• Giant impacts
might also
explain the
different
rotation axes
of some
planets.
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Review of the
nebular theory
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Thought Question
How would the solar system be different if the
temperature everywhere in the proto planetary disc
was halved?
A. Jovian planets would have formed closer to
the Sun.
B. There would be no asteroids.
C. There would be no comets.
D. Terrestrial planets would be larger.
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Thought Question
Which of these facts is NOT explained by the
nebular theory?
A. There are two main types of planets: terrestrial
and jovian.
B. Planets orbit in the same direction and plane.
C. Asteroids and comets exist.
D. There are four terrestrial and four jovian planets.
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When did the planets form?
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We cannot find the age of a planet, but we
can find the ages of the rocks that make it
up.
We can determine the age of a rock
through careful analysis of the proportions
of various atoms and isotopes within it.
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Radioactive Decay
• Some isotopes
decay into
other nuclei.
• A half-life is
the time for
half the nuclei
in a substance
to decay.
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Thought Question
Suppose you find a rock originally made of potassium40, half of which decays into argon-40 every 1.25
billion years. You open the rock and find 15 atoms of
argon-40 for every atom of potassium-40. How long
ago did the rock form?
A.
B.
C.
D.
1.25 billion years ago
2.5 billion years ago
3.75 billion years ago
5 billion years ago
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Dating the Solar System
Age dating of
meteorites that are
unchanged since they
condensed and
accreted tells us that
the solar system is
about 4.6 billion
years old.
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Dating the Solar System
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Radiometric dating tells us that the oldest
moon rocks are 4.4 billion years old.
The oldest meteorites are 4.55 billion
years old.
Planets probably formed 4.5 billion years
ago.
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Meteorites from the Moon and Mars
• A few meteorites arrive on Earth from the
Moon and Mars.
• Composition differs from the asteroid
fragments.
• This is a cheap (but slow) way to acquire moon
rocks and Mars rocks.
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9.2 Comets
Our goals for learning:
• How do comets get their tails?
• Where do comets come from?
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How do comets get their tails?
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Comet Facts
• Formed beyond the frost line, comets are icy
counterparts to asteroids.
• The nucleus of a comet is like a “dirty snowball.”
• Most comets do not have tails.
• Most comets remain perpetually frozen in the
outer solar system.
• Only comets that enter the inner solar system grow
tails.
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Nucleus of Comet
• A “dirty snowball”
• Source of material
for comet’s tail
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Anatomy of a Comet
• Coma is atmosphere
that comes from
heated nucleus.
• Plasma tail is gas
escaping from coma,
pushed by solar
wind (it always
points way from the
Sun).
• Dust tail is pushed
by photons.
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Growth of Tail
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Comets eject small particles that follow the comet around in its
orbit and cause meteor showers when Earth crosses the comet’s
orbit.
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Where do comets come from?
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Only a tiny number of
comets enter the inner
solar system; most
stay far from the Sun.
Oort Cloud:
Comets on random
orbits extending to
about 50,000 AU
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Kuiper Belt:
Comets on orderly
orbits at 30-100 AU
in disk of solar
system
How did they get there?
• Kuiper Belt comets formed in the Kuiper Belt.
– Flat plane aligned with the plane of planetary orbits
– Orbiting in the same direction as the planets
• Oort Cloud comets were once closer to the Sun, but they
were kicked farther out by gravitational interactions with
jovian planets.
– Spherical distribution
– Orbiting in any direction
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Pluto and Other Kuiper Belt Objects
• Kuiper Belt objects are basically icy bodies
that are the same as comets.
• Most comets and asteroids are not spherical.
• Only asteroids and larger than about 500km
in diameter are spherical.
• For these large objects their own gravity is
strong enough to make them spherical.
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How big can a comet be?
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Pluto’s Orbit
• Pluto’s orbit is tilted and significantly elliptical.
• Neptune orbits three times during the time Pluto orbits
twice—resonance prevents a collision.
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Is Pluto a planet?
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Much smaller than the eight major planets
Not a gas giant like the outer planets
Has an icy composition like a comet
Has a very elliptical, inclined orbit
Pluto has more in common with comets than with the
eight major planets.
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Discovering Large Iceballs
• In summer 2005,
astronomers
discovered Eris, an
iceball even larger
than Pluto.
• Eris even has a
moon: Dysnomia.
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Other Icy Bodies
• There are many icy
objects like Pluto on
elliptical, inclined
orbits beyond
Neptune.
• The largest ones are
comparable in size
to Earth’s Moon.
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Kuiper Belt Objects
• These large, icy
objects have orbits
similar to the
smaller objects in
the Kuiper Belt that
become short period
comets.
• So are they very
large comets or very
small planets?
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Is Pluto a planet?
• In 2006, the International Astronomical Union
decided to call Pluto and objects like it “dwarf
planets.”
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What is Pluto like?
• Its largest moon, Charon, is nearly as large as
Pluto itself.
• Pluto is very cold (40 K).
• Pluto has a thin nitrogen atmosphere.
– The atmosphere refreezes onto the surface as Pluto’s
orbit takes it farther from the Sun.
• Its axial tilt is small compared to that of the Earth.
• Its ellipticity is 0.25 (larger than Earth’s 0.016).
– Its perihelion distance is about 30AU.
– Its aphelion distance is about 50AU.
• Unlike the Earth, Pluto’s seasons are cause by its
changing distance from the Sun.
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HST’s View of Pluto and Its Moons
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Other Kuiper Belt Objects
• Most have been discovered very recently so
little is known about them.
• Halley's Comet is an example of a comet
that is thought to have originally formed in
the Kuiper belt and now has an orbit that
brings it near the Earth every 76 years.
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Summary
— The Kuiper Belt from which comets come
contains objects as large as Pluto.
— The icy composition of Pluto and other
“dwarf planets” is more like large comets
than like the major planets.
— Large objects in the Kuiper Belt have tilted,
elliptical orbits and icy compositions like
those of comets.
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Collisions In the Solar System
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Recent Impacts in the Solar System
• Comet Shoemaker–Levy 9 collided with
Jupiter in the mid-1990s.
• It was about 10km in size and if it had struct
the Earth it might have ended all life on our
planet.
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Impact sites in infrared light
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Impact plume
from a fragment
of comet SL9
rises high above
Jupiter’s surface.
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Dusty debris at an impact site
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Several impact sites
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Did an impact kill the dinosaurs?
Mass Extinctions
• Fossil record shows occasional large dips in
the diversity of species: mass extinctions.
• The most recent was 65 million years ago,
ending the reign of the dinosaurs.
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Iridium: Evidence of an Impact
• Iridium is very rare in Earth surface rocks
but is often found in meteorites.
• Luis and Walter Alvarez found a worldwide
layer containing iridium, laid down 65
million years ago, probably by a meteorite
impact.
• Dinosaur fossils all lie below this layer.
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Iridium Layer
No dinosaur fossils
in upper rock layers
Thin layer
containing the rare
element iridium
Dinosaur fossils in
lower rock layers
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Consequences of an Impact
• A meteorite 10 km in size would leave a crater
about 200km (20 times larger than the meteorite).
• The impact sends large amounts of debris into the
atmosphere.
• Debris would reduce the amount of sunlight
reaching Earth’s surface.
• The resulting climate change (called impactwinter) may have caused mass extinction.
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Likely Impact Site
• Geologists found a large
subsurface crater about 65
million years old in
Mexico.
• 200km crater suggests
impacting object was ~10
km in diameter.
• Impact of such a large
object would have ejected
debris high into Earth’s
atmosphere.
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Tunguska Impact – Siberia 1908
A meteorite of about 40m in
diameter exploded about
5km about the Tunguska
forest with the force of 15
Megaton bomb and leveling
nearly 1000sq miles of
Siberian forest.
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Meteor Crater, Arizona: 50,000 years ago (50-meter object)
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Facts About Impacts
• Asteroids and comets have hit Earth.
• A major impact is only a matter of time: not IF but
WHEN.
• Major impacts are very rare.
• Extinction level events ~ millions of years
• Major damage ~ tens to hundreds of years
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Frequency of Impacts
• We can determine how often
impacts occur by counting
the number of craters of
different sizes on the moon
and other planets.
• Small impacts happen
almost daily.
• Impacts large enough to
cause mass extinctions are
many millions of years
apart.
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The Asteroid with Our Name on It
• We have found about 80% of the estimated 10,000 Near
Earth Objects (NEOs).
• About 1200 are potentially hazardous.
• For large asteroids we may get a few years of warning.
• Some hazardous asteroids will come close but none will
strike the Earth in the next few years.
• How can we avoid the danger of a big impact.
– Deflection is more probable with years of advance warning.
– Breaking a big asteroid into a bunch of little asteroids is unlikely to
help.
• We get less advance warning of a killer comet.
– They only become bright when the near the sun.
– Their highly elliptical orbit means they move very fast close to the
sun.
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What are we doing about it?
• Stay tuned to
http://impact.arc.nasa.gov
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How do other planets affect
impact rates and life on Earth?
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Influence of Jovian Planets
The gravity of a jovian planet (especially Jupiter) can
redirect a comet.
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Influence of Jovian Planets
Jupiter has directed some comets toward Earth but has
ejected many more into the Oort Cloud.
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Was Jupiter necessary for life on
Earth?
Impacts can extinguish life.
But were they necessary for
“life as we know it”?
Were comet impacts the source
of the Earth’s water?
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