Chapter 9 Remnants of Rock and Ice

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Chapter 9Remnants of Rock and Ice

• Asteroids, Comets, and Pluto

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9.1 Asteroids and Meteorites

• Our Goals for Learning
• Why is there an asteroid belt?
• How are meteorites related to asteroids?

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Asteroid Facts

• Asteroids are rocky leftovers of planet
  formation.
• Largest is Ceres, diameter 1,000 km
• 150,000 in catalogs, and probably over a million
  with diameter gt1 km.
• Small asteroids are more common than large
  asteroids.
• All the asteroids in the solar system wouldnt
  add up to even a small terrestrial planet.

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Asteroids are cratered and not round
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Why is there an asteroid belt?
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More than 150,000 asteroids at their predicted
locations for Jan 1 2004 On this scale,
asteroids are much smaller than the dots used to
represent them
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Why are there very few asteroids beyond Jupiters
orbit?

1. There was no rocky material beyond Jupiters
   orbit.
2. The heaviest rocks sank towards the center of the
   solar system.
3. Ice could form in the outer solar system.
4. A passing star probably stripped away all of
   those asteroids, even if they were there at one
   time.

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Why are there very few asteroids beyond Jupiters
orbit?

• There was no rocky material beyond Jupiters
  orbit.
• The heaviest rocks sank towards the center of the
  solar system.
• Ice could form in the outer solar system.
• A passing star probably stripped away all of
  those asteroids, even if they were there at one
  time.

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Which explanation seems to be the most plausible?

1. The belt is where all the asteroids happened to
   form.
2. The belt is the remnant of a large terrestrial
   planet that used to be between Mars and Jupiter.
3. The belt is where all the asteroids happened to
   survive.

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Which explanation seems to be the most plausible?

1. The belt is where all the asteroids happened to
   form.
2. The belt is the remnant of a large terrestrial
   planet that used to be between Mars and Jupiter.
3. The belt is where all the asteroids happened to
   survive.

But WHY didnt they form a little planet?
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Rocky planetesimals survived in the asteroid belt
between Mars and Jupiter because they did not
accrete into a planet. Jupiters gravity, stirs
up the asteroid orbits and prevents their planet
formation.
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How are meteorites related to asteroids?
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How are meteorites related to asteroids?
Meteorites are pieces of asteroids - or sometimes
planets or the Moon.
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Meteor The bright tail of hot debris from the
rock Meteorite A rock from space that reaches
Earths surface
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Peekskill, NY October 9, 1992
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Pieces of AsteroidsMeteorite Types

1. Primitive Unchanged in composition since they
   first formed 4.6 billion years ago.
2. Processed Younger, have experienced processes
   like volcanism or differentiation.

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Primitive Meteorites simple, all ingredients
mixed together
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Processed Meteorites shattered fragments of
larger objects
Iron from a core
Volcanic rock from a crust or mantle
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What do we learn from meteorites?

• primitive meteorites tell us when solar system
  formation began.
• Processed meteorites tell us what asteroids are
  like on the inside.
• Processed meteorites provide direct proof that
  differentiation and volcanism happened on
  asteroids.

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Meteorites from Moon and Mars

• A few meteorites arrive from the Moon and Mars
• Composition differs from the asteroid fragments.
• A cheap (but slow) way to acquire moon rocks and
  Mars rocks.
• One Mars meteorite generated a stir when
  scientists claimed evidence for microscopic life
  in it.

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What have we learned?

• Why is there an asteroid belt?
• Orbital resonances with Jupiter disrupted the
  orbits of planetesimals, preventing them from
  accreting into a planet. Those that were not
  ejected from this region make up the asteroid
  belt today. Most asteroids in other regions of
  the inner solar system accreted into one of the
  planets.

• How are meteorites related to asteroids?
• Most meteorites are pieces of asteroids.
  Primitive meteorites are essentially unchanged
  since the birth of the solar system. Processed
  meteorites are fragments of larger asteroids that
  underwent differentiation.

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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 frostline, comets are icy
  counterparts to asteroids.
• Dirty snowballs the nucleus
• Most comets do not have tails.
• Most comets remain perpetually frozen in the
  outer solar system. Only a few enter the inner
  solar system, where they can grow tails.

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When a comet nears the Sun, its ices can
sublimate into gas and carry off dust, creating a
coma and long tails.
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Draw This Picture
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Comets eject small particles that follow the
comet around in its orbit This can cause meteor
showers when Earth crosses the comets orbit.
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Meteors in a shower appear to emanate from the
same area of sky because of Earths motion
through space
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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 On random orbits extending to about
50,000 AU
Kuiper belt On orderly orbits from 30-100 AU in
disk of solar system
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How did they get there?

• Kuiper belt comets align with the plane of planet
  orbits
• Oort Cloud Comets were kicked out of the solar
  system by the gravity from jovian planets random
  orbits

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What have we learned?

• How do comets get their tails?
• The vast majority of comets do not have tails.
  Only those few comets that enter the solar system
  grow tails. As the comet approaches the Sun its
  nucleus heats up. Some of the comets ice
  sublimates into gas, and the escaping gases carry
  along some dust. The gas and dust form a coma and
  two tails a plasma tail of ionized gas and a
  dust tail. Larger particles can also escape,
  becoming the particles that cause meteors and
  meteor showers on Earth.

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What have we learned?

• Where do comets come from?
• Comets that enter the solar system come from one
  of two reservoirs in the outer solar system the
  Kuiper belt and the Oort cloud. The Kuiper belt
  comets still reside in the region beyond Neptune
  in which they formed during the birth of the
  solar system. The Oort cloud comets are thought
  to have formed in the region of the jovian
  planets, and were kicked out to the great
  distance of the Oort cloud by gravitational
  encounters with the planets.

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9.3 Pluto Lone Dog or Part of a Pack?

• Our Goals for Learning
• What is Pluto like?
• Is Pluto a planet or a Kuiper belt comet?

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Pluto the exception

• Not a gas giant like the other outer planets.
• Has a very elliptical, inclined orbit.
• By far the smallest planet, and smaller than
  several moons.
• Has a surprisingly large moon Charon, probably
  formed by a huge comet collision with Pluto.

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Pluto will never collide with Neptune because of
a 32 orbital resonance.
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What is Pluto like?

• 1978 discovery of Plutos moon Charon Plutos
  mass from Newtons orbital law.
• It has a thin nitrogen atmosphere that will
  refreeze onto the surface as Plutos orbit takes
  it farther from the Sun.
• Pluto is the largest Solar System object that has
  not been visited by spacecraft.

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HSTs view of Pluto Charon
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Brightness variations during eclipsing orbits
showed dirty ice - like comets.
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Is Pluto a planet or a Kuiper Belt comet?
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Is Pluto a planet or a Kuiper Belt comet?

• Pluto is well beyond Neptune, in the Kuiper Belt.
• Inclined orbit is typical of Kuiper Belt comets.
• Composition is typical of Kuiper Belt comets, but
  not any of the other planets.

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Is Pluto a planet or a Kuiper Belt comet?

• Kuiper Belt objects have been found that approach
  Plutos size.
• Kuiper Belt comets have similar orbital
  resonances with Neptune.
• Kuiper Belt comets can have moons.
• Triton (a captured moon) is even larger than
  Pluto.

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What have we learned?

• Is Pluto a planet or a Kuiper belt comet?
• Whether Pluto should be called a planet is a
  matter of opinion, but its properties suggest
  that it is a Kuiper belt comet. Its composition
  and orbital properties match those of other
  Kuiper belt comets and do not fit in with the
  other planets. It is the largest known Kuiper
  belt comet today, but there may be larger ones
  still awaiting discovery.

• What is Pluto like?
• Pluto is much smaller than any other planet, with
  an orbit more elliptical and more inclined to the
  ecliptic plane than that of any other planet. It
  is made mostly of ices and has a very thin
  atmosphere of gases that are expected to freeze
  onto the surface as Pluto moves farther from the
  Sun in its 248-year orbit. It has a moon, Charon,
  with a slightly lower density than Pluto,
  suggesting that Charon may have been formed in a
  giant impact.

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9.4 Cosmic Collisions small bodies vs. the
planets
Our Goals for Learning Have we ever witnessed a
major impact? Did an impact kill the
dinosaurs? Is the impact threat a real danger
or just media hype? How do other planets affect
impact rates and life on Earth?
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Have we ever witnessed a major impact?
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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
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Impact plume rises high above Jupiters surface
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Did an impact kill the dinosaurs?
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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.
• Was it caused by an impact?
• How would it have happened?

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No dinosaur fossils in these rock layers
Thin layer containing iridium from impactor
Dinosaur fossils in lower rock layers
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Iridium - evidence of an impact

• Iridium is very rare in Earth surface rocks but
  often found in meteorites.
• Luis and Walter Alvarez found a worldwide layer
  containing iridium, laid down 65 million years
  ago.

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Comet or asteroid about 10km in diameter
approaches Earth
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An iridium-rich sediment layer and an impact
crater on the Mexican coast 65 million years
ago. shows that a large impact occurred at the
time the dinosaurs died out,
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The Impact Threat Real danger or media hype?
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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
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Meteor Crater, Arizona 50,000 years ago (50
meter object)
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This slide is placeholder in case you would like
to download and show the movie noted below (notes
field).
An object 1km in size would produce a mile-high
tidal wave
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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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The asteroid with our name on it

• We havent seen it yet.
• 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

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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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Gravity from Jovial planets can influence the
path of comets and Asteroids. They could protect
us or steer one in our direction
Fig 9.20
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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?
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What have we learned?

• Have we ever witnessed a major impact?
• In 1994, we observed the impacts of comet
  ShoemakerLevy 9 on Jupiter. The comet had
  fragmented into a string of individual nuclei, so
  there was a string of impacts that left Jupiters
  atmosphere scarred for months

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What have we learned?

• Did an impact kill the dinosaurs?
• We are not certain whether an impact was the sole
  cause, but a major impact clearly coincided with
  the mass extinction in which the dinosaurs died
  out, about 65 million years ago. Sediments from
  the time show clear evidence of an impact, and an
  impact crater of the right age has been found
  near the coast of Mexico.

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What have we learned?

• Is the impact threat a real danger or just
  media hype?
• Impacts certainly pose a threat, though the
  probability of a major impact in our lifetimes is
  fairly low.

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What have we learned?

• How do other planets affect impact rates and
  life on Earth?
• Impacts of asteroids and comets are always linked
  in at least some way to the gravitational
  influences of Jupiter and the other jovian
  planets. These gravitational influences have
  shaped the asteroid belt, the Kuiper belt, and
  the Oort cloud, and sometimes still help
  determine when an object is flung our way.