9.4 Cosmic Collisions: small bodies vs. the planets

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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 show that a large
impact occurred at the time the dinosaurs died
out, 65 million years ago.
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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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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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Jovian planets determine where asteroids and
comets are today They might protect Earth from
most of bombardment
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.