Lecture 16

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					Impact Cratering
       Read Ch. 7




 ASTR 1304 – Solar System
  Dr. David A. Wood, Jr.
                Impact Cratering
• Where do impacts occur?
• How are impact craters produced?
• Airbursts
• Why isn’t Earth covered with craters?
• Terrestrial impacts
• Extinction of the dinosaurs
• Impact threat today
Where do Impacts Occur?
    How are Impact Craters Produced?
•   Pre-Impact Stage
     Ø Object approaches target at speeds typically ranging
       from 20 -70 km/sec
     Ø Target is typically composed of several different
       geologic layers with the oldest layer on the bottom
       and the youngest layer at top
•   Contact and Compression Stage
     Ø When impactor collides with target, all KE must be
       rapidly converted to other forms
     Ø Shock wave in impactor causes it to explode
     Ø Shock wave is generated in target material
     Ø Remainder of energy is released as heat and KE of
       excavated material
•   Excavation Stage – Ejecta Stack
     Ø Shock wave expands into target and “fluidizes”
       target material
     Ø Difference in pressure between surface and shock
       drives material upwards and outwards
     Ø Since upper layers are ejected first, they land first
       producing an overturn layer at crater rim
     Ø Target material is shock metamorphosed creating
       materials unique to impact craters
    How are Impact Craters Produced?
•   Modification Stage (Part 1)
     Ø Transient crater is unstable over long timescales
     Ø If enough material is excavated from crater, then
       underlying layers rebound to produce a central peak
     Ø Melted target rock otherwise accumulates in the
       center of the crater leaving a flat, shallow floor
     Ø Crater walls are too steep to support weight of
       overlying material on the rim so the walls slump
•   Modification Stage (Part 2)
     Ø Over geologic timescales, wind and water erosion,
       combined with plate tectonics and volcanism weather
       and erode the crater
     Ø Rim deposits are often particularly susceptible to
       erosion since they sit higher than the crater’s
       surrounding landscape
•   Final Crater
     Ø An impact crater on an airless, waterless world may
       be preserved indefinitely in a pristine state
     Ø On worlds with erosion or other geologic activity, the
       crater may eventually be worn almost completely
       away, leaving little evidence of its existence
 Why Isn’t Earth Covered With Craters?
• Earth
   Ø 4 times larger than the Moon
   Ø Surface gravity 6 times greater
   Ø More likely to be struck than the Moon


• Vain Earth syndrome
   Ø   Atmosphere - most small objects burn up
   Ø   Oceans - can’t retain craters in liquid
   Ø   Erosion - craters that exist are eventually destroyed by wind/water processes
   Ø   Plate Tectonics - Earth’s surface is stretched and even recycled
   Ø   Volcanism – magma fills in or buries craters so that they are no longer
       visible from the surface
                     Terrestrial Impacts
• Meteor Crater – Arizona
   Ø   Crater diameter =1.2 km
   Ø   Impactor diameter ~30 m (likely iron)
   Ø   Impactor completely vaporized
   Ø   20,000 – 50,000 yrs old
   Ø   Best preserved impact crater on Earth

• Tunguska
   Ø June 30, 1908
   Ø No crater
   Ø Impactor diameter ~ 30 m (rocky or
     perhaps icy)
   Ø 2000 km2 of forest snapped like
     toothpicks
   Ø Objects this size hit Earth once per
     century
                                      Airbursts
•   Effects of Atmospheres – NY Fireball
     Ø Atmospheres absorb some of the
       impactor’s kinetic energy
     Ø If atmosphere is sufficiently strong or
       if impactor is small or weak, the
       atmosphere can absorb all the KE
     Ø If impactor deposits significant KE to
       atmosphere in a short period of time,
       then an airburst results

•   Effects of Airbursts
     Ø Tunguska was an airburst that leveled a
       large swath of forest on Earth
     Ø On Venus, atmosphere is so dense that
       airbursts are much stronger
     Ø Airbursts on Venus can crush rock near
       ground zero
     Ø Shock winds from the airburst can
       scour the surface
     Ø Radar dark and bright “splotches”
       support this hypothesis
                       Terrestrial Impacts
•   Chicxulub crater - Yucatan
     Ø Crater diameter ~200 km
     Ø Impactor diameter ~10 km
     Ø 65 million yrs old
     Ø Impactor completely vaporized as well as large
       amount of water and soil
     Ø Yucatan deposits found as far away as Colorado
     Ø Worldwide iridium layer
     Ø Dinosaur-killer
                    K-T Boundary
• Worldwide layer 1 to 2 cm thick
• Rich in iridium and shocked quartz
   Ø Iridium is a heavy metal rare in Earth’s crust, but abundant in
     meteorites
   Ø Highly shocked quartz implies rapid heating and sudden
     quenching (abrupt event)
   Ø Dinosaurs, plants, and other fossils found below the layer but
     not above it
   Ø Climate change and different flora/fauna fossils found above
     the layer
• 65 million years old
• End of Cretaceous / Beginning of Tertiary
Iridium Layer
              Extinction of the Dinosaurs
• Effects of Impact
   Ø   Killed instantly within ≈ 1000 km
   Ø   Tidal waves on coastlines
   Ø   Continent-wide forest fires
   Ø   Nuclear winter
   Ø   Acid rain
   Ø   Greenhouse effect

• Why did mammals survive?
   Ø Big animals need lots of food, little
     animals can survive on scraps for much
     longer
   Ø Even most little animals died, but a few
     that burrowed underground might have
     survived
   Ø After things settle down, mammals spread
     out to occupy the empty ecological niche
Impact Cratering & Extinctions
               •   Impact cratering occurs on all planetary
                   surfaces including Earth
               •   Cometary impacts brought organics and
                   water to Earth allowing life to start
               •   Impacts may be responsible for mass
                   extinctions every ~ 26 Myr
               •   Extinction of dinosaurs ~ 65 Myr ago
Impact Threat
Response to Impact Threat
             Impact Effects Calculator
             http://www.lpl.arizona.edu/impacteffects
             http://down2earth.eu/impact_calculator/




             Asteroid 1950 DA will come perilously
             close to Earth on March 16, 2880.
Response to Impact Threat
                        Summary
• Where do impacts occur?
• How are impact craters produced?
• Airbursts
• Why isn’t Earth covered with craters?
• Terrestrial impacts
• Extinction of the dinosaurs
• Impact threat today

				
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posted:7/19/2013
language:English
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