Meteoroid Impact and Planetary Evolution

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Meteoroid Impact and Planetary Evolution

• H. J. Melosh
• Lunar and Planetary Lab
• University of Arizona
• Tucson, AZ 85721 USA

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The study of impact craters has a well-defined
beginning 1610
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Galileos small telescope and limited field of
view did not permit him to view the entire moon
at once, so his global maps were distorted

• Nevertheless, he recognized a pervasive landform
  that he termed spots
• He described them as circular, rimmed depressions
• But declined to speculate on their origin

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Robert Hooke had a better telescope in 1665

• Hooke make good drawings of Hipparchus and
  speculated on the origin of the lunar pits. He
  considered impact, but dismissed it because he
  could not imagine a source for the impactors. In
  the end, he opted for a volcanic origin.
• For the next 300 years astronomers accepted this
  origin.

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In 1893 American Geologist G. K. Gilbert proposed
an impact origin for lunar craters
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Gilbert established a size-morphology progression
for craters

• Small craters are simple bowls
• Larger craters have central peaks and wreaths of
  terraces
• Craters are of different ages Some are fresh,
  others old and degraded

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The first impact crater recognized on Earth was
the Arizona Meteor crater
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• D. M. Barringer established a meteoritic origin
  in 1906, but until the end of his life he
  believed that the iron meteorite that created the
  crater was buried beneath its floor

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Impact-Explosion Analogy

• In the early 20th century, a number of workers
  realized that a high-speed impact resembled an
  explosion. Öpik (1916), Ives (1919), Moulton
  (1929) and Gifford (1924) all promoted this idea.
  Giffords arguments eventually carried the day.

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Crypto-Volcanic Structures

• Geologists Boone and Albritton re-interpreted
  Crypto-Volcanic structures as impacts in 1937
  and defined the geologic characteristics of
  impact craters

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In 1964 Robert Dietz proposed the first
shock-metamorphic feature diagnostic of impact
Shatter Cones
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High pressure polymorphs and shock features in
quartz followed quickly
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Modern Size-Morphology Progression

• Simple, bowl-shaped craters. Less than 15 km
  diameter on the moon
• Complex craters with central peaks and terraced
  rims. From 30 to 200 km diameter on the moon

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The simple-complex transition is accompanied by a
sudden decrease in crater depth
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• Peak-ring craters. About 300 km diameter on the
  moon
• Multi-ring basins. Largest sizes, more than 500
  km diameter on the moon

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Another type of Multi-ring structure on Callisto
and Europa
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Studies of cratering were advanced by three areas
of research

• Nuclear weapons effects,

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Numerical simulations of impacts and explosions,
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and experimental studies of the impact process
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Crater MechanicsContact and Compression
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The shock wave accelerates material that will
eventually open into a crater
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Streamlines cut across shock contours
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It is important to understand where materials
originate--the depth of excavation does not equal
the depth of the crater!
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Impact melt volume scales as a function of crater
size
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Excavation follows a simple progression from
hemispherical to parabolic
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Don Gaults 1968 view of this process
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Simple crater excavation and collapse
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Complex crater excavation and collapse
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Impact craters on Earth are sparse
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But may be of considerable local and economic
importance

• Elgygytgyn, Siberia Vredefort, South
  Africa

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On other plants, craters are often the dominant
landform
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The population of impact craters reaches an
equilibrium
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The degradation pattern depends on the slope of
the size-frequency relation
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Impacts have brought us rocks from Mars and the
Moon
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1.25 mm wide, Xpolar
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The process of spallation is responsible for
ejecting material at high speed but low shock
pressure
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The interaction of the stress wave with the free
surface leads to a characteristic pattern of
fractures
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An impact caused the greatest biological
extinction in the last 100 Million years
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• Global wildfires, sun-obscuring aerosols, acid
  rain, ozone loss, etc, etc, killed half the
  animal families alive before the impact

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But left tangible evidence in the form of a
global, Ir-rich spherule layer,
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The high-speed ejecta was dispersed ballistically
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Reentry of the ejecta caused immediate wildfires
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As well as a large impact crater--Chicxulub
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Early in Earths history, a gigantic collision
with a Mars-size protoplanet may have created the
Moon
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But this is only the tail end of a much bigger
story

• The Earth and planets grew out of a large swarm
  of smaller planetesimals. As growth continued,
  a hierarchical spectrum of planetary bodies
  resulted

The present-day distribution of asteroid sizes
reflects the same distribution
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The resulting D-2 distribution has special
properties
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• Thus, while the smaller bodies may be much more
  numerous than the larger ones, most of the mass
  and energy is contributed by the largest impacts
• This is the definition of a Catastrophe The
  largest impacts produce more geologic change than
  all of the small ones combined

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• The final stages of planetary growth were
  characterized by extreme violence, with global
  melting episodes followed by periods of cooling.
  This may have happened dozens of times in Earth
  history.

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The surfaces, and indeed the very existence, of
the Earth, Moon and the rest of the solar system
is affected by the impact of solid objects