Impact Cratering

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Impact Cratering

• Virginia Pasek
• September 18, 2008

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Astronomical Observations

• Galileo first noted craters on the Moon 1610
• Robert Hooke, 1665, speculated about the origins
  of the lunar craters
• Couldnt be impactors space was empty
• J. H. Schröter first formal use of word crater
  in 1791
• Concluded volcanic origins after seven years of
  study
• Beer and Mädler 1876
• Scientists believed that the moon was covered
  with extinct volcanoes until 1930

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Space is not so Empty

• Meteorites
• 1819 by Chladni
• Supported by April 26, 1803 fall in LAigle near
  Paris
• Scientifically accepted by 1880
• Discovery of asteroids around 1820
• Connection made between large impacts and
  meteorites in 1906
• Meteorite Crater

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The Bowl-shaped Problem

• Incidence angle was a problem
• Objects with low incidence angles should produce
  elongated impacts

Right?
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High-velocity Impacts

• E. J. Öpik, 1916
• Like explosions, high-velocity meteoroids produce
  circular craters for most incidence angles
• Again in 1919 by H. E. Ives of Langly Field
• Even noted central peaks
• Countered by W. W. Campbell of Lick Observatory
• Third times the charm Finally, two papers by A.
  C. Gifford in 1924 and 1930 fixed the bowl-shaped
  problem forever

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World War II

• Recent and poorly documented
• Much research still classified
• Driven by threat of nuclear weapons and
  high-velocity impacts
• Dangers to satellites in low-earth orbit

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Converging Lines of Study

• Astronomical study of the origins of the lunar
  craters
• The acceptance of meteorites as impactors circa
  1880
• Military testing associated with WWII
• Study of Impact Cratering

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What is Impact Cratering?

• The study of the physics of impact and explosion
  craters joined with astronomical and geological
  study of impact craters
• Recent field of study - decades
• Spurred forward by space travel and Apollo program

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Craters Everywhere!

• Grieve, 1987, lists 116 impact craters on Earth
• Craters found on nearly every solid body in the
  Solar System

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Cratering Mechanics

• Contact
• Compression
• Excavation
• Modification

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Contact and Compression

• Briefest of the stages
• Lasts only a few times longer that it takes for
  the impactor to traverse its own diameter
• Transfers energy and momentum to underlying rocks
• Impactor is slowed and compressed
• Surface is pushed downward and outward
• Material at the boundary moves at same velocity

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Excavation

• Shockwave expands and weakens moving as fast or
  faster than speed of sound
• Attains shape of hemisphere as it expands through
  the target rocks
• High shock is confined to surface of hemisphere
• Interior has already decompressed
• High pressure minerals such as stishovite and
  coenite form

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Excavation

• Surface pressure is zero. Shock pressure from
  contact and compression.
• Thin layer of surface rocks thrown upward at very
  high velocity
• Debris is lightly shocked or unshocked
• Only 1 - 3 of total mass excavated
• May be origin of lunar meteorites and SNC
  meteorites from Mars

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Modification

• Motion halts, then moved downward and back toward
  the crater
• Due to gravity and occasionally elastic rebound
• Simple craters
• Debris and drainback
• Complex craters
• Complete alteration of form, including floor
  rise, central peak, rim sinking into wide stepped
  terraces
• Mountain ranges and pits in the largest complex
  craters
• Begins almost immediately after formation of
  transient crater

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Crater Morphology

• Simple craters
• Complex craters
• Multiring basins
• Abberant crater types

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Simple Crater Facts

• Common at
• Rim height 4 of D
• Rim-to-floor depth 1/5 of D
• Ejecta blanket extends one D from rim
• Secondary craters and bright ray ejecta
• Floor underlain by breccia
• Contains shocked quartz i.e. coesite and
  stishovite
• Floor typically 1/2 to 1/3 of rim-to-floor depth

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Simple Craters Schematic
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Simple Craters on Earth

• First to be identified on Earth
• Not always completely circular
• Faults
• Common at 3 km to 6 km diameter

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Simple Crater on Moon

• Moltke crater, a simple crater, was photographed
  by Apollo 10 astronauts in 1969. The depression,
  about 7 km (4.3 miles) in diameter.
• Common up to 15 km diameter

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Transition to Complex Craters

• Transition diameter scales as g-1, where g is the
  acceleration of gravity at the planets surface
• On moon, transition is about 20 km

• Because
• Earth gravity 9.8 m/s2
• Moon gravity 1.6 m/s2

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Complex Craters

• Formed by collapse of bowl-shaped crater
• Observed on Moon, Mars, Earth, and Mercury
• Uplift beneath centers
• Structural uplift to crater diameter by

• Diameter of central peak approx 22 of rim-to-rim
  diameter on terrestrial planets
• Depth increases slowly
• Depth from 3 - 6 km
• Diameters from 20 - 400 km
• Diameter may increase as much as 60 during
  collapse

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Complex Crater Schematic
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Complex Crater on Mercury
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Complex Crater on Moon
The far side of Earth's Moon. Crater 308. It
spans about 30 kilometers (19 miles) and was
photographed by the crew of Apollo 11 as they
circled the Moon in 1969
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More Complex
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More Complex Facts

• Transition to central ring at approx 140 km
  diameter on Moon
• Still follows the g-1 rule
• Central ring generally about half of rim-to-rim
  diameter for terrestrial planets

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Central Ring Crater

• Barton crater on Venus
• Discontinuous central ring
• Very close to transition diameter
• 50 km ring

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Multiring basins

• Valhalla basin on Callisto
• 4000 km
• Only central bright stop believed to be formed by
  impact
• Outward facing scarps

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Multiring basins

• Orientale basin on Moon
• Youngest and best preserved
• Approx 930 km diameter
• 2 km depth
• Inward facing scarps

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Characteristics of Multiring Basins

• Most likely caused by circular normal faults
• Normal fault is result of crustal extension
• Ring diameter ratios of roughly
• No longer function of g-1
• Possibly influenced by the internal structure of
  the planet

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Valhalla
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Multiring Schematic
The ring tectonic theory suggests that in layered
media in which the strength decreases with
increasing depth, one or more ring fractures
arise outside the rim of the original crater
(figure 5) (Melosh and McKinnon, 1978). This
suggests that for the formation of multiring
basins to occur there must be a high
brittle-ductile thickness ratio in the impacted
material i.e. where thick crust exists over a
deeper ductile layer (Allemand and Thomas, 1999).
www.spacechariots.biz/ creaters.htm
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Aberrant Crater Types

• Unusual formation conditions
• Either in impactor or planetary body
• Very low impact angles - 6 from horizontal
• Circular crater with asymmetric ejecta blankets
• Elliptical craters with butterfly eject patterns
• Smaller impactors on Earth and Venus tend to form
  clusters of craters, reflecting atmospheric
  breakup

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References

• Encyclopedia of Planetary Sciences, pp. 326,
  Impact Cratering by H. J. Melosh
• Impact Cratering A Geologic Process, H. J.
  Melosh, Oxford University Press, 1989
• Encyclopedia of the Solar System, Ch 43, Grieve
  et al
• Google