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Impact Cratering. Virginia Pasek September 18, 2008. Astronomical Observations. Galileo first noted craters on the Moon ~1610 Robert Hooke, 1665, speculated about the origins of the lunar craters Couldn’t be impactors; space was empty

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impact cratering

Impact Cratering

Virginia Pasek

September 18, 2008

astronomical observations
Astronomical Observations
  • Galileo first noted craters on the Moon ~1610
  • Robert Hooke, 1665, speculated about the origins of the lunar craters
    • Couldn’t 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
space is not so empty
Space is not so Empty
  • Meteorites
    • 1819 by Chladni
    • Supported by April 26, 1803 fall in L’Aigle near Paris
    • Scientifically accepted by 1880
  • Discovery of asteroids around 1820
  • Connection made between large impacts and meteorites in 1906
    • Meteorite Crater
the bowl shaped problem
The Bowl-shaped Problem
  • Incidence angle was a problem
    • Objects with low incidence angles should produce elongated impacts

Right?

high velocity impacts
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
world war ii
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
converging lines of study
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

what is impact cratering
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
craters everywhere
Craters Everywhere!
  • Grieve, 1987, lists 116 impact craters on Earth
  • Craters found on nearly every solid body in the Solar System
cratering mechanics
Cratering Mechanics
  • Contact
  • Compression
  • Excavation
  • Modification
contact and compression
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
excavation
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
excavation1
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
modification
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
crater morphology
Crater Morphology
  • Simple craters
  • Complex craters
  • Multiring basins
  • Abberant crater types
simple crater facts
Simple Crater Facts
  • Common at < 15 km rim-to-rim diameter, D, on moon
  • 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
simple craters on earth
Simple Craters on Earth
  • First to be identified on Earth
  • Not always completely circular
    • Faults
  • Common at 3 km to 6 km diameter
simple crater on moon
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
transition to complex craters
Transition diameter scales as g-1, where g is the acceleration of gravity at the planet’s surface

On moon, transition is about 20 km

Because…

Earth gravity 9.8 m/s2

Moon gravity 1.6 m/s2

Transition to Complex Craters
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

Complex Craters
complex crater on moon
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

more complex facts
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
central ring crater
Central Ring Crater
  • Barton crater on Venus
  • Discontinuous central ring
  • Very close to transition diameter
    • 50 km ring
multiring basins
Multiring basins
  • Valhalla basin on Callisto
  • 4000 km
    • Only central bright stop believed to be formed by impact
  • Outward facing scarps
multiring basins1
Multiring basins
  • Orientale basin on Moon
  • Youngest and best preserved
  • Approx 930 km diameter
  • 2 km depth
  • Inward facing scarps
characteristics of multiring basins
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
multiring schematic
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

aberrant crater types
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
references
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