MORPHOLOGY of IMPACT CRATERS

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MORPHOLOGYof IMPACT CRATERS

• Henrik Hargitai
• hargitai_at_emc.elte.hu

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Origins

• Lunar Craters
• Volcanic (17-19th century) (Galilei)
• Impact (20th century) (Wegener, Gilbert)
• Great Basins

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Morphology depends

• Impact energy
• E1/2mv2
• Original impacting body usually evaporated during
  a hypervelocity impact event
• Crater is formed by shock wave from the released
  energy
• Energy of shock wave depends on kinetic energy
  (1/2mv2)
• Temperature and pressure are also related to the
  potential energy (Epmgh)

Data for Mars g3.97, height of the impacting
body velocity of impctor asteroid 7 km/s,
cometary body 42 km/s
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Formationstages

• Contact/
• compression
• Excavation
• Modification

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SimpleCrater

• Small (3-10 km)
• Bowl-shaped
• Da apparent depth
• Dt true depth

Fallout ejecta
Rim crest
Ejecta blanket
Crater fill sediment
Breccia lens
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Complex craters

• Elastic rebound
• Central peak (structural uplift SU)
• Ring depression (flat floor/annular basin)

rim
Ejecta
Terrace/slump
peak
sediment
Melt sheet
Allochton Polimict breccia
Monomict Autochton breccia
breccia
Shatter cones
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Flat floor crater

• walled plains
• Sediment /
• lava-filled

Plato, Moon
Dawes-type
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Central ring crater

• Complex crater with internal ring
• gt4 km on Earth

W Clearwater, Québec, Canada
Barton, Venus
Lowell, Mars
Schrödinger, Moon
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Giant Multiringed Basins

• Impact-related inner,
• Tectonics related outer rings
• Lava-fill possible
• Valhalla-type
• 20 rings
• Young elastic thin crust
• Global effects

Mare Orientale, Moon
Valhalla, Callisto
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Doublet craters

• Physical or optical
• Source Double asteroids

Clearwater
Toutatis
Venus
Possible
Optical
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Catena (crater chain)

• Source distrupted comets
• (Shoemaker Levy 9)
• (impact to Jupiter, 1994)

Ganymede
Davy Catena
Mars Volcanic origin
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Crater cluster

• Multiple asteroid or
• Synchronous impact of
• Exploded incoming body
• In the atmosphere

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Central pit/domecraters

• Pit volatile rich material explodes / released
  (ice melted)
• Dome Mars polar areas
• Ice/snow deposits

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Erosion

• Buried / Ghost craters
• Lava or sediment

Crater under ice polygons (Mars)
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Rayed crater

• Ejecta jets
• Fresh material (colourdifference)
• Mars above the dust layer
• Optical freshness1 Gy

Tycho, Moon
Unnamed, Mars
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Petal Ejecta

• On Venus
• P90 atm, CO2 atmosphere
• Extreme pressure
• supercritical state
• Fluidized atmosphere/rock interaction
• With missing segment (at incoming direction)

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Lobate ejecta

• Single Lobe Ejecta
• Double Lobe Ejecta
• Rampart
• Regolith Ice Layers
• Fluidized ejecta
• Eroded pedestal
• Also Pancake craters

Pedestal
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Butterfly ejecta

• Observed on Mars
• Grazing impact
• lt5 impact angle
• Also Oval craters
• Rio Cuarto, Argentine
• Mars

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Impact with no crater

• Splotche (Dark spot)
• Atmospheric explosion
• Air Blast /Shock Wave
• 1908 Tunguzka event
• comet explosion at 8 km?
• Penetration Crater
• Just a pit (not hypervelocity impact)

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Secondaries

• Secondary impacts
• Often V shaped
• Small craters on Mars all secondaries?

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Relaxed craters

• Ice in regolith
• Softened terrain
• melted craters
• Freeze-thaw cycle

Enceladus
Mars
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Paimpsest

• On Icy moons
• Albedo difference
• Relaxed (no topography)
• Early age viscous relaxation
• Bright material from underneath
• Remnant topography Penepalimpsest (crust not
  viscous)
• Geographic term Facula

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Cometary craters

• Pit halo structures
• Ejecta, Microgravity, homogenous material
• Flat floor structures
• No ejecta, steep slope porous material

P/Wild 2
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Tempel 1 / Deep Impact

• Thank you
• Henrik Hargitai
• hargitai_at_emc.elte.hu