MORPHOLOGY
of IMPACT CRATERS
Henrik Hargitai
hargitai@emc.elte.hu
Origins
• Lunar Craters
• Volcanic (17-19th century) (Galilei)
• Impact (20th century) (Wegener, Gilbert)
• Great Basins
Morphology
depends:
• Impact energy
• E=1/2mv2
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 (Ep=mgh)
Data for Mars: g=3.97, h[eight of the
impacting body] v[elocity of impctor]
asteroid: ~7 km/s, cometary body:
~42 km/s
Formation
stages
•
•
•
•
Contact/
compression
Excavation
Modification
Simple
Crater
•Small (3-10 km)
•Bowl-shaped
•Da apparent depth
•Dt true depth
Fallout ejecta
Ejecta blanket
Rim crest
Crater fill
sediment
Breccia lens
Complex craters
• Elastic rebound
• Central peak (structural uplift [SU])
• Ring depression (flat floor/annular
basin)
rim
Monomict
Autochton
breccia
Terrace/slump
sediment
peak
Ejecta
Melt sheet
Shatter cones
Allochton
breccia Polimict
breccia
Flat floor crater
• „walled plains”
• Sediment /
• lava-filled
Plato, Moon
Dawes-type
Central ring crater
• Complex crater
with internal ring
• >4 km on Earth
Schrödinger, Moon
Lowell, Mars
W Clearwater,
Québec, Canada
Barton, Venus
Giant
Multiringed
Basins
• Impact-related inner,
• Tectonics related outer
rings
• Lava-fill possible
• Valhalla-type
Mare Orientale, Moon
• 20+ rings
• Young elastic thin crust
• Global effects
Valhalla, Callisto
Doublet craters
• Physical or „optical”
• Source: Double asteroids
Toutatis
Possible
Optical
Clearwater
Venus
Catena
(crater chain)
• Source: distrupted comets
• (Shoemaker Levy 9)
(impact to Jupiter, 1994)
Ganymede
Davy Catena
Mars: Volcanic origin
Crater cluster
• Multiple asteroid or
• Synchronous impact of
• Exploded incoming body
• In the atmosphere
Central pit/dome
craters
• Pit: volatile rich material
explodes / released (ice melted)
• Dome: Mars polar areas
• Ice/snow deposits
Erosion:
• Buried / Ghost craters
• Lava or sediment
Crater under
ice polygons (Mars)
Rayed crater
• Ejecta jets
• Fresh material (colour
difference)
• Mars: above the dust
layer
• Optical freshness:
1 Gy
Tycho, Moon
Unnamed, Mars
Petal Ejecta
• On Venus
• P=90 atm, CO2
atmosphere
• Extreme pressure
• „supercritical state”
• Fluidized atmosphere/rock
interaction
• With missing segment (at
incoming direction)
Lobate ejecta
•
•
•
•
•
•
•
Single Lobe Ejecta
Double Lobe Ejecta
Rampart
Regolith Ice Layers
Fluidized ejecta
Eroded: pedestal
Also: Pancake craters
Pedestal
Butterfly ejecta
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•
•
•
•
•
Observed on Mars
„Grazing impact”
<5° impact angle
Also: Oval craters:
Rio Cuarto, Argentine
Mars
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)
Secondaries
• Secondary
impacts
• Often V shaped
• Small craters on
Mars all
secondaries?
Relaxed craters
•
•
•
•
Ice in regolith
Softened terrain
„melted craters”
Freeze-thaw cycle
Enceladus
Mars
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
Cometary craters
Pit halo structures
• Ejecta, Microgravity,
homogenous material
P/Wild 2
Flat floor structures
• No ejecta, steep slope:
porous material
Tempel 1 / Deep Impact
• Thank you
• Henrik Hargitai
• hargitai@emc.elte.hu