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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 energyEnergy 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
Formationstages
• Contact/• compression• Excavation• Modification
SimpleCrater•Small (3-10 km)•Bowl-shaped•Da apparent depth •Dt true depth
Fallout ejectaEjecta blanket
Breccia lens
Rim crest
Crater fill sediment
Complex craters• Elastic rebound• Central peak (structural uplift [SU])• Ring depression (flat floor/annular
basin)
rim Ejecta Terrace/slump
sedimentpeak
Melt sheet
brecciaShatter cones
Monomict Autochtonbreccia
Allochton Polimictbreccia
Flat floor crater
• „walled plains”• Sediment / • lava-filled
Dawes-typePlato, Moon
Central ring crater
• Complex crater with internal ring
• >4 km on EarthSchrödinger, Moon Lowell, Mars Barton, Venus
W Clearwater,Québec, Canada
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
Doublet craters
• Physical or „optical”• Source: Double asteroids
Toutatis Venus
Clearwater
Possible Optical
Catena (crater chain)
• Source: distrupted comets• (Shoemaker Levy 9) (impact to Jupiter, 1994)
Davy Catena
Ganymede
Mars: Volcanic origin
Crater cluster
• Multiple asteroid or• Synchronous impact of
• Exploded incoming body• In the atmosphere
Central pit/domecraters
• 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
• 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
P/Wild 2
Pit halo structures• Ejecta, Microgravity, homogenous material
Flat floor structures• No ejecta, steep slope: porous material
