PYTS/ASTR 206 – Craters1z Homework #1 returned Grades were well distributed –Average was a high CAtilt15Average question results 1 –5 were 74%, 72%, 77%, 57%, 59% We’re happy to talk about the homework – tomorrow! Solutions posted after this lecturep No discussions with us for 24 hoursz Homework #2 posted on website after this lecture One week to finishPYTS/ASTR 206 – Craters2CratersPTYS/ASTR 206 – The Golden Age of Planetary ExplorationShane Byrne – [email protected]/ASTR 206 – Craters3In this lecture…In this lecture…z Introduction to cratersz Characteristics of cratersBowls rims and ejecta blanketsBowls, rims and ejecta blankets Nuclear test results Simple vs complex cratersCt f tizCrater formation Impacts and Energy Excavation Relaxation e.g. Meteor crater, Chicxulubz Atmospheric effects E.g. Tunguskaz Crater populationsDating a planetary surfaceDating a planetary surfacePYTS/ASTR 206 – Craters4z Where do we find craters? – Everywhere!Cratering is the one geologic process that every solid solar system bodyCratering is the one geologic process that every solid solar system body experiences…MercuryVenusMoonEarth Mars AsteroidsPYTS/ASTR 206 – Craters5z Morphology changes as craters get biggerPit→ Bowl Shape→ Central Peak → Central Peak Ring → Multi-ringPit → Bowl Shape→ Central Peak → Central Peak Ring → Multiring BasinMoltke – 1km10 micronsEuler – 28kmSchrödinger – 320kmOrientale – 970kmPYTS/ASTR 206 – Craters6z Origin of impactor craters Asteroid fragments leave the main asteroid belt From collisions with each other Become Near-Earth Asteroids Kuiper Belt Objects leave the Kuiper belt From collisions with each other Become Jupiter Family CometszSteady trickle of thezSteady trickle of the objects Less common today than billions of years agoPYTS/ASTR 206 – Craters7Characteristics of cratersCharacteristics of cratersz Simple vs. complexMoltke–1kmEuler – 28kmMelosh, 1989PYTS/ASTR 206 – Craters8z Common crater featuresOverturned flap at edgeOverturned flap at edge Gives the crater a raised rim Reverses stratigraphyEject blanketEject blanket Continuous for ~1 Rc Breccia Pulverized rock on crater floorMelosh, 1989Meteor Crater – 1.2 kmPYTS/ASTR 206 – Craters9z Craters are point-source explosions Was fully realized in 1940s and 1950s test explosionsMeteor Crater – 1200mSedan Crater – 300mz Three main implications:Crater depends on theimpactor’skinetic energy–NOT JUST SIZECrater depends on the impactor skinetic energy –NOT JUST SIZE Impactor is much smaller than the crater it produces Meteor crater impactor was ~50m in sizeOblique impacts still make circular cratersOblique impacts still make circular craters Unless they hit the surface at an extremely grazing angle (<5°)PYTS/ASTR 206 – Craters10z Lunar craters – volcanoes or impacts?Thi t ttl d i f f i t l l b i tThis argument was settled in favor of impacts largely by comparison to weapons tests Many geologists once believed that the lunar craters were extinct volcanoesWhi h f th i l i ld ?zWhich of these is a volcanic caldera?PYTS/ASTR 206 – Craters11z Lunar craters – volcanoes or impacts?Thi t ttl d i f f i t l l b i tThis argument was settled in favor of impacts largely by comparison to weapons tests Many geologists once believed that the lunar craters were extinct volcanoesWhi h f th i l i ld ?zWhich of these is a volcanic caldera?VOLCANICImpactRaised Rim – from explosionNo Raised Rim – formed by collapsePYTS/ASTR 206 – Craters12Simple ComplexBowl shaped Flat-flooredCentral peakWall terracesLittle melt Some Meltdepth/D ~ 0.2Size independentdepth/D smallerSize dependentSmall sizes Larger sizesgPushes most rocks downward and outwardMove most rocks outside the craterSize limited by strength of rocksSize limited by weight of rocksPYTS/ASTR 206 – Craters13z Crater size depends on impactor energySi filtd d thtthft t kzSize of a simple crater depends on the strengthof target rock Small craters are in the so called ‘strength regime’ The stronger the rocks, the smaller the craterTh i ht f th k i ’t i t tThe weight of the rocks isn’t importantz Size of a complex crater depends on the weight of the target rock Large craters are in the so called ‘gravity regime’ Weight of target rocks depends on gravity and target-rock density The strength of the rocks isn’t importantMoltke – 1kmEuler – 28kmPYTS/ASTR 206 – Craters14z When do you switch from the strength regime to gravity regime?Titidit(D)Transition diameter (DT) Y=rock strength ρ=rock densitylt itgYDTρ≈z Rock strength and density don’t vary much g=planetary gravity …but gravity varies quite a bit Earth: DT~ 3km Moon: DT~ 18kmMoltke – 1kmEuler – 28kmPYTS/ASTR 206 – Craters15z An example:T pical rock strength is 108PaTypical rock strength is 108Pa Typical rock density is 3000 kg m-3 What’s the transition diameter from simple to complex craters on M?Mars? Martian gravity is 3.72 ms-2YD8870108 About 8.9 km()mgYDT887072.3300010=×=≈ρz What about an impact into martian ice Strength 107Pa & Density 1000 kg m-3()mgYDT269072.31000107=×=≈ρ About 2.7 kmPYTS/ASTR 206 – Craters16Formation of cratersFormation of cratersz How to build a craterzThree stageszThree stages Contact and explosion Excavation Collapse z Total energy is ½mv2 m is the mass v is the impactor velocity v is at least 11 km s-1(Earth’s escape velocity) v is at most 72 km s-1(A head-on collision with a comet)PYTS/ASTR 206 – Craters17z Contact stageIthitthftlitImpactor hits the surface –traveling at several km s-1 Shockwave start propagating through the impactor and targetpg Impactor penetrates the surface Shockwave reaches the other side of the impactor – impactor explodes Like an underground point-source explosionTrinity Nuclear Test – 0.03 secondsPYTS/ASTR 206 – Craters18z Excavation stage Bowl shaped cavity formsMaterial ejected in a coneMaterial ejected in a cone Particles on balastic trajectories Cone appears to expandPYTS/ASTR 206 – Craters19z Simulations can extend lab work Oslo University, Physics Dept.PYTS/ASTR 206 – Craters20z Some blocks of ejecta can be very large Can form secondary cratersPYTS/ASTR 206
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