PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36PTYS 554Evolution of Planetary SurfacesImpact Cratering IImpact Cratering IPYTS 554 – Impact Cratering I2Impact Cratering ISize-morphology progressionPropagation of shocksHugoniotEjecta blankets - Maxwell Z-modelFloor rebound, wall collapseImpact Cratering IIThe population of impacting bodiesRescaling the lunar cratering rateCrater age datingSurface saturationEquilibrium crater populationsImpact Cratering IIIStrength vs. gravity regimeScaling of impactsEffects of material strengthImpact experiments in the labHow hydrocodes workPYTS 554 – Impact Cratering I3Where do we find craters? – Everywhere!Cratering is the one geologic process that every solid solar system body experiences…MercuryVenusMoonEarth Mars AsteroidsPYTS 554 – Impact Cratering I4Jupiter continues to perturb asteroidsMutual velocities remain highCollisions cause fragmentation not agglomerationFragments stray into Kirkwood gaps This material ends up in the inner solar systemPYTS 554 – Impact Cratering I5How much energy does an impact deliver?Projectile energy is all kinetic = ½mv2 ~ 2 ρ r3 v2Most sensitive to size of objectSize-frequency distribution is a power lawSlope close to -2Expected from fragmentation mechanicsMinimum impacting velocity is the escape velocityOrbital velocity of the impacting body itselfLowest impact velocity ~ escape velocity (~11 km s-1 for Earth)Highest velocity from a head-on collision with a body falling from infinityLong-period comet~78 km s-1 for the Earth~50 times the energy of the minimum velocity case1kg of TNT = 4.7 MJ – equivalent to 1kg of rock traveling at ~3 kms-1A 1km rocky body at 12 kms-1 would have an energy of ~ 1020J~20,000 Mega-Tons of TNTLargest bomb ever detonated ~50 Mega-Tons (USSR, 1961)2007 earthquake in Peru (7.9 on Richter scale) released ~10 Mega-Tons of TNT equivalentHarris et al. Vesc=GMpRp V = GM*2r-1aæ è ç ö ø ÷PYTS 554 – Impact Cratering I6Lunar craters – volcanoes or impacts?This argument was settled in favor of impacts largely by comparison to weapons testsMany geologists once believed that the lunar craters were extinct volcanoesPYTS 554 – Impact Cratering I7Overturned flap at edgeGives the crater a raised rimReverses stratigraphyEject blanketContinuous for ~1 RcBrecciaPulverized rock on crater floorShock metamorphosed mineralsStishoviteCoesiteTektitesSmall glassy blobs, widely distributedMelosh, 1989Meteor Crater – 1.2 kmPYTS 554 – Impact Cratering I8Craters are point-source explosionsWas fully realized in 1940s and 1950s test explosionsThree main implications:Crater depends on the impactor’s kinetic energy – NOT JUST SIZEImpactor is much smaller than the crater it producesMeteor crater impactor was ~50m in sizeOblique impacts still make circular cratersUnless they hit the surface at an extremely grazing angle (<5°)Meteor Crater – 1200mSedan Crater – 300mPYTS 554 – Impact Cratering I9Morphology changes as craters get biggerPit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring BasinMoltke – 1km10 micronsEuler – 28kmSchrödinger – 320kmOrientale – 970kmPYTS 554 – Impact Cratering I10Simple vs. complexCharacteristics of cratersCharacteristics of cratersMoltke – 1kmEuler – 28kmMelosh, 1989PYTS 554 – Impact Cratering I11H =0.2DhR=0.2H =0.04DInterior bowl: parabolicRim+Ejecta falls off as distance cubedBreccia lens thickness ~0.5HShape is size independent e.g. H/DMelosh, 1989Complex craterPYTS 554 – Impact Cratering I12Central peaks of complex craters have upturned stratigraphyUpheaval dome, UtahUnnamed crater,Marshuplift=0.086D1.03Grieve and Pilkington (1996)PYTS 554 – Impact Cratering I13Simple to complex transition varies from planet to planet and material to materialMoltke – 1kmEuler – 28kmPYTS 554 – Impact Cratering I14Simple to complex transitionAll these craters start as a transient quasi-hemispheric cavitySimple cratersIn the strength regimeMost material pushed downwardsSize of crater limited by strength of rockEnergy ~ Complex cratersIn the gravity regimeSize of crater limited by gravityEnergy ~ At the transition diameter you can use either methodi.e. Energy ~ ~So: The transition diameter is higher whenThe material strength is higherThe density is lowerThe gravity is lowerY ~ 100 MPa and ρ ~ 3x103 kg m-3 for rocky planetsDT is ~3km for the Earth and ~18km for the MoonCompares well to observations 23p r3( )Y 23p r3( )r gD 23p rT3( )Y 23p rT3( )r g DT Y » r g DTor DT»Yr gPYTS 554 – Impact Cratering I15Shockwaves in solidsOnly Longitudinal waves important in crater formation~7 km s-1 in crustal rocksWhere K is the bulk modulus, μ is the shear modulusOnly one pulse, compression in one direction affects the othersCreates shear stress τ, pressure PSo:CL=K +43m( )r ¶2uL¶t2=CL2¶2uL¶x2sL=rouLCLsT=n1- næèçöø÷sLt =12sL- sT( )=121- 2n( )1- n( )sLP =13sL+sT+sT( )=131+n( )1- n( )sLt =321- 2n( )1+n( )P » 0.6PShockwaves in SolidsPYTS 554 – Impact Cratering I16Ductile failure wheni.e.Point of failure is the Hugoniot Elastic limitPermanent deformationAfter failing, the rock looses shear strengthShear Modulus declinesLongitudinal waves slow downInitial elastic wave now splits into an elastic and slower plastic wavesL- sT( )=Y sL>1- n( )1- 2n( )YPYTS 554 – Impact Cratering I17K is a function of pressureHigher pressure means higher K and faster wavesHigh enough stresses means wave speed can be even faster than the elastic caseWhen the longitudinal stress is very largeTypical impacts have 100s GPa peak pressuresWave speed exceeds elastic case and becomes a shock frontShocks are pretty narrow~mm in pure metals~10s m in rocks under impactsPYTS 554 – Impact Cratering I18Shocked minerals producedShock metamorphosed minerals produced from quartz-rich (SiO2) target