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PTYS 411/511 Geology and Geophysics of the Solar System Shane Byrne – Background is from NASA Planetary Photojournal PIA00094 Impact Cratering Mechanics and Morphologies

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PYTS 411/511 – Cratering Mechanics and Morphologies 2 l Crater morphologies n Morphologies of impacts rim, ejecta etc n Energies involved in the impact process n Simple vs. complex craters l Shockwaves in Solids l Cratering mechanics n Contact and compression stage wTektites n Ejection and excavation stage wSecondary craters wBright rays n Collapse and modification stage l Atmospheric Interactions In This Lecture

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PYTS 411/511 – Cratering Mechanics and Morphologies 3 l Where do we find craters? – Everywhere! n Cratering is the one geologic process that every solid solar system body experiences… Mercury Venus Moon EarthMarsAsteroids

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PYTS 411/511 – Cratering Mechanics and Morphologies 4 l Morphology changes as craters get bigger n Pit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring Basin Moltke – 1km 10 microns Euler – 28km Schrödinger – 320km Orientale – 970km

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PYTS 411/511 – Cratering Mechanics and Morphologies 5 l How much energy does an impact deliver? l Projectile energy is all kinetic = ½mv 2 ~ 2 ρ r 3 v 2 n Most sensitive to size of object n Size-frequency distribution is a power law wSlope close to -2 wExpected from fragmentation mechanics n Minimum impacting velocity is the escape velocity n Orbital velocity of the impacting body itself n Planet’s orbital velocity around the sun (~30 km s -1 for Earth) n Lowest impact velocity ~ escape velocity (~11 km s -1 for Earth) n Highest velocity from a head-on collision with a body falling from infinity wLong-period comet w~78 km s -1 for the Earth w~50 times the energy of the minimum velocity case n A 1km rocky body at 12 kms -1 would have an energy of ~ J w~20,000 Mega-Tons of TNT wLargest bomb ever detonated ~50 Mega-Tons (USSR, 1961) wRecent earthquake in Peru (7.9 on Richter scale) released ~10 Mega-Tons of TNT equivalent Harris et al.

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PYTS 411/511 – Cratering Mechanics and Morphologies 6 l Planetary craters similar to nuclear test explosions l Craters are products of point-source explosions n Oblique impacts still make round craters Meteor Crater – 1.2 km Sedan Crater – 0.3 km l Overturned flap at edge n Gives the crater a raised rim n Reverses stratigraphy l Eject blanket n Continuous for ~1 R c l Breccia n Pulverized rock on crater floor l Shock metamorphosed minerals n Shistovite n Coesite l Tektites n Small glassy blobs, widely distributed

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PYTS 411/511 – Cratering Mechanics and Morphologies 7 l Differences in simple and complex morphologies SimpleComplex Bowl shapedFlat-floored Central peak Wall terraces Little meltSome Melt d/D ~ 0.2d/D much smaller Diameter dependent Small sizesLarger sizes Moltke – 1km Euler – 28km

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PYTS 411/511 – Cratering Mechanics and Morphologies 8 l Simple to complex transition n All these craters start as a transient hemispheric cavity l Simple craters n In the strength regime n Most material pushed downwards n Size of crater limited by strength of rock n Energy ~ l Complex craters n In the gravity regime n Size of crater limited by gravity n Energy ~ l At the transition diameter you can use either method n i.e. Energy ~ ~ n So: n The transition diameter is higher when wThe material strength is higher wThe density is lower wThe gravity is lower n Y ~ 100 MPa and ρ ~ 3x10 3 kg m -3 for rocky planets n D T is ~3km for the Earth and ~18km for the Moon wCompares well to observations

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PYTS 411/511 – Cratering Mechanics and Morphologies 9

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10 Dimensional Analysis and Pi-Scaling V – Volume of the crater Projectile: a – radius U – velocity - density Target: - density Y – strength g – gravity acc. By dimensional analysis we obtain: or The impactor act as a “point source”. The coupling parameter: Strength regime Gravity regime

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PYTS 411/511 – Cratering Mechanics and Morphologies 11 Strength RegimeGravity Regime Cratering law

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PYTS 411/511 – Cratering Mechanics and Morphologies 12 Simple scaling model Crater size = F [ {impactor prop}, {target prop}, {env. prop.} ] V = F [ aU , , Y, g ] Strength-regime: /2 ) ( ) Y U2U2 ( VV m VV m ga/U 2 Gravity-regime: -3 /(2+ ) ) ( ) ga U2U2 ( 2+ VV m (from Housen 2003)

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PYTS 411/511 – Cratering Mechanics and Morphologies 13 Cratering in metals Ref: Holsapple and Schmidt (1982) JGR, 87, Regression gives =0.4, =0.5

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PYTS 411/511 – Cratering Mechanics and Morphologies 14 Regímenes de Impacto

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PYTS 411/511 – Cratering Mechanics and Morphologies 15 Regímenes de Impacto II

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PYTS 411/511 – Cratering Mechanics and Morphologies 16

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PYTS 411/511 – Cratering Mechanics and Morphologies 18 Radius of transient crater depth R final = 1.3 R d/D final ~ 0.2 d=d=

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PYTS 411/511 – Cratering Mechanics and Morphologies 19 In the gravity regime Diameter of transient crater Diameter of final rim Depth of transient crater Depth of final rim (Collins et al. 2005)

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PYTS 411/511 – Cratering Mechanics and Morphologies 20 l Why impact craters are not just holes in the ground… n Energy is transported through solids via waves n Away from free surfaces, two types of wave exist n Shear (S) waves with velocity n Pressure (P) waves with velocity n ρ is the density, μ is the shear modulus (rigidity), and K is the bulk modulus n P waves are faster, but typically only about 7 km s -1 in crustal rock l An impact transports energy faster than the sound speed n Causes a shockwave in both target and projectile v >> v p l Projectile is slowed, target material is accelerated downward l Shockwaves cause irreversible damage to material they pass through Shockwaves in Solids

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PYTS 411/511 – Cratering Mechanics and Morphologies 21 l Hugoniot – a locus of shocked states n When a material is shocked it’s pressure and density can be predicted n Need to know the initial conditions… n …and the shock wave speed l Rankine-Hugoniot equations n Conservation equations for: wMass wMomentum wEnergy n Need an equation of state (P as a function of T and ρ) n Equations of state come from lab measurements n Phase changes complicate this picture Melosh, 1989 l Material can bounce back if it stays within the coulomb failure envelope n Permanent deformation occurs when stress > H.E.L. n Material flows plastically n Material fails outright when stress > Y

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PYTS 411/511 – Cratering Mechanics and Morphologies 22 l Material jumps into shocked state as compression wave passes through n Shock-wave causes near-instantaneous jump to high-energy state (along Rayleigh line) n Compression energy represented by area (in blue) on a pressure-volume plot l Decompression allows release of some of this energy (green area) n Decompression follows adiabatic curve n Used mostly to mechanically produce the crater l Difference in energy-in vs. energy-out (pink area) n Heating of target material – material is much hotter after the impact n Irreversible work – like fracturing rock

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PYTS 411/511 – Cratering Mechanics and Morphologies 23 l Shockwave starts traveling backward through projectile n In that time the projectile moves forward so it gets flattened n Shock takes < 1sec to travel through object D/v l Target material gets accelerated away from contact site n Hemispheric cavity forms n Jets of material expelled n Projectile material deforms to line the cavity l Rarefaction wave follows shock n Unloading of pressure causes massive heating n Some target material melted n Projectile usually vaporized n Vapor plume (fireball) expands upward l Material begins to move out of the crater n Rarefaction wave provides the energy n Hemispherical transient crater cavity forms Contact and compression Stage

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PYTS 411/511 – Cratering Mechanics and Morphologies 24 l Plume of molten silica expands l Tektites n Drops of impact melt are swept up n Freeze during flight – aerodynamic forms n Cool quickly – glassy composition l Minimum size n Balance surface tension and velocity l Maximum size n Balance surface tension with aerodynamic forces n Surface tension (σ) typically 0.3 N/m n v Jet is < impact velocity n Δv is the difference between gas and droplet velocity in plume l Minimum size close to 1 nm l Maximum size depends on how well coupled the gas and particles are l Tektites rain out over a large area

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PYTS 411/511 – Cratering Mechanics and Morphologies 25 l Vaporization and melting n Peak pressures of 100’s of GPa are common n Usually enough to melt material n Some target material also vaporized l Shocked minerals produced n Shock metamorphosed minerals produced from quartz-rich (SiO 2 ) target rock n Shistovite – forms at 15 GPa, > 1200 K n Coesite – forms at 30 GPa, > 1000 K n Dense phases of silica formed only in impacts

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PYTS 411/511 – Cratering Mechanics and Morphologies 26 l Material begins to move out of the crater n Rarefaction wave provides the energy n Hemispherical transient crater cavity forms n Time of excavate crater in gravity regime: n For a 10 Km crater on Earth, t ~ 32 sec l Material forms an inverted cone shape n Fastest material from crater center n Slowest material at edge forms overturned flap n Ballistic trajectories with range: n Material escapes if ejected faster than n Craters on asteroids generally don’t have ejecta blankets Ejection and Excavation Stage

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PYTS 411/511 – Cratering Mechanics and Morphologies 27 l Only the top ~⅓ of the original material is ejected n Most material is displaced downwards n Interaction of shock with surface produces spall zone l Large chunks of ejecta can cause secondary craters n Commonly appear in chains radial to primary impact n Eject curtains of two secondary impacts can interact wChevron ridges between craters – herring-bone pattern n Shallower than primaries: d/D~0.1 n Asymmetric in shape – low angle impacts l Contested! n Distant secondary impacts have considerable energy and are circular n Secondaries complicate the dating of surfaces n Very large impacts can have global secondary fields wSecondaries concentrated at the antipode

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PYTS 411/511 – Cratering Mechanics and Morphologies 28 l Oblique impacts n Crater stays circular unless projectile impact angle < 10 deg n Ejecta blanket can become asymmetric at angles ~45 deg l Rampart craters n Fluidized ejecta blankets n Occur primarily on Mars n Ground hugging flow that appears to wrap around obstacles n Perhaps due to volatiles mixed in with the Martian regolith n Atmospheric mechanisms also proposed l Bright rays n Occur only on airless bodies n Removed quickly by impact gardening n Lifetimes ~1 Gyr n Associated with secondary crater chains n Brightness due to fracturing of glass spherules on surface n …or addition of more crystalline material Carr, 2006 Unusual Ejecta

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PYTS 411/511 – Cratering Mechanics and Morphologies 29 l Previous stages produces a hemispherical transient crater l Simple craters collapse from d/D of ~0.5 to ~0.2 n Bottom of crater filled with breccia n Extensive cracking to great depths l Peak versus peak-ring in complex craters n Central peak rebounds in complex craters n Peak can overshoot and collapse forming a peak-ring n Rim collapses so final crater is wider than transient bowl n Final d/D < 0.1 Melosh, 1989 Collapse and Modification Stage

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PTYS 411/511 Geology and Geophysics of the Solar System Shane Byrne – Background is from NASA Planetary Photojournal PIA00094 Impact Cratering Dating and the Planetary Record

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PYTS 411/511 – Cratering Mechanics and Morphologies 31 l Older surfaces have more craters l Small craters are more frequent than large craters l Relate crater counts to a surface age, if: n Impact rate is constant n Landscape is far from equilibrium i.e. new craters don’t erase old craters n No other resurfacing processes n Target area all has one age n You have enough craters wNeed fairly old or large areas l Techniques developed for Lunar Maria n Telescopic work established relative ages n Apollo sample provided absolute calibration Mercury – Young and Old

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PYTS 411/511 – Cratering Mechanics and Morphologies 32 l Crater population is counted n Need some sensible criteria e.g. geologic unit, lava flow etc… n Tabulate craters in diameter bins n Bin size limits are some ratio e.g. 2 ½ l Size-frequency plot generated n In log-log space n Frequency is normalized to some area l Piecewise linear relationship: n Slope (64km

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PYTS 411/511 – Cratering Mechanics and Morphologies 33 l Cumulative plots n Tend to mask deviations from the ideal l R-plots n Size-frequency plot with -2 slope removed n Highlights differences from the ideal l Fractional area covered n Area covered by craters of a certain size n Differs from R-plot by a numerical factor

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PYTS 411/511 – Cratering Mechanics and Morphologies 34 l Plotting styles compared for Phobos craters n Hartmann and Neukum, 2001 Differential Cumulative R-plot

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PYTS 411/511 – Cratering Mechanics and Morphologies 35 l Geometric saturation: n You can’t fit in more craters than the hexagonal packing (P f = 90.5% efficiency) of area allows n A mix of crater diameters allows N s ~ 1.54 D -2 n No surface ever reaches this theoretical limit. n Saturation sets in long beforehand (typically a few % of the geometric value) n Mimas reaches 13% of geometric saturation – an extreme case l Craters below a certain diameter exhibit saturation n This diameter is higher for older terrain – 250m for lunar Maria

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PYTS 411/511 – Cratering Mechanics and Morphologies 36 l When a surface is saturated no more age information is added n Number of craters stops increasing

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PYTS 411/511 – Cratering Mechanics and Morphologies 37 l Typical size-frequency curve n Steep-branch for sizes <1-2 km n Saturation equilibrium for sizes <250m

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PYTS 411/511 – Cratering Mechanics and Morphologies 38 l Moon is divided into two terrain types n Light-toned Terrae (highlands) – plagioclase feldspar n Dark-toned Mare – volcanic basalts n Maria have ~200 times fewer craters l Apollo and Luna missions n Sampled both terrains n Mare ages Ga n Terrae ages all Ga l Lunar meteorites n Confirm above ages are representative of most of the moon. Linking Crater Counts to Age

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PYTS 411/511 – Cratering Mechanics and Morphologies 39 l Crater counts had already established relative ages n Samples of the impact melt with geologic context allowed absolute dates to be connected to crater counts l Lunar cataclysm? n Impact melt from large basins cluster in age wImbrium 3.85Ga wNectaris Ga n Highland crust solidified at ~4.45Ga l Cataclysm or tail-end of accretion? n Lunar mass favors cataclysm n Impact melt >4Ga is very scarce n Pb isotope record reset at ~3.8Ga l Cataclysm referred to as ‘Late Heavy Bombardment’ } weak

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PYTS 411/511 – Cratering Mechanics and Morphologies 40 l Last stages of planetary accretion n Many planetesimals left over n Most gone in a ~100 Myr n We’re still accreting the last of these bodies today l Jupiter continues to perturb asteroids n Mutual velocities remain high n Collisions cause fragmentation not agglomeration n Fragments stray into Kirkwood gaps n This material ends up in the inner solar system

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PYTS 411/511 – Cratering Mechanics and Morphologies 41 l The worst is over… n Late heavy bombardment Ga n Impacts still occurring today though n Jupiter was hit by a comet ~15 years ago n Chain impacts occur due to Jupiter’s high gravity n e.g. Callisto

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PYTS 411/511 – Cratering Mechanics and Morphologies 42 l Impacting bodies can explode or be slowed in the atmosphere l Significant drag when the projectile encounters its own mass in atmospheric gas: n Where P s is the surface gas pressure, g is gravity and ρ i is projectile density n If impact speed is reduced below elastic wave speed then there’s no shockwave – projectile survives l Ram pressure from atmospheric shock Crater-less impacts n If P ram exceeds the yield strength then projectile fragments n If fragments drift apart enough then they develop their own shockfronts – fragments separate explosively (pancake model) n Weak bodies at high velocities (comets) are susceptible n Tunguska event on Earth n Crater-less ‘powder burns’ on venus

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PYTS 411/511 – Cratering Mechanics and Morphologies 43

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PYTS 411/511 – Cratering Mechanics and Morphologies 44 The sounds Two sounds: Sonic Boom sónico: minutes after fireball Electrofonic noise: simultaneous with fireball

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PYTS 411/511 – Cratering Mechanics and Morphologies 45 Infrasound records Fireball of the European Network Fireball Park Forest

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PYTS 411/511 – Cratering Mechanics and Morphologies 46 Seismic records of the airblast

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PYTS 411/511 – Cratering Mechanics and Morphologies 47 Seismic detections of Carancas First seismic detection of an extraterrestrial impact on Earth

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PYTS 411/511 – Cratering Mechanics and Morphologies 48 l Craters occur on all solar system bodies l Crater morphology changes with impact energy l Impact craters are the result of point source explosions Morphology l Craters form from shockwaves l Contact and compression <1 s l Excavation of material 10’s of seconds l Craters collapse from a transient cavity to their final form l Ejecta blankets are ballistically emplaced l Low-density projectiles can explode in the atmosphere Mechanics

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PYTS 411/511 – Cratering Mechanics and Morphologies 49 Summary of recognized impact features l Primary crater l Ejecta blanket l Secondary impact craters l Rays l Rings and multirings l Breccia l Shock metamorphism: Planar Deformation Features (PDFs) l Melt glasses l Tektites l Regolith l Focusing effects in the antipodes l Erosion and catastrophic disruption

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PYTS 411/511 – Cratering Mechanics and Morphologies 50 Ejecta blanket

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PYTS 411/511 – Cratering Mechanics and Morphologies 51 Secondary craters

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PYTS 411/511 – Cratering Mechanics and Morphologies 52 Crater rays

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PYTS 411/511 – Cratering Mechanics and Morphologies 53 Rings and multirings

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PYTS 411/511 – Cratering Mechanics and Morphologies 54 Focusing in the antipoe

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