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Impact Cratering

Impact Cratering. Where do we find craters? – Everywhere! Cratering is the one geologic process that every solid solar system body experiences…. Mercury. Venus. Moon. Earth. Mars. Asteroids. Harris et al. Projectile energy is all kinetic = ½mv 2 Most sensitive to size of object

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Impact Cratering

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  1. Impact Cratering

  2. Where do we find craters? – Everywhere! • Cratering is the one geologic process that every solid solar system body experiences… Mercury Venus Moon Earth Mars Asteroids

  3. Harris et al. • Projectile energy is all kinetic = ½mv2 • 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 • 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 equivalent

  4. Morphology changes as craters get bigger • Pit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring Basin Euler – 28km 10 microns Moltke – 1km Schrödinger – 320km Orientale – 970km

  5. Characteristics of craters • Simple vs. complex Moltke – 1km Melosh, 1989 Euler – 28km

  6. 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 volcanoes

  7. Impact craters are point-source explosions • Was fully realized in 1940s and 1950s test explosions • Three main implications: • Crater depends on the impactors 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°) Sedan Crater – 300m Meteor Crater – 1200m

  8. Sedan Crater – 0.3 km Meteor Crater – 1.2 km • 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 • Shistovite • Coesite • Tektites • Small glassy blobs, widely distributed Melosh, 1989

  9. Euler – 28km Moltke – 1km

  10. Central peaks have upturned stratigraphy Upheaval dome, Utah Unnamed crater, Mars

  11. Simple craters have a fixed shape that scales up or down • Simple to complex transition varies from planet to planet and material to material

  12. Simple to complex transition • All these craters start as a transient 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

  13. Stages of impact • Contact and compression • Lasts Dprojectile/vprojectile • Excavation flow • Lasts (Dcrater/g)0.5 • Grows like a hemisphere • Produces a transient cavity • Depth stops growing but crater still gets wider • Final depth/diameter of transient crater 1/4 to 1/3 • Collapse • Shallows the bowl-shaped simple crater so depth/diameter ~ 1/5 • Diameter enlarged • Causes wall terraces in normal craters • Normal Faults in multiring basins • Uplifts central peaks

  14. Shocked minerals produced • Shock metamorphosed minerals produced from quartz-rich (SiO2) target rock • Shistovite – forms at 15 GPa, > 1200 K • Coesite – forms at 30 GPa, > 1000 K • Dense phases of silica formed only in impacts Planar deformation features

  15. Hugoniot – a locus of shocked states • When a material is shocked its pressure and density can be predicted • Need to know the initial conditions… • …and the shock strength • Rankine-Hugoniot equations • Conservation equations for: • Need an equation of state (P as a function of T and ρ) • Equations of state come from lab measurements • Phase changes complicate this picture • Slope of the Rayleigh line related to shock speed • Change in material energy… • Let Po ~ 0 • Energy added by shock is ½(P-Po)(Vo-V) • Area of triangle under the Rayleigh line Melosh, 1989

  16. Material jumps into shocked state as compression wave passes through • Shock-wave causes near-instantaneous jump to high-energy state (along Rayleigh line) • Compression energy represented by area (in blue) on a pressure-volume plot • Final specific volume > initial specific volume • Decompression allows release of some of this energy (green area) • Decompression follows adiabatic curve • Used mostly to mechanically produce the crater • Difference in energy-in vs. energy-out (pink area) • Heating of target material – material is much hotter after the impact • Irreversible work – like fracturing rock, collapsing pore space, phase changes

  17. Refraction wave follows shock wave • Starts when shock reaches rear of projectile • Adiabatically releases shocked material • Refraction wave speed faster than shock speed • Eventually catches up and lowers the shock • Particle velocity not reduced to zero by the refraction wave though • A consequence of not being able to undo the irreversible work done • It’s this residual velocity that excavates the crater

  18. Adiabatic decompression can cause melting • The higher the peak shock, the more melting • Shock strength dies of quickly with distance • Not much material melted like this Ponded and pitted terrain in Mojave crater, Mars

  19. Mass of melt and vapor (relative to projectile mass) • Increases as velocity squared • Melt-mass/displaced-mass α (gDat)0.83 vi0.33 • Very large craters dominated by melt Earth, 35 km s-1

  20. Material flows down and out • Shock expands as a hemisphere • Near surface material sees a high pressure gradient • Spallation • Deepest material excavated • Exits the crater at its edge • Exits the slowest • Slowest material forms overturned flap • Maxwell Z-model • Streamlines follow • Theta = 0 for straight down, ro is intersection with surface • Z=3 is a pretty good match to impacts and explosions • Ejecta exist at ~45° • ro = D/2 is the material that barely makes it out of the crater • Maximum depth D/8 • In forming transient craters most material is displaced downwards and not ejected

  21. Material begins to move out of the crater • Rarefaction wave provides the energy • Hemispherical transient crater cavity forms • Time of excavate crater in gravity regime: • For a 10 Km crater on Earth, t ~ 32 sec • Material forms an inverted cone shape • Fastest material from crater center • Slowest material at edge forms overturned flap • Ballistic trajectories with range: • Material escapes if ejected faster than • Craters on asteroids generally don’t have ejecta blankets

  22. Ejecta blankets are rough and obliterate pre-existing features… • Radial striations are common

  23. Large chunks of ejecta can cause secondary craters • Commonly appear in chains radial to primary impact • Eject curtains of two secondary impacts can interact • Chevron ridges between craters – herring-bone pattern • Shallower than primaries: d/D~0.1 • Asymmetric in shape – low angle impacts • Contested! • Distant secondary impacts have considerable energy and are circular • Secondaries complicate the dating of surfaces • Very large impacts can have global secondary fields • Secondaries concentrated at the antipode

  24. Unusual Ejecta • Oblique impacts • Crater stays circular unless projectile impact angle < 10 deg • Ejecta blanket can become asymmetric at angles ~45 deg • Rampart craters • Fluidized ejecta blankets • Occur primarily on Mars • Ground hugging flow that appears to wrap around obstacles • Perhaps due to volatiles mixed in with the Martian regolith • Atmospheric mechanisms also proposed • Bright rays • Occur only on airless bodies • Removed by space weathering • Lifetimes ~1 Gyr • Associated with secondary crater chains • Brightness due to fracturing of glass spherules on surface Carr, 2006

  25. Previous stages produce a parabolic transient crater • Simple craters collapse from d/D of ~0.37 to ~0.2 • Bottom of crater filled with breccia • Diameter enlarges • Melt sheet buried • Profile (z vs r) of transient crater is a parabola • Ejecta thickness (δ vs r) falls off as distance cubed • Constant (40) chosen so that total volume is conserved • Derive breccia thickness • Observed Hb/H ~ 0.5, so D/Dt is ~1.19 • So craters get a little bigger, but a lot shallower

  26. Layering in the target can upset this nice picture

  27. Peak versus peak-ring in complex craters • Central peak rebounds in complex craters • Peak can overshoot and collapse forming a peak-ring • Rim collapses so final crater is wider than transient bowl • Final d/D < 0.1 Melosh, 1989

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