PTYS 411 511 cratering mechanics morphologies

Impact CrateringMechanics and Morphologies: 

Impact Cratering Mechanics and Morphologies

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Crater morphologies Morphologies of impacts rim, ejecta etc Energies involved in the impact process Simple vs. complex craters Shockwaves in Solids Cratering mechanics Contact and compression stage Tektites Ejection and excavation stage Secondary craters Bright rays Collapse and modification stage Atmospheric Interactions In This Lecture

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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

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Morphology changes as craters get bigger 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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Last stages of planetary accretion Many planetesimals left over Most gone in a ~100 Myr We’re still accreting the last of these bodies today 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 system

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The worst is over… Late heavy bombardment 3.7-3.9 Ga Impacts still occurring today though Jupiter was hit by a comet ~15 years ago Chain impacts occur due to Jupiter’s high gravity e.g. Callisto Next lecture will look at: Dating using impact craters Solar system history from the impact record

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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 Planet’s orbital velocity around the sun (~30 km s-1 for Earth) 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 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) Recent earthquake in Peru (7.9 on Richter scale) released ~10 Mega-Tons of TNT equivalent Harris et al.

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Planetary craters similar to nuclear test explosions Craters are products of point-source explosions Oblique impacts still make round craters Meteor Crater – 1.2 km Sedan Crater – 0.3 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

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Differences in simple and complex morphologies Moltke – 1km Euler – 28km

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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

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

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Hugoniot – a locus of shocked states When a material is shocked it’s pressure and density can be predicted Need to know the initial conditions… …and the shock wave speed Rankine-Hugoniot equations Conservation equations for: Mass Momentum Energy 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 Melosh, 1989 Material can bounce back if it stays within the coulomb failure envelope Permanent deformation occurs when stress > H.E.L. Material flows plastically Material fails outright when stress > Y

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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 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

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

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

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Vaporization and melting Peak pressures of 100’s of GPa are common Usually enough to melt material Some target material also vaporized 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

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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 Ejection and Excavation Stage

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Only the top ~⅓ of the original material is ejected Most material is displaced downwards Interaction of shock with surface produces spall zone 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

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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 quickly by impact gardening Lifetimes ~1 Gyr Associated with secondary crater chains Brightness due to fracturing of glass spherules on surface …or addition of more crystalline material Carr, 2006 Unusual Ejecta

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Previous stages produces a hemispherical transient crater Simple craters collapse from d/D of ~0.5 to ~0.2 Bottom of crater filled with breccia Extensive cracking to great depths 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 Collapse and Modification Stage

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

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