Impact Cratering Mechanics and Morphologies

1. Impact CrateringMechanics and Morphologies

2. 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

3. 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

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. 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.

6. Planetary craters similar to nuclear test explosions • Craters are products of point-source explosions • Oblique impacts still make round craters 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

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

8. 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

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: Gravity regime Strength regime

11. Strength Regime Gravity Regime Cratering law

12. Gravity-regime: rV rV m m -3m/(2+m) r ga ( ) ( ) µ d U2 rV m 2+m-6n 2+m Simple scaling model Crater size = F [ {impactor prop}, {target prop}, {env. prop.} ] V = F [ aUmdn, r, Y, g ] Strength-regime: -3m/2 r 1-3n Y ( ) ( ) µ d rU2 ga/U2 (from Housen 2003) †

13. Cratering in metals Regression gives n=0.4, m=0.5 Ref: Holsapple and Schmidt (1982) JGR, 87, 1849-1870.

14. Regímenes de Impacto

15. Regímenes de Impacto II

18. Radius of transient crater d= depth Rfinal = 1.3 R d/Dfinal~ 0.2

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)

20. v >> vp Shockwaves in Solids • 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

21. 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 • 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

22. 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

23. Contact and compression Stage • 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

24. 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

25. 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

26. Ejection and Excavation Stage • 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

27. 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

28. 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 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

29. Collapse and Modification Stage • 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

30. Impact CrateringDating and the Planetary Record

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

32. Crater population is counted • Need some sensible criteria e.g. geologic unit, lava flow etc… • Tabulate craters in diameter bins • Bin size limits are some ratio e.g. 2½ • Size-frequency plot generated • In log-log space • Frequency is normalized to some area • Piecewise linear relationship: • Slope (64km<D, b ~ 2.2 • Slope (2km<D<64km), b ~ 1.8 • Slope (250m<D<2km), b ~ 3.8 • Primary vs. Secondary Branch • Vertical position related to age • These lines are isochrones • Actual data = production function - removal An ideal case…

33. Cumulative plots • Tend to mask deviations from the ideal • R-plots • Size-frequency plot with -2 slope removed • Highlights differences from the ideal • Fractional area covered • Area covered by craters of a certain size • Differs from R-plot by a numerical factor

34. Plotting styles compared for Phobos craters • Hartmann and Neukum, 2001 Cumulative R-plot Differential

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

36. When a surface is saturated no more age information is added • Number of craters stops increasing

37. Typical size-frequency curve • Steep-branch for sizes <1-2 km • Saturation equilibrium for sizes <250m

38. Linking Crater Counts to Age • Moon is divided into two terrain types • Light-toned Terrae (highlands) – plagioclase feldspar • Dark-toned Mare – volcanic basalts • Maria have ~200 times fewer craters • Apollo and Luna missions • Sampled both terrains • Mare ages 3.1-3.8 Ga • Terrae ages all 3.8-4.0 Ga • Lunar meteorites • Confirm above ages are representative of most of the moon.

39. Crater counts had already established relative ages • Samples of the impact melt with geologic context allowed absolute dates to be connected to crater counts • Lunar cataclysm? • Impact melt from large basins cluster in age • Imbrium 3.85Ga • Nectaris 3.9-3.92 Ga • Highland crust solidified at ~4.45Ga • Cataclysm or tail-end of accretion? • Lunar mass favors cataclysm • Impact melt >4Ga is very scarce • Pb isotope record reset at ~3.8Ga • Cataclysm referred to as ‘Late Heavy Bombardment’ } weak

40. 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

41. 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

42. Crater-less impacts • 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 • If Pram exceeds the yield strength then projectile fragments • If fragments drift apart enough then they develop their own shockfronts – fragments separate explosively (pancake model) • Weak bodies at high velocities (comets) are susceptible • Tunguska event on Earth • Crater-less ‘powder burns’ on venus

44. The sounds • Two sounds: • Sonic Boom sónico: minutes after fireball • Electrofonic noise: simultaneous with fireball

45. Infrasound records Fireball of the European Network Fireball Park Forest

46. Seismic records of the airblast

47. Seismic detectionsof Carancas First seismic detection of an extraterrestrial impact on Earth

48. Morphology • Craters occur on all solar system bodies • Crater morphology changes with impact energy • Impact craters are the result of point source explosions Mechanics • 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

49. Summary of recognized impact features • Primary crater • Ejecta blanket • Secondary impact craters • Rays • Rings and multirings • Breccia • Shock metamorphism: Planar Deformation Features (PDFs) • Melt glasses • Tektites • Regolith • Focusing effects in the antipodes • Erosion and catastrophic disruption

50. Ejecta blanket