1 / 29

Impact Cratering II

Impact Cratering II. 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

cassia
Download Presentation

Impact Cratering II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Impact Cratering II

  2. 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 work

  3. 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

  4. An ideal case… • 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

  5. There are at least 4 ways to represent crater count data • Bin spacing should be geometric, √2 is most common • Plots from craterstats (Michael & Neukum, EPSL, 2010) • Definitions from the “CRATER ANALYSIS TECHNIQUES WORKING GROUP” (Icarus, 37, 1979) Incremental Cumulative Relative Differential

  6. Cumulative plots • Tends to mask deviations from the ideal • Not binned • Incremental plots • The ‘standard’ plot… Incremental Cumulative

  7. Incremental plots with √2 diameter bin spacing is favored by Hartmann • Isochrons have become relatively standardized for Mars Hartmann, 2005

  8. Cumulative plots • Differential plots Differential Cumulative

  9. R-plots • Size-frequency plot with slope removed - Highlights differences from the ideal • Area of craters: • Rarely used Cumulative Relative (R-Plot)

  10. R-plots reveal different populations of cratering bodies • Young surfaces are flat • close to a -2 slope in log(N) vs. log(D) • Older surfaces show a different impacting population • More on this later Strom et al., 2005

  11. When a surface is saturated no more age information is added • Number of craters stops increasing • The whole premise of crater dating is that c (or k) increases linearly with time

  12. Geometric saturation • Hexagonal packing allows craters to fill 90.5% of available area (Pf) • A mix of crater diameters allows Ns = 1.54 D-2 • Crater arrays separated by a factor of two in diameter For equal sized craters Log (N) Log (D)

  13. Equilibrium saturation: • No surface ever reaches the geometrically saturated 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 • This saturation diameter increases with time

  14. Multiple slope breaks • Summary of a classic crater size-frequency distribution • Typical size-frequency curve • Steep-branch for sizes <1-2 km • Saturation equilibrium for sizes <250m Sample of Mare Orientale

  15. In general, it’s hardly ever as neat and tidy as the lunar mare. • Craters can get removed as fast as they arrive – an equilibrium population • production x lifetime = population • production & population known • Can find the crater lifetime… • Usually crater lifetime is a power-law of diameter: a Dx • If x=0, then the crater lifetime is the surface age i.e. all craters are preserved • If x=1, then crater lifetime is proportional to depth… e.g. constant infill rate

  16. Viscous relaxation of icy topography can make craters undetectable • Maxwell time • Stress causes elastic deformation and creep • Time after which creep strain equals elastic strain • tM = εel / (Δεcreep/t) = η/μ • μ is the shear modulus (rigidity), η is the viscosity • On Earth • tM for rock >109 years • tM for ice ~ 100s sec • Ganymede ice is intermediate Pathare and Paige, 2005

  17. Relaxed craters • Penepalimpset → Palimpset Viscous relaxation on the icy Galilean satellites Images by Paul Schenk Lunar and Planetary Institute

  18. Secondary craters confuse the picture • Steep-branch of lunar production function caused controversy • Are these true secondaries or collisional fragments generated in space • Asteroid Gaspra • Also has steep-branch • Definitely lacks true secondaries • Case closed? Not really…

  19. Analysis of Zunil by McEwen et al. • Modeling suggests this one crater can account for all craters a few 10’s of meters in size • They suggest most small craters on Mars should be secondaries • Secondary distribution • Lumpy in space and time • Can’t use these craters for dating a surface

  20. 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.

  21. 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? • Highland crust solidified at ~4.45Ga • Impact melt from large basins cluster in age • Imbrium 3.85Ga • Nectaris 3.9-3.92 Ga

  22. Before and after the late heavy bombardment • 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

  23. Origin of the late heavy bombardment projectiles • Convert crater size distribution to projectile size distribution • Using Pi scaling laws • Display both as R-plots to highlight structure • LHB – matches main-belt asteroids • Post LHB craters – match the near-Earth asteroid population • LHB caused by surge of asteroidal material entering the inner solar system • Migration of Jupiter can move orbital-resonances through the asteroid belt Strom et al., 2005

  24. Lunar impact rates can be scaled to other planets • Must assume the same projectile population i.e. this doesn’t work for the outer solar system where a different projectile population dominates • Two-step process – e.g. Mars • Rbolide is the ratio of projectile fluxes • Comes from dynamical studies ~2.6 (very uncertain) • Rcrater is the ratio of crater sizes formed by the same projectile • Impact energy ratio come from dynamical studies ~ 0.71 • Ratio of gravities = 2.3 • Rcrater ~ 0.75 Hartmann, 2005 Schmitt and Housen, 1987 Hartmann, 2005

  25. The problem is that we can’t date martian materials in the lab… • But we can start to test these impact rates on Mars…. June 4th 2008 August 10th 2008

  26. ~190 impact events recognized so far • Crater sizes from a few meters to a few decameters • Effective diameter of clusters reconstructed from • Very biased and incomplete sample Daubar et al. 2012

  27. Crater flux close to what we expect, but we’re not seeing all impacts… • Efficiency of atmospheric screening also not well known Daubar et al. 2012

  28. Titan Cratering Neish and Lorenz, 2011 • Outer solar system chronology relies entirely on dynamical models • E.g. Titan shows a global ‘age’ of <1 Gyr

  29. 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 work

More Related