PYTS 411– Impact Cratering
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PTYS 411 Geology and Geophysics of the Solar System Impact Cratering PYTS 411– Impact Cratering 2 Where do we find craters? – Everywhere! Cratering is the one geologic process that every solid solar system body experiences… Mercury Earth Venus Moon Mars Asteroids PYTS 411– Impact Cratering 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 Vesc = GM p Rp Orbital velocity of the impacting body itself æ2 1ö V = GM*ç - ÷ è r aø 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 MegaTons of TNT equivalent 3 Harris et al. PYTS 411– Impact Cratering 4 Morphology changes as craters get bigger Pit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring Basin Moltke – 1km 10 microns Orientale – 970km Euler – 28km Schrödinger – 320km PYTS 411– Impact Cratering 5 Characteristics of craters Simple vs. complex Moltke – 1km Melosh, 1989 Euler – 28km PYTS 411– Impact Cratering 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 6 PYTS 411– Impact Cratering Impact craters are point-source explosions Was fully realized in 1940s and 1950s test explosions Meteor Crater – 1200m Sedan Crater – 300m 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°) 7 PYTS 411– Impact Cratering Meteor Crater – 1.2 km 8 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 Melosh, 1989 PYTS 411– Impact Cratering 9 Simple Complex Bowl shaped Flat-floored Central peak Wall terraces Moltke – 1km Little melt Some Melt depth/D ~ 0.2 depth/D smaller Size independent Size dependent Small sizes Larger sizes Pushes most rocks downward and outward Move most rocks outside the crater Size limited by rock strength Size limited by rock weight Euler – 28km PYTS 411– Impact Cratering Central peaks have upturned stratigraphy Upheaval dome, Utah Unnamed crater, Mars 10 PYTS 411– Impact Cratering Simple craters have a fixed shape that scales up or down Simple to complex transition varies from planet to planet and material to material 11 PYTS 411– Impact Cratering 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 ~ 2 p r 3 Y ( ) 3 Complex craters In the gravity regime Size of crater limited by gravity 3 Energy ~ 2 3 p r r g D ( ) At the transition diameter you can use either method i.e. Energy ~ 2 p rT 3 Y ~ 2 p rT 3 r g DT 3 3 ( So: Y » r g DT ) ( or DT » Y ) rg 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 12 PYTS 411– Impact Cratering 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 13 PYTS 411– Impact Cratering 14 PYTS 411– Impact Cratering 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 PYTS 411– Impact Cratering 16 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: Mass : r (U - up ) = roU Momentum : P - Po = roupU æ 1 1ö Energy : E - Eo = 1 2 ( P + Po ) ç - ÷ è ro r ø 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 U2 = 1 P - Po ro2 Vo - V 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 PYTS 411– Impact Cratering 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 PYTS 411– Impact Cratering 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 PYTS 411– Impact Cratering 19 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 PYTS 411– Impact Cratering 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 PYTS 411– Impact Cratering 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 1 ( z-2) r = ro (1- cosq ) 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 PYTS 411– Impact Cratering 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 t D g Material forms an inverted cone shape Fastest material from crater center Slowest material at edge forms overturned flap Ballistic trajectories with range: æ [v 2 R g] sinFcosF ö p ÷ 2Rp tan-1çç 2 2 ÷ è 1- [v Rp g] cos F ø Material escapes if ejected faster than ve GM P RP Craters on asteroids generally don’t have ejecta blankets 22 PYTS 411– Impact Cratering Ejecta blankets are rough and obliterate pre-existing features… Radial striations are common 23 PYTS 411– Impact Cratering 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 PYTS 411– Impact Cratering 25 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 PYTS 411– Impact Cratering 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 3 æ 2r ö2 H æ Dö z = Hç ÷ and d = ç ÷ è Dø 40 è r ø 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 PYTS 411– Impact Cratering Layering in the target can upset this nice picture 27 PYTS 411– Impact Cratering 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 28
