PTYS 411 Geology and Geophysics of the Solar System Impact Cratering

PYTS 411– Impact Cratering 2 l Where do we find craters? – Everywhere! n Cratering is the one geologic process that every solid solar system body experiences… Mercury Venus Moon EarthMarsAsteroids

PYTS 411– Impact Cratering 3 l Projectile energy is all kinetic = ½mv 2 n Most sensitive to size of object n Size-frequency distribution is a power law wSlope close to -2 wExpected from fragmentation mechanics n Minimum impacting velocity is the escape velocity n Orbital velocity of the impacting body itself n Highest velocity from a head-on collision with a body falling from infinity wLong-period comet w~78 km s -1 for the Earth w~50 times the energy of the minimum velocity case n 1kg of TNT = 4.7 MJ – equivalent to 1kg of rock traveling at ~3 kms -1 n A 1km rocky body at 12 kms -1 would have an energy of ~ J w~20,000 Mega-Tons of TNT wLargest bomb ever detonated ~50 Mega-Tons (USSR, 1961) w2007 earthquake in Peru (7.9 on Richter scale) released ~10 Mega- Tons of TNT equivalent Harris et al.

PYTS 411– Impact Cratering 4 l Morphology changes as craters get bigger n Pit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring Basin Moltke – 1km 10 microns Euler – 28km Schrödinger – 320km Orientale – 970km

PYTS 411– Impact Cratering 5 l Simple vs. complex Characteristics of craters Moltke – 1km Euler – 28km Melosh, 1989

PYTS 411– Impact Cratering 6 l Lunar craters – volcanoes or impacts? n This argument was settled in favor of impacts largely by comparison to weapons tests n Many geologists once believed that the lunar craters were extinct volcanoes

PYTS 411– Impact Cratering 7 l Impact craters are point-source explosions n Was fully realized in 1940s and 1950s test explosions l Three main implications: n Crater depends on the impactors kinetic energy – NOT JUST SIZE n Impactor is much smaller than the crater it produces wMeteor crater impactor was ~50m in size n Oblique impacts still make circular craters wUnless they hit the surface at an extremely grazing angle (<5°) Meteor Crater – 1200m Sedan Crater – 300m

PYTS 411– Impact Cratering 8 Sedan Crater – 0.3 km l Overturned flap at edge n Gives the crater a raised rim n Reverses stratigraphy l Eject blanket n Continuous for ~1 R c l Breccia n Pulverized rock on crater floor l Shock metamorphosed minerals n Shistovite n Coesite l Tektites n Small glassy blobs, widely distributed Melosh, 1989 Meteor Crater – 1.2 km

PYTS 411– Impact Cratering 9 SimpleComplex Bowl shapedFlat-floored Central peak Wall terraces Little meltSome Melt depth/D ~ 0.2 Size independent depth/D smaller Size dependent Small sizesLarger sizes Pushes most rocks downward and outward Move most rocks outside the crater Size limited by rock strengthSize limited by rock weight Moltke – 1km Euler – 28km

PYTS 411– Impact Cratering 10 l Central peaks have upturned stratigraphy Upheaval dome, Utah Unnamed crater, Mars

PYTS 411– Impact Cratering 11 l Simple craters have a fixed shape that scales up or down l Simple to complex transition varies from planet to planet and material to material

PYTS 411– Impact Cratering 12 l Simple to complex transition n All these craters start as a transient hemispheric cavity l Simple craters n In the strength regime n Most material pushed downwards n Size of crater limited by strength of rock n Energy ~ l Complex craters n In the gravity regime n Size of crater limited by gravity n Energy ~ l At the transition diameter you can use either method n i.e. Energy ~ ~ n So: n The transition diameter is higher when wThe material strength is higher wThe density is lower wThe gravity is lower n Y ~ 100 MPa and ρ ~ 3x10 3 kg m -3 for rocky planets n D T is ~3km for the Earth and ~18km for the Moon wCompares well to observations

PYTS 411– Impact Cratering 13 l Stages of impact n Contact and compression n Lasts D projectile /v projectile n Excavation flow n Lasts (D crater /g) 0.5 n Grows like a hemisphere n Produces a transient cavity n Depth stops growing but crater still gets wider n Final depth/diameter of transient crater 1/4 to 1/3 n Collapse n Shallows the bowl-shaped simple crater so depth/diameter ~ 1/5 n Diameter enlarged n Causes wall terraces in normal craters n Normal Faults in multiring basins n Uplifts central peaks

PYTS 411– Impact Cratering 14

PYTS 411– Impact Cratering 15 l Shocked minerals produced n Shock metamorphosed minerals produced from quartz-rich (SiO 2 ) target rock n Shistovite – forms at 15 GPa, > 1200 K n Coesite – forms at 30 GPa, > 1000 K n Dense phases of silica formed only in impacts Planar deformation features

PYTS 411– Impact Cratering 16 l Hugoniot – a locus of shocked states n When a material is shocked its pressure and density can be predicted n Need to know the initial conditions… n …and the shock strength l Rankine-Hugoniot equations n Conservation equations for: n Need an equation of state (P as a function of T and ρ) n Equations of state come from lab measurements n Phase changes complicate this picture n Slope of the Rayleigh line related to shock speed Melosh, 1989 l Change in material energy… n Let P o ~ 0 n Energy added by shock is ½(P-P o )(V o -V) n Area of triangle under the Rayleigh line

PYTS 411– Impact Cratering 17 l Material jumps into shocked state as compression wave passes through n Shock-wave causes near-instantaneous jump to high-energy state (along Rayleigh line) n Compression energy represented by area (in blue) on a pressure-volume plot n Final specific volume > initial specific volume l Decompression allows release of some of this energy (green area) n Decompression follows adiabatic curve n Used mostly to mechanically produce the crater l Difference in energy-in vs. energy-out (pink area) n Heating of target material – material is much hotter after the impact n Irreversible work – like fracturing rock, collapsing pore space, phase changes

PYTS 411– Impact Cratering 18 l Refraction wave follows shock wave n Starts when shock reaches rear of projectile n Adiabatically releases shocked material n Refraction wave speed faster than shock speed n Eventually catches up and lowers the shock l Particle velocity not reduced to zero by the refraction wave though l A consequence of not being able to undo the irreversible work done l It’s this residual velocity that excavates the crater

PYTS 411– Impact Cratering 19 l Adiabatic decompression can cause melting n The higher the peak shock, the more melting n Shock strength dies of quickly with distance wNot much material melted like this Ponded and pitted terrain in Mojave crater, Mars

PYTS 411– Impact Cratering 20 l Mass of melt and vapor (relative to projectile mass) n Increases as velocity squared n Melt-mass/displaced-mass α (gD at ) 0.83 v i 0.33 n Very large craters dominated by melt Earth, 35 km s -1

PYTS 411– Impact Cratering 21 l Material flows down and out n Shock expands as a hemisphere n Near surface material sees a high pressure gradient wSpallation l Deepest material excavated n Exits the crater at its edge n Exits the slowest n Slowest material forms overturned flap l Maxwell Z-model n Streamlines follow n Theta = 0 for straight down, r o is intersection with surface n Z=3 is a pretty good match to impacts and explosions n Ejecta exist at ~45° n r o = D/2 is the material that barely makes it out of the crater n Maximum depth D/8 l In forming transient craters most material is displaced downwards and not ejected

PYTS 411– Impact Cratering 22 l Material begins to move out of the crater n Rarefaction wave provides the energy n Hemispherical transient crater cavity forms n Time of excavate crater in gravity regime: n For a 10 Km crater on Earth, t ~ 32 sec l Material forms an inverted cone shape n Fastest material from crater center n Slowest material at edge forms overturned flap n Ballistic trajectories with range: l Material escapes if ejected faster than n Craters on asteroids generally don’t have ejecta blankets

PYTS 411– Impact Cratering 23 l Ejecta blankets are rough and obliterate pre-existing features… n Radial striations are common

PYTS 411– Impact Cratering 24 l Large chunks of ejecta can cause secondary craters n Commonly appear in chains radial to primary impact n Eject curtains of two secondary impacts can interact wChevron ridges between craters – herring-bone pattern n Shallower than primaries: d/D~0.1 n Asymmetric in shape – low angle impacts l Contested! n Distant secondary impacts have considerable energy and are circular n Secondaries complicate the dating of surfaces n Very large impacts can have global secondary fields wSecondaries concentrated at the antipode

PYTS 411– Impact Cratering 25 l Oblique impacts n Crater stays circular unless projectile impact angle < 10 deg n Ejecta blanket can become asymmetric at angles ~45 deg l Rampart craters n Fluidized ejecta blankets n Occur primarily on Mars n Ground hugging flow that appears to wrap around obstacles n Perhaps due to volatiles mixed in with the Martian regolith n Atmospheric mechanisms also proposed l Bright rays n Occur only on airless bodies n Removed by space weathering n Lifetimes ~1 Gyr n Associated with secondary crater chains n Brightness due to fracturing of glass spherules on surface Carr, 2006 Unusual Ejecta

PYTS 411– Impact Cratering 26 l Previous stages produce a parabolic transient crater l Simple craters collapse from d/D of ~0.37 to ~0.2 n Bottom of crater filled with breccia n Diameter enlarges n Melt sheet buried l Profile (z vs r) of transient crater is a parabola l Ejecta thickness (δ vs r) falls off as distance cubed n Constant (40) chosen so that total volume is conserved l Derive breccia thickness n Observed H b /H ~ 0.5, so D/D t is ~1.19 n So craters get a little bigger, but a lot shallower

PYTS 411– Impact Cratering 27 l Layering in the target can upset this nice picture

PYTS 411– Impact Cratering 28 l Peak versus peak-ring in complex craters n Central peak rebounds in complex craters n Peak can overshoot and collapse forming a peak-ring n Rim collapses so final crater is wider than transient bowl n Final d/D < 0.1 Melosh, 1989