impact cratering
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Impact Cratering. 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. Harris et al. Projectile energy is all kinetic = ½mv 2 Most sensitive to size of object - PowerPoint PPT PresentationTRANSCRIPT
PTYS 411
Geology and Geophysics of the Solar System
Impact CrateringImpact 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 Venus Moon
Earth Mars Asteroids
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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
Orbital velocity of the impacting body itself
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 Mega-
Tons of TNT equivalent
Harris et al.
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Morphology changes as craters get bigger Pit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring Basin
Moltke – 1km10 microns Euler – 28km
Schrödinger – 320kmOrientale – 970km
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Simple vs. complex
Characteristics of cratersCharacteristics of craters
Moltke – 1km
Euler – 28km
Melosh, 1989
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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
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Impact craters are point-source explosions Was fully realized in 1940s and 1950s test explosions
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°)
Meteor Crater – 1200m Sedan Crater – 300m
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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
Meteor Crater – 1.2 km
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Simple Complex
Bowl shaped Flat-floored
Central peak
Wall terraces
Little melt Some Melt
depth/D ~ 0.2
Size independent
depth/D smaller
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
Moltke – 1km Euler – 28km
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Central peaks have upturned stratigraphy
Upheaval dome, Utah
Unnamed crater,Mars
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Simple craters have a fixed shape that scales up or down
Simple to complex transition varies from planet to planet and material to material
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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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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
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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
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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:
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
Melosh, 1989
Change in material energy… Let Po ~ 0 Energy added by shock is ½(P-Po)(Vo-V) Area of triangle under the Rayleigh line
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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 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
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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
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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
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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
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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 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
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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
gDt
P
Pe R
GMv
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Ejecta blankets are rough and obliterate pre-existing features… Radial striations are common
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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 by space weathering Lifetimes ~1 Gyr Associated with secondary crater chains Brightness due to fracturing of glass spherules
on surface
Carr, 2006
Unusual Ejecta
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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
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
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Layering in the target can upset this nice picture
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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