Impact Cratering

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


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.


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


Simple vs. complex

Characteristics of cratersCharacteristics of craters

Moltke – 1km

Euler – 28km

Melosh, 1989


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


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


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


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


Central peaks have upturned stratigraphy

Upheaval dome, Utah

Unnamed crater,Mars


Simple craters have a fixed shape that scales up or down

Simple to complex transition varies from planet to planet and material to material


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


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


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


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


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


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


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


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


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


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


Ejecta blankets are rough and obliterate pre-existing features… Radial striations are common


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


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


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


Layering in the target can upset this nice picture


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

Impact Cratering

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