PTYS 554 Evolution of Planetary Surfaces Impact Cratering I

PTYS 554

Evolution of Planetary Surfaces

Impact Cratering IImpact Cratering I

PYTS 554 – Impact Cratering I 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


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


Jupiter continues to perturb asteroids Mutual velocities remain high Collisions cause fragmentation not agglomeration Fragments stray into Kirkwood gaps This material ends up in the inner solar system


How much energy does an impact deliver?

Projectile energy is all kinetic = ½mv2 ~ 2 ρ r3 v2

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

Lowest impact velocity ~ escape velocity (~11 km s-1 for Earth) 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.


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


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 Stishovite Coesite

Tektites Small glassy blobs, widely distributed

Melosh, 1989

Meteor Crater – 1.2 km


Craters are point-source explosions Was fully realized in 1940s and 1950s test explosions

Three main implications: Crater depends on the impactor’s 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


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


Interior bowl: parabolic

Rim+Ejecta falls off as distance cubed

Breccia lens thickness ~0.5H

Shape is size independent e.g. H/D

Melosh, 1989

Complex crater


Central peaks of complex craters have upturned stratigraphy

Upheaval dome, Utah

Unnamed crater,Mars

Grieve and Pilkington (1996)


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

Moltke – 1km Euler – 28km


Simple to complex transition All these craters start as a transient quasi-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


Shockwaves in solids Only Longitudinal waves important in crater formation ~7 km s-1 in crustal rocks

Where K is the bulk modulus, μ is the shear modulus

Only one pulse, compression in one direction affects the others

Creates shear stress τ, pressure P

So:

Shockwaves in Solids


Ductile failure when i.e.

Point of failure is the Hugoniot Elastic limit Permanent deformation

After failing, the rock looses shear strength Shear Modulus declines Longitudinal waves slow down

Initial elastic wave now splits into an elastic and slower plastic wave


K is a function of pressure Higher pressure means higher K and faster waves High enough stresses means wave speed can be even faster than

the elastic case

When the longitudinal stress is very large Typical impacts have 100s GPa peak pressures Wave speed exceeds elastic case and becomes a shock front Shocks are pretty narrow

~mm in pure metals ~10s m in rocks under impacts


Shocked minerals produced Shock metamorphosed minerals produced from quartz-rich

(SiO2) target rock Stishovite – forms at 15 GPa, > 1200 K Coesite – forms at 30 GPa, > 1000 K Dense phases of silica formed only in impacts

Shatter cones produced at lower pressures

Planar deformation features


Rankine-Hugoniot equations relate conditions on either side of the shock

Conservation equations for:

Need an equation of state (P as a function of T and ρ) Equations of state come from lab measurements

Hugoniot – a locus of shocked states Phase changes complicate this picture Slope of the Rayleigh line related to shock speed

Area under Rayleigh line is the kinetic energy imparted to the material

Melosh, 1989

Change in material energy… Let Po ~ 0 Energy added by shock is ½P(V-Vo) Area of triangle under the Rayleigh line


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

This residual velocity excavates the crater


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


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


Material flows down and out

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

Fastest ejecta

Slowestejecta


Most material does not get ejected Downward displacement raises crater rim

Deepest material excavated… Exits the crater at its edge Exits the slowest Slowest material forms overturned flap


Layering in the target can upset this nice picture


Ormo et al., 2013 Oblique impacts can shift inner cavity uprange


Preexisting weaknesses can lead to non-circular craters


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


Courtesy of Brendan Hermalyn, Univ. Hawaii


Ejecta blankets are rough and obliterate pre-existing features…


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


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

Created by high-speed jets in the initial contact stage

Unusual Ejecta


Oblique impact ejecta even when crater is still circular

>45°

30-45°

20-30°

10-20°

0-10°

Ranges are very approximate

downrange


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 buried

Profile of transient crater also a parabola

Derive transient diameter from breccia thickness

Observed Hb/H ~ 0.5, so: Simple craters get a little wider, but a lot shallower


Complex craters collapse extensively

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 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

Documents

PTYS 554 Evolution of Planetary Surfaces Impact Cratering I