1. Impact Cratering I

2. Impact Cratering I Impact Cratering II Impact Cratering III
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

3. Mercury Venus Moon Earth Mars Asteroids
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

4. 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

5. 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.

6. 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

7. Overturned flap at edge
Meteor Crater – 1.2 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
Stishovite
Coesite
Tektites
Small glassy blobs, widely distributed
Melosh, 1989

8. 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

9. Morphology changes as craters get bigger
Pit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring Basin
10 microns
Moltke – 1km
Euler – 28km
Orientale – 970km
Schrödinger – 320km

10. Characteristics of craters
Simple vs. complex
Moltke – 1km
Melosh, 1989
Euler – 28km

11. Interior bowl: parabolic Rim+Ejecta falls off as distance cubed
Complex crater
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

12. Grieve and Pilkington (1996)
Central peaks of complex craters have upturned stratigraphy
Upheaval dome, Utah
Grieve and Pilkington (1996)
Unnamed crater,
Mars

13. Simple to complex transition varies from planet to planet and material to material
Moltke – 1km
Euler – 28km

14. 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
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

15. Shockwaves in Solids 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:

16. After failing, the rock looses shear strength
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

17. 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

18. Planar deformation features
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

19. Hugoniot – a locus of shocked states
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
Change in material energy…
Let Po ~ 0
Energy added by shock is ½P(V-Vo)
Area of triangle under the Rayleigh line
Melosh, 1989

20. 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

21. 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

22. Ponded and pitted terrain in Mojave crater, Mars
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

23. Slowest ejecta Fastest ejecta
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
Slowest
ejecta
Fastest ejecta

24. Most material does not get ejected Deepest material excavated…
Downward displacement raises crater rim
Deepest material excavated…
Exits the crater at its edge
Exits the slowest
Slowest material forms overturned flap

25. Layering in the target can upset this nice picture

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

27. Preexisting weaknesses can lead to non-circular craters

28. 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

29. Courtesy of Brendan Hermalyn, Univ. Hawaii

30. Ejecta blankets are rough and obliterate pre-existing features…

31. 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

32. Unusual Ejecta Rampart craters Bright rays 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

33. Oblique impact ejecta even when crater is still circular
>45°
30-45°
20-30°
10-20°
0-10°
Ranges are very approximate
downrange

34. 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

35. 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

36. Impact Cratering I Impact Cratering II Impact Cratering III
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