Impact Cratering Mechanics and Morphologies

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Impact CrateringMechanics and Morphologies
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In This Lecture

• Crater morphologies
• Morphologies of impacts rim, ejecta etc
• Energies involved in the impact process
• Simple vs. complex craters
• Shockwaves in Solids
• Cratering mechanics
• Contact and compression stage
• Tektites
• Ejection and excavation stage
• Secondary craters
• Bright rays
• Collapse and modification stage
• Atmospheric Interactions

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• 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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• Morphology changes as craters get bigger
• Pit ? Bowl Shape? Central Peak ? Central Peak
  Ring ? Multi-ring Basin

Euler 28km
Moltke 1km
10 microns
Schrödinger 320km
Orientale 970km
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• Last stages of planetary accretion
• Many planetesimals left over
• Most gone in a 100 Myr
• Were still accreting the last of these bodies
  today
• Jupiter continues to perturb asteroids
• Mutual velocities remain high

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• The worst is over
• Late heavy bombardment 3.7-3.9 Ga
• Impacts still occurring today though
• Jupiter was hit by a comet 15 years ago

• Chain impacts occur due to Jupiters high gravity
• e.g. Callisto
• Next lecture will look at
• Dating using impact craters
• Solar system history from the impact record

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• How much energy does an impact deliver?
• Projectile energy is all kinetic ½mv2 2 ? 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
• Planets orbital velocity around the sun (30 km
  s-1 for Earth)
• 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

Harris et al.
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Characteristics of craters

• Simple vs. complex

Moltke 1km
Melosh, 1989
Euler 28km
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• Common crater features
• Overturned flap at edge
• Gives the crater a raised rim
• Reverses stratigraphy
• Eject blanket
• Continuous for 1 Rc
• Breccia
• Pulverized rock on crater floor

Melosh, 1989
Meteor Crater 1.2 km
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• 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 (lt5)

Meteor Crater 1200m
Sedan Crater 300m
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• Planetary craters similar to nuclear test
  explosions
• Craters are products of point-source explosions
• Oblique impacts still make round craters

Sedan Crater 0.3 km
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
• Shistovite
• Coesite
• Tektites
• Small glassy blobs, widely distributed

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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
• Which of these is a volcanic caldera?

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• Crater size depends on impactor energy
• Size of a simple crater depends on the strength
  of target rock
• Small craters are in the so called strength
  regime
• The stronger the rocks, the smaller the crater
• The weight of the rocks isnt important
• Size of a complex crater depends on the weight of
  the target rock
• Large craters are in the so called gravity
  regime
• Weight of target rocks depends on gravity and
  target-rock density
• The strength of the rocks isnt important

Euler 28km
Moltke 1km
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• When do you switch from the strength regime to
  gravity regime?
• Transition diameter (DT)
• Yrock strength
• ?rock density
• gplanetary gravity

• Rock strength and density dont vary much
• but gravity varies quite a bit
• Earth DT 3km
• Moon DT 18km

Moltke 1km
Euler 28km
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• Differences in simple and complex morphologies

Euler 28km
Moltke 1km
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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

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Shockwaves in Solids

• Why impact craters are not just holes in the
  ground
• Energy is transported through solids via waves
• Away from free surfaces, two types of wave exist
• Pressure (P) waves with velocity
• Shear (S) waves with velocity
• ? is the density, µ is the shear modulus
  (rigidity), and K is the bulk modulus
• P waves are faster, but typically only about 7 km
  s-1 in crustal rock
• An impact transports energy faster than the sound
  speed
• Causes a shockwave in both target and projectile

• Projectile is slowed, target material is
  accelerated downward
• Shockwaves cause irreversible damage to material
  they pass through

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• Material can bounce back if it stays within the
  coulomb failure envelope
• Permanent deformation occurs when stress gt H.E.L.
• Material flows plastically
• Material fails outright when stress gt Y

• Shatter cones
• Point to impact center

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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 wave speed
• Rankine-Hugoniot equations
• Conservation equations for
• Mass
• Momentum
• Energy
• Need an equation of state (P as a function of T
  and ?)
• Equations of state come from lab measurements
• Phase changes complicate this picture

Melosh, 1989
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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
• 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

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Contact and compression Stage

• Shockwave starts traveling backward through
  projectile
• In that time the projectile moves forward so it
  gets flattened
• Shock takes lt 1sec to travel through object D/v
• Target material gets accelerated away from
  contact site
• Hemispheric cavity forms
• Jets of material expelled
• Projectile material deforms to line the cavity
• Rarefaction wave follows shock
• Unloading of pressure causes massive heating
• Some target material melted
• Projectile usually vaporized
• Vapor plume (fireball) expands upward
• Material begins to move out of the crater
• Rarefaction wave provides the energy
• Hemispherical transient crater cavity forms

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• Plume of molten silica expands
• Tektites
• Drops of impact melt are swept up
• Freeze during flight aerodynamic forms
• Cool quickly glassy composition

• Minimum size
• Balance surface tension and velocity
• Usually close to 1mm
• Maximum size
• Balance surface tension with aerodynamic forces
• Depends on speed relative to the plume vapor

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• Vaporization and melting
• Peak pressures of 100s of GPa are common
• Usually enough to melt material
• Some target material also vaporized

Mohave crater impact induced melting of ground
ice

• Shocked minerals produced
• Shock metamorphosed minerals produced from
  quartz-rich (SiO2) target rock
• Shistovite forms at 15 GPa, gt 1200 K
• Coesite forms at 30 GPa, gt 1000 K
• Dense phases of silica formed only in impacts

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Ejection and Excavation Stage

• 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 dont have ejecta
  blankets

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• Ejecta blankets are rough and obliterate
  pre-existing features

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• Only the top ? of the original material is
  ejected
• Most material is displaced downwards
• Interaction of shock with surface produces spall
  zone
• 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/D0.1
• 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

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Unusual Ejecta

• Oblique impacts
• Crater stays circular unless projectile impact
  angle lt 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 quickly by impact gardening
• Lifetimes 1 Gyr
• Associated with secondary crater chains
• Brightness due to fracturing of glass spherules
  on surface
• or addition of more crystalline material

Carr, 2006
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Collapse and Modification Stage

• Previous stages produces a hemispherical
  transient crater
• Simple craters collapse from d/D of 0.5 to 0.2
• Bottom of crater filled with breccia
• Extensive cracking to great depths
• 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 lt 0.1

Melosh, 1989
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• Meteor crater as an example of a simple crater
• Occurred about 50,000 year ago
• Impactor was an iron asteroid 50m in diameter
• Crater is about 1200m in diameter
• Energy 30,000 kilotons of TNT
• Hiroshima 15 Kilotons
• In a modern city?
• Depends on terrain
• Compete destruction death
• Out to several km
• Out to 10s of km
• Mostly destroyed
• Few survivors
• Is this common?
• Every 10,000 years or so
• Most of them over the oceans

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• Chicxulub as an example
• Occurred about 65 Million year ago
• Impactor was an asteroid 10 km in diameter
• Crater is about 200 Km in diameter
• Local region was devastated for 1000km

• Debris blasted into orbit
• Reenters atmosphere and causes global wild-fires
• Heat radiation from hot debris boils animals
  alive
• Evidence from global soot layer enriched in
  iridium
• Sunlight diminished plants die
• Corresponds to the KT boundary
• Cretaceous Tertiary
• Break in the fossil record where 75 of species
  went extinct

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Crater-less impacts

• Impacting bodies can explode or be slowed in the
  atmosphere
• Significant drag when the projectile encounters
  its own mass in atmospheric gas
• Where Ps is the surface gas pressure, g is
  gravity and ?i is projectile density
• If impact speed is reduced below elastic wave
  speed then theres no shockwave projectile
  survives
• Ram pressure from atmospheric shock

• If Pram exceeds the yield strength then
  projectile fragments
• If fragments drift apart enough then they develop
  their own shockfronts fragments separate
  explosively
• Weak bodies at high velocities (comets) are
  susceptible
• Tunguska event on Earth
• Crater-less powder burns on venus
• Crater clusters on Mars

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• Crater clusters on Mars
• Atmospheric breakup allows clusters to form here
• Screened out on Earth and Venus
• No breakup on Moon or Mercury

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Morphology

• Craters occur on all solar system bodies
• Crater morphology changes with impact energy
• Impact craters are the result of point source
  explosions

Mechanics

• Craters form from shockwaves
• Contact and compression lt1 s
• Excavation of material 10s of seconds
• Craters collapse from a transient cavity to their
  final form
• Ejecta blankets are ballistically emplaced
• Low-density projectiles can explode in the
  atmosphere