Transcript PYTS 411– Impact Cratering

PTYS 411
Geology and Geophysics of the Solar System
Impact Cratering
PYTS 411– Impact Cratering
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Where do we find craters? – Everywhere!
Cratering is the one geologic process that every solid solar system body experiences…
Mercury
Earth
Venus
Moon
Mars
Asteroids
PYTS 411– Impact Cratering
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
Vesc =
GM p
Rp
Orbital velocity of the impacting body itself
æ2 1ö
V = GM*ç - ÷
è r aø
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 MegaTons of TNT equivalent
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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 – 1km
10 microns
Orientale – 970km
Euler – 28km
Schrödinger – 320km
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Characteristics of craters
Simple vs. complex
Moltke – 1km
Melosh, 1989
Euler – 28km
PYTS 411– Impact Cratering
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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PYTS 411– Impact Cratering
Impact craters are point-source explosions
Was fully realized in 1940s and 1950s test explosions
Meteor Crater – 1200m
Sedan Crater – 300m
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°)
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PYTS 411– Impact Cratering
Meteor Crater – 1.2 km
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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
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Simple
Complex
Bowl shaped
Flat-floored
Central peak
Wall terraces
Moltke – 1km
Little melt
Some Melt
depth/D ~ 0.2
depth/D smaller
Size independent
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
Euler – 28km
PYTS 411– Impact Cratering
Central peaks have upturned stratigraphy
Upheaval
dome, Utah
Unnamed
crater,
Mars
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PYTS 411– Impact Cratering
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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PYTS 411– Impact Cratering
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 ~ 2 p r 3 Y
(
)
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Complex craters
In the gravity regime
Size of crater limited by gravity
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Energy ~ 2 3 p r r g D
(
)
At the transition diameter you can use either method
i.e. Energy ~ 2 p rT 3 Y ~ 2 p rT 3 r g DT
3
3
(
So:
Y » r g DT
)
(
or DT » Y
)
rg
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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PYTS 411– Impact Cratering
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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PYTS 411– Impact Cratering
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:
Mass : r (U - up ) = roU
Momentum : P - Po = roupU
æ 1 1ö
Energy : E - Eo = 1 2 ( P + Po ) ç - ÷
è ro r ø
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
U2 =
1 P - Po
ro2 Vo - V
Change in material energy…
Let Po ~ 0
Energy added by shock is ½(P-Po)(Vo-V)
Area of triangle under the Rayleigh line
Melosh, 1989
PYTS 411– Impact Cratering
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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PYTS 411– Impact Cratering
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
PYTS 411– Impact Cratering
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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PYTS 411– Impact Cratering
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
1 ( z-2)
r = ro (1- cosq )
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
t D
g
Material forms an inverted cone shape
Fastest material from crater center
Slowest material at edge forms overturned flap
Ballistic trajectories with range:
æ [v 2 R g] sinFcosF ö
p
÷
2Rp tan-1çç
2
2
÷
è 1- [v Rp g] cos F ø
Material escapes if ejected faster than
ve  GM P
RP
Craters on asteroids generally don’t have ejecta blankets
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PYTS 411– Impact Cratering
Ejecta blankets are rough and obliterate pre-existing features…
Radial striations are common
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PYTS 411– Impact Cratering
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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Unusual Ejecta
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
PYTS 411– Impact Cratering
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
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æ 2r ö2
H æ Dö
z = Hç ÷
and d =
ç ÷
è Dø
40 è r ø
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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PYTS 411– Impact Cratering
Layering in the target can upset this nice picture
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PYTS 411– Impact Cratering
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
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