ptys 411/511 geology and geophysics of the solar system shane byrne – [email protected]...
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PTYS 411/511 Geology and Geophysics of the Solar System
Shane Byrne – [email protected] is from NASA Planetary Photojournal PIA00094
Impact CrateringMechanics and Morphologies
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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
In This Lecture
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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
Moltke – 1km10 microns Euler – 28km
Schrödinger – 320kmOrientale – 970km
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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
Planet’s 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 ~50 times the energy of the minimum velocity case
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) Recent earthquake in Peru (7.9 on Richter scale) released ~10 Mega-Tons of TNT equivalent
Harris et al.
p
pesc R
GMV
arGMVesc
12*
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Planetary craters similar to nuclear test explosions
Craters are products of point-source explosions Oblique impacts still make round craters
Meteor Crater – 1.2 km 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
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Differences in simple and complex morphologies
Simple Complex
Bowl shaped Flat-floored
Central peak
Wall terraces
Little melt Some Melt
d/D ~ 0.2 d/D much smaller
Diameter dependent
Small sizes Larger sizes
Moltke – 1km
Euler – 28km
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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:
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
Yr3
32
Dgr 3
32
YrT3
32 TT Dgr 3
32
gYDorDgY TT
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Dimensional Analysis and Pi-Scaling
V – Volume of the craterProjectile: a – radius U – velocity - densityTarget: - density Y – strength g – gravity acc.By dimensional analysis we obtain:
or
The impactor act as a “point source”. The coupling parameter:
Strength regime Gravity regime
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Strength Regime Gravity Regime
Cratering law
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Simple scaling model
Crater size = F [ {impactor prop}, {target prop}, {env. prop.} ]
V = F [ aU, , Y, g ]
Strength-regime:
1-3 -3/2) ( )Y
U2(Vm
Vm
ga/U2
Gravity-regime:
-3/(2+)) ( )ga
U2(2+-6
2+Vm
(from Housen 2003)
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Cratering in metals
Ref: Holsapple and Schmidt (1982) JGR, 87, 1849-1870.
Regression gives =0.4, =0.5
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Regímenes de Impacto
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Regímenes de Impacto II
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Radius of transient crater
depth
Rfinal = 1.3 R
d/Dfinal ~ 0.2
d=
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In the gravity regime
Diameter of transient crater
Diameter of final rim
Depth of transient crater
Depth of final rim
(Collins et al. 2005)
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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 Shear (S) waves with velocity
Pressure (P) waves with velocity
ρ 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
Sv
S
P
PKwhere
Kv
3
4
v >> vp
Projectile is slowed, target material is accelerated downward
Shockwaves cause irreversible damage to material they pass through
Shockwaves in Solids
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Hugoniot – a locus of shocked states When a material is shocked it’s 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 ρ) Equations of state come from lab measurements Phase changes complicate this picture
oo
ooo
poo
op
VandVwhere
VVPPEE
vvPP
vvv
112
1
Melosh, 1989
Material can bounce back if it stays within the coulomb failure envelope
Permanent deformation occurs when stress > H.E.L. Material flows plastically Material fails outright when stress > Y
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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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Shockwave starts traveling backward through projectile In that time the projectile moves forward so it gets flattened Shock takes < 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
Contact and compression Stage
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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
22
108
vD
v gasJetmelt
Minimum size Balance surface tension and velocity
Maximum size Balance surface tension with aerodynamic
forces
Surface tension (σ) typically 0.3 N/m vJet is < impact velocity Δv is the difference between gas and droplet velocity in plume
Minimum size close to 1 nm
Maximum size depends on how well coupled the gas and particles are
Tektites rain out over a large area
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Vaporization and melting Peak pressures of 100’s of GPa are common Usually enough to melt material Some target material also vaporized
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
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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
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
Ejection and Excavation Stage
gDt
22
21
cos1
cossintan2
gRv
gRvR
p
pp
P
Pe R
GMv
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Only the top ~⅓ 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/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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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 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
Unusual Ejecta
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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 < 0.1
Melosh, 1989
Collapse and Modification Stage
PTYS 411/511 Geology and Geophysics of the Solar System
Shane Byrne – [email protected] is from NASA Planetary Photojournal PIA00094
Impact CrateringDating and the Planetary Record
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Older surfaces have more craters
Small craters are more frequent than large craters
Relate crater counts to a surface age, if: Impact rate is constant Landscape is far from equilibrium
i.e. new craters don’t erase old craters No other resurfacing processes Target area all has one age You have enough craters
Need fairly old or large areas
Techniques developed for Lunar Maria Telescopic work established relative ages Apollo sample provided absolute calibration
Mercury – Young and Old
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Crater population is counted Need some sensible criteria
e.g. geologic unit, lava flow etc… Tabulate craters in diameter bins Bin size limits are some ratio e.g. 2½
Size-frequency plot generated In log-log space Frequency is normalized to some area
Piecewise linear relationship:
Slope (64km<D, b ~ 2.2 Slope (2km<D<64km), b ~ 1.8 Slope (250m<D<2km), b ~ 3.8 Primary vs. Secondary Branch
Vertical position related to age
These lines are isochrones
Actual data = production function - removal
bkDDDN )2,(
An ideal case…
oo DDD 2
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Cumulative plots Tend to mask deviations from the ideal
R-plots Size-frequency plot with -2 slope removed Highlights differences from the ideal
Fractional area covered Area covered by craters of a certain size Differs from R-plot by a numerical factor
b
cumcum
bcum
kc
DNDNDDNwhere
cDDN
21
)2()(2,
)(
DNDNDDR cumcum 212
2)( 243
DRDF
DNDNDDF cumcum
49
2
2
12
242)(
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Plotting styles compared for Phobos craters Hartmann and Neukum, 2001
Differential Cumulative R-plot
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Geometric saturation:
You can’t fit in more craters than the hexagonal packing (P f = 90.5% efficiency) of area allows A mix of crater diameters allows Ns ~ 1.54 D-2
No surface ever reaches this theoretical limit. Saturation sets in long beforehand (typically a few % of the geometric value) Mimas reaches 13% of geometric saturation – an extreme case
DPAreaNor
DD
PAreaN
fSAT
fSAT
log24
log/log
15.14/ 22
Craters below a certain diameter exhibit saturation This diameter is higher for older terrain – 250m for lunar Maria
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When a surface is saturated no more age information is added Number of craters stops increasing
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Typical size-frequency curve Steep-branch for sizes <1-2 km Saturation equilibrium for sizes
<250m
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Moon is divided into two terrain types Light-toned Terrae (highlands) – plagioclase feldspar Dark-toned Mare – volcanic basalts Maria have ~200 times fewer craters
Apollo and Luna missions Sampled both terrains Mare ages 3.1-3.8 Ga Terrae ages all 3.8-4.0 Ga
Lunar meteorites Confirm above ages are representative of most of the moon.
Linking Crater Counts to Age
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Crater counts had already established relative ages Samples of the impact melt with geologic context allowed
absolute dates to be connected to crater counts
Lunar cataclysm? Impact melt from large basins cluster in age
Imbrium 3.85Ga Nectaris 3.9-3.92 Ga
Highland crust solidified at ~4.45Ga
Cataclysm or tail-end of accretion? Lunar mass favors cataclysm Impact melt >4Ga is very scarce Pb isotope record reset at ~3.8Ga
Cataclysm referred to as ‘Late Heavy Bombardment’
} weak
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Last stages of planetary accretion Many planetesimals left over Most gone in a ~100 Myr We’re still accreting the last of these bodies today
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
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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 Jupiter’s high gravity
e.g. Callisto
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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 ρi is projectile density
If impact speed is reduced below elastic wave speed then there’s no shockwave – projectile survives
Ram pressure from atmospheric shock
Crater-less impacts
iPSi gPDei 23..
ATM
Hz
SATMram
atmosphereram
gkTHwhere
eHg
PvzP
TkvPconstTif
vP
22
2
.
If Pram exceeds the yield strength then projectile fragments If fragments drift apart enough then they develop their own
shockfronts – fragments separate explosively (pancake model)
Weak bodies at high velocities (comets) are susceptible Tunguska event on Earth Crater-less ‘powder burns’ on venus
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The sounds
Two sounds:•Sonic Boom sónico: minutes after fireball•Electrofonic noise: simultaneous with fireball
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Infrasound records
Fireball of the European Network
Fireball Park Forest
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Seismic records of the airblast
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Seismic detections of Carancas
First seismic detection of an extraterrestrial impact on Earth
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Craters occur on all solar system bodies
Crater morphology changes with impact energy
Impact craters are the result of point source explosions
Morphology
Craters form from shockwaves
Contact and compression <1 s
Excavation of material 10’s 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
Mechanics
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Summary of recognized impact features Primary crater Ejecta blanket Secondary impact craters Rays Rings and multirings Breccia Shock metamorphism: Planar Deformation Features (PDFs) Melt glasses Tektites Regolith Focusing effects in the antipodes Erosion and catastrophic disruption
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Ejecta blanket
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Secondary craters
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Crater rays
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Rings and multirings
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Focusing in the antipoe