ptys 554 evolution of planetary surfaces impact cratering i
Post on 19-Dec-2015
220 Views
Preview:
TRANSCRIPT
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
PYTS 554 – Impact Cratering I 3
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
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 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.
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 7
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
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 9
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
PYTS 554 – Impact Cratering I 10
Simple vs. complex
Characteristics of cratersCharacteristics of craters
Moltke – 1km
Euler – 28km
Melosh, 1989
PYTS 554 – Impact Cratering I 11
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
PYTS 554 – Impact Cratering I 12
Central peaks of complex craters have upturned stratigraphy
Upheaval dome, Utah
Unnamed crater,Mars
Grieve and Pilkington (1996)
PYTS 554 – Impact Cratering I 13
Simple to complex transition varies from planet to planet and material to material
Moltke – 1km Euler – 28km
PYTS 554 – Impact Cratering I 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 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
PYTS 554 – Impact Cratering I 15
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
PYTS 554 – Impact Cratering I 16
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
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 18
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
PYTS 554 – Impact Cratering I 19
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
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 22
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 554 – Impact Cratering I 23
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
PYTS 554 – Impact Cratering I 24
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
PYTS 554 – Impact Cratering I 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
gDt
P
Pe R
GMv
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 32
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
PYTS 554 – Impact Cratering I 33
Oblique impact ejecta even when crater is still circular
>45°
30-45°
20-30°
10-20°
0-10°
Ranges are very approximate
downrange
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 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
PYTS 554 – Impact Cratering I 36
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
top related