ptys 411/511 geology and geophysics of the solar system shane byrne – [email protected]...

PTYS 411/511 Geology and Geophysics of the Solar System

Shane Byrne – [email protected] is from NASA Planetary Photojournal PIA00094

Impact CrateringMechanics and Morphologies

PYTS 411/511 – Cratering Mechanics and Morphologies 2

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


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


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


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*


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


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


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


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


Strength Regime Gravity Regime

Cratering law


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)


Cratering in metals

Ref: Holsapple and Schmidt (1982) JGR, 87, 1849-1870.

Regression gives =0.4, =0.5


Regímenes de Impacto


Regímenes de Impacto II


Radius of transient crater

depth

Rfinal = 1.3 R

d/Dfinal ~ 0.2

d=


In the gravity regime

Diameter of transient crater

Diameter of final rim

Depth of transient crater

Depth of final rim

(Collins et al. 2005)


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


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


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


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


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


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


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


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


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


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


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


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


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)(


Plotting styles compared for Phobos craters Hartmann and Neukum, 2001

Differential Cumulative R-plot


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


When a surface is saturated no more age information is added Number of craters stops increasing


Typical size-frequency curve Steep-branch for sizes <1-2 km Saturation equilibrium for sizes

<250m


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


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


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


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


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


The sounds

Two sounds:•Sonic Boom sónico: minutes after fireball•Electrofonic noise: simultaneous with fireball


Infrasound records

Fireball of the European Network

Fireball Park Forest


Seismic records of the airblast


Seismic detections of Carancas

First seismic detection of an extraterrestrial impact on Earth


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


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


Ejecta blanket


Secondary craters


Crater rays


Rings and multirings


Focusing in the antipoe

ptys 411/511 geology and geophysics of the solar system shane byrne – [email protected]...

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