meteoritical evidence and constraints on impacts and disruption

Guy Consolmagno SJSpecola Vaticana

Meteoritical evidence and constraints on

impacts and disruption

Meteoritical evidence and constraints on

impacts and disruption

Catastrophic Disruptionshave played a central role

in the life of meteorites

compacted/lithified the meteorites

produced shock minerals, shock blackening

turned their parent bodies into rubble

dispersed the pieces and sent them to Earth

1

3

2

4

compare McKinney to Rio Negro

• shock blackening• shock effects

1. Meteorites have seen Catastrophic

Disruptions…

Shock Stage Pressure GPa T Increase

S1 < 4 - 5 10 - 20 K

S2 5 - 10 20 - 50 K

S3 15 - 20 100 - 150 K

S4 30-35 250 - 350 K

S5 45 - 55 600 - 850 K

S6 70 - 901500 - 1750

K

Stöffler, Keil, and Scott, GCA 55, 3845

Shock Stage Pressure GPa % (N)

S1 < 4 - 5 11.6% (257)

S2 5 - 10 34.0% (753)

S3 15 - 20 34.8% (770)

S4 30-35 12.9% (286)

S5 45 - 55 4.2% (94)

S6 70 - 90 2.5% (55)

Statistics from Grady 2000 (Catalog of Meteorites)

Galactic CRs (range of a few 10s of cm) produce 3He, 21Ne, 36Cl, etc.

Collisional breakup starts the clock (samples no longer buried and shielded)

Uncertainties: partial shielding, gas loss, GCR rate… addressed in recent years

2. Cosmic Ray Exposure Ages: evidence for breakup and orbital evolution

from Wasson 1985

Meteorites spend most of their lives shielded in parent bodies

L, H ages not random, but indicate distinct collision times

Irons > stones; implies irons from asteroids, stones from the Moon!

Wood’s 1968 interpretation(a cautionary tale!):

From Wood, 1968

H

L

Irons

Wasson (1985) interprets iron data:

IIIABs = 650 ± 100 MaIVAs = 400 ± 100 MaIAB, IVB ages scatterFew low ages; selection effectFew data, big error bars…

45% of all H chondrites were involved in collisional events around 7 Ma ago

Maybe two distinct parent objects/collisions 7.6 Ma and 7.0 Ma ago

A detailed look at H chondrites

Graf and Marti, 1995(JGR 100, 21247)

Graf et al., 2001 (Icarus 150, 181)

Alexeev, 2001(SoSysRes 35, 458)

Correlates with time-of-day for meteorite fall

Suggestion: many H5’s were heated by the Sun at small perihelion distances

Hence they had a “distinct orbital evolution”

Implies nu-6 or 3:1 resonance orbits?

Comparing He- ages with Ne- ages suggests some meteorites

experienced heating after breakup

Meteorites densities can be directly measured in the lab

Meteorite porosity can be modeled to look through effects of terrestrial weathering

Comparison with asteroids is striking…

3. Meteorite vs. asteroid densities: clues to asteroid collisional history

Most meteorites have a bulk density of around 3 to 3.5 times the density of water. CI, CM, and CR meteorites are rich in water, but CRs also are rich in iron. (H, L and LL =ordinary

chondrites.)8

7

6

5

4

3

2

1

Densi

ty,

g/c

c

CI, C

M

CR

,CV

,CO H L LL

Ach

St-

Ir

Iron

Epinal H5

Fell, September 13,

1822, in Vosges,

France

After correcting for weathering effects, a “model” porosity can be estimated.

For all ordinary chondrite types, the

average model porosity is ~10% ±

5%

100

80

60

40

20

0

0%

5%

10%

15%

20%

25%

30%

35%

This OC average

model porosity of

~10% is

independent of

petrographic

type or shock

state

35%

30%

25%

20%

15%

10%

5%

0%

3 4 5 6

S1 S2 S3 S4 S5 S6

Mass from moons

To the right: AO images of Eugenia and Antiope from Merline et al.

Volume from radiometric diameters, lightcurves

Averages for C, S types from Mars perturbations

Asteroid densities

QuickTime™ and aGIF decompressorare needed to see this picture.

QuickTime™ and aGIF decompressorare needed to see this picture.

Estimated Macroporosity

1E+14

1E+16

1E+18

1E+20

1E+22

0% 10% 20% 30% 40% 50% 60% 70% 80%Macroporosity

Mass in Kg (log scale)

45 Eugenia (C)

Phobos

Deimos

1 Ceres (G)

2 Pallas (B)4 Vesta (V)

253 Mathilde (C)243 Ida (S)

433 Eros (S)

Average CAverage S

16 Psyche (M)121 Hermione (C)

90 Antiope (C)762 Pulcova (F)

Loosely Consolidated"Rubble-Pile"

Asteroids

Transition ZoneFractured Asteroids

Coherent Asteroids

87 Sylvia (P)

22 Kalliope (M)11 Parthenope (S)

20 Massalia

Most of the dark, low-density asteroids

measured to date have no water

bands…

if they are made of dry (high

density) material, they are very underdense!

Dark Asteroid Macroporosity

1E+14

1E+16

1E+18

1E+20

1E+22

0% 10% 20% 30% 40% 50% 60% 70% 80% 90%

Macroporosity, assuming CO ρ

( )Mass in Kg log scale

1 Ceres (G)

2 Pallas (B)

87 Sylvia (P)121 Hermione (C) 45 Eugenia (C)

762 Pulcova (F)90 Antiope (C)

Average C

253 Mathilde (C)

Phobos

DeimosTransition Zone

The many large craters on the dark asteroid Mathilde, imaged by NEAR, imply that it must be made of soft material that can absorb heavy blows without flying

apart.

4. Did collisions form

well-compacted

meteorites in the

solar nebula?

How did dust in a vacuum become a low-porosity stony

meteorite?

Epinal H5

Fell, September 13,

1822, in Vosges,

France

It takes many GigaPa to

squeeze pore space out of a

porous powder or sandstone.

Where, and how, did

meteorites lose their porosity?

Why aren’t meteorites fluffy?

results of shock experiments on sandstone (above, Menéndez et al. 1996, J.

Struct. Geol. 18, 1) and meteoritic powders (left, Hirata et al. 1998, LPSC XXIX)

Lithification of sandstones on Earth requires either heat, water, or static pressures on the order of 500 Mpa – 1 Gpa

Ordinary chondrites have not experienced such heat or water; and you’d have to go to the center of Ceres to find such high static pressures.

Could collisions (impacts between porous parent bodies) be the source of the energy needed to compact meteorites?

Eccentricity of 0.05 ≈ collisional speed of 1 km/s ≈ 1 GPa shock pressure

Porous impact experiment described inHousen et al., Nature, 1999

from: De Carli, Bowden, and Seaman (2001) “Shock compaction and porosity in meteorites”

paper given at the

2001 Meteoritical

Society meeting,

Rome

“ ‘Natural’ shock compaction, via

impacts in space, will also

subsequently create porosity.”

10 km/s collision?P > 80 Gpa

Waste Heat >12000 J/gBut… rapid shock attenuation

Model Porosity vs. ShockM

odel Poro

sity

S1 S2 S3 S4 S5 S6

25%

20%

15%

10%

5%

0%

Jupiter Forms in the Solar Nebula:

100-km planetesimals not near a major resonance perturbed to eccentricities fluctuating from 0 to 0.1

(resonant bodies attain much higher e’s, destroy targets on collision)

10-km bodies attain eccentricities of 0.05

smaller bodies damped to low eccentricity until nebular gas dissipated

Jupiter in nebula also induces shock waves that can form chondrules

Collisions Induced by Jupiter Perturbations:

perturbed bodies hit at speeds many times the target body’s escape velocity

similar-sized bodies disrupted

collisions with smaller impactors allow the target to survive.

A series of impacts produce lithified regions in porous unconsolidated matrix.

Subsequent disruptions dissipate this matrix

Lithified regions survive to the present.