meteoritical evidence and constraints on impacts and disruption
DESCRIPTION
Meteoritical evidence and constraints on impacts and disruption. Guy Consolmagno SJ Specola Vaticana. Catastrophic Disruptions have played a central role in the life of meteorites. compacted/lithified the meteorites produced shock minerals, shock blackening - PowerPoint PPT PresentationTRANSCRIPT
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.