Meteoritical evidence and constraints on impacts and disruption

1. Meteoritical evidence and constraints on impacts and disruption • Guy Consolmagno SJ • Specola Vaticana

2. 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 4 1 3 2

3. 1. Meteorites have seen Catastrophic Disruptions… • • shock blackening • • shock effects compare McKinney to Rio Negro

4. 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 - 90 1500 - 1750 K Stöffler, Keil, and Scott, GCA55, 3845

5. 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)

6. 2. Cosmic Ray Exposure Ages: evidence for breakup and orbital evolution • 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 from Wasson 1985

7. Wood’s 1968 interpretation(a cautionary tale!): H L Irons • 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! From Wood, 1968

8. Wasson (1985) interprets iron data: • IIIABs = 650 ± 100 Ma • IVAs = 400 ± 100 Ma • IAB, IVB ages scatter • Few low ages; selection effect • Few data, big error bars…

9. A detailed look at H chondritesGraf and Marti, 1995(JGR 100, 21247)Graf et al., 2001 (Icarus 150, 181)Alexeev, 2001(SoSysRes 35, 458) • 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

10. Comparing He- ages with Ne- ages suggests some meteorites experienced heating after breakup • 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?

11. 3. Meteorite vs. asteroid densities: clues to asteroid collisional history • 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…

12. 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 CI, CM CR,CV,CO H L LL Ach St-Ir Iron Density, g/cc

13. Epinal H5 Fell, September 13, 1822, in Vosges, France

14. 100 80 60 40 20 0 After correcting for weathering effects, a “model” porosity can be estimated. For all ordinary chondrite types, the average model porosity is ~10% ± 5% 0% 5% 10% 15% 20% 25% 30% 35%

15. 35% 30% 25% 20% 15% 10% 5% 0% 3 4 5 6 This OC average model porosity of ~10% is independent of petrographic type or shock state S1 S2 S3 S4 S5 S6

16. Asteroid densities • 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

18. 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!

20. 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.

21. 4. Did collisions form well-compacted meteorites in the solar nebula? How did dust in a vacuum become a low-porosity stony meteorite?

22. Epinal H5 Fell, September 13, 1822, in Vosges, France

23. 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)

24. 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 in Housen et al., Nature, 1999

25. 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/g But… rapid shock attenuation

26. Model Porosity vs. Shock 25% 20% 15% 10% 5% 0% Model Porosity S1 S2 S3 S4 S5 S6

27. 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 impactsproduce lithified regions in porous unconsolidated matrix. • Subsequentdisruptions dissipate this matrix • Lithified regionssurvive to the present.