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Hit-and-run planetary collisions


Terrestrial planet formation is believed to have concluded in our Solar System with about 10 million to 100 million years of giant impacts, where hundreds of Moon- to Mars-sized planetary embryos acquired random velocities through gravitational encounters and resonances with one another and with Jupiter. This led to planet-crossing orbits and collisions that produced the four terrestrial planets, the Moon and asteroids. But here we show that colliding planets do not simply merge, as is commonly assumed. In many cases, the smaller planet escapes from the collision highly deformed, spun up, depressurized from equilibrium, stripped of its outer layers, and sometimes pulled apart into a chain of diverse objects. Remnants of these ‘hit-and-run’ collisions are predicted to be common among remnant planet-forming populations, and thus to be relevant to asteroid formation and meteorite petrogenesis.

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  1. 1

    Agnor, C. & Asphaug, E. Accretion efficiency during planetary collisions. Astrophys. J. 613, L157–L160 (2004)

  2. 2

    Wetherill, G. W. Occurrence of giant impacts during the growth of the terrestrial planets. Science 228, 877–879 (1985)

  3. 3

    Agnor, C. B., Canup, R. M. & Levison, H. F. On the character and consequences of large impacts in the late stage of terrestrial planet formation. Icarus 142, 219–237 (1999)

  4. 4

    Greenberg, R., Hartmann, W. K., Chapman, C. R. & Wacker, J. F. Planetesimals to planets—Numerical simulations of collisional evolution. Icarus 35, 1–26 (1978)

  5. 5

    Weidenschilling, S. J., Spaute, D., Davis, D. R., Marzari, F. & Ohtsuki, K. Accretion evolution of a planetesimals swarm. Icarus 128, 429–455 (1997)

  6. 6

    Kokubo, E. & Ida, S. Formation of protoplanetary systems and diversity of planetary systems. Astrophys. J. 581, 666–680 (2002)

  7. 7

    Pierazzo, E. & Melosh, H. J. Hydrocode modelling of oblique impacts: The fate of the projectile. Meteorit. Planet. Sci. 35, 117–130 (2000)

  8. 8

    Stevenson, D. J. Origin of the moon—The collision hypothesis. Annu. Rev. Earth Planet. Sci. 15, 271–315 (1987)

  9. 9

    Canup, R. M. Dynamics of lunar formation. Annu. Rev. Astron. Astrophys. 42, 441–475 (2004)

  10. 10

    Canup, R. & Asphaug, E. Origin of the Moon in a giant impact near the end of the Earth's formation. Nature 412, 708–712 (2001)

  11. 11

    Asphaug, E. & Benz, W. Size, density, and structure of comet Shoemaker-Levy 9 inferred from the physics of tidal breakup. Icarus 121, 225–248 (1996)

  12. 12

    Jeffreys, H. The relation of cohesion to Roche's limit. Mon. Not. R. Astron. Soc. 107, 260–262 (1947)

  13. 13

    McKinnon, W. B. & Schenk, P. M. Estimates of comet fragment masses from impact crater chains on Callisto and Ganymede. Geophys. Res. Lett. 22, 1829–1832 (1995)

  14. 14

    Boss, A. P., Cameron, A. G. W. & Benz, W. Tidal disruption of inviscid planetesimals. Icarus 92, 165–178 (1991)

  15. 15

    Rettig, T. W., Mumma, M. J., Sobczak, G. J., Hahn, J. M. & DiSanti, M. The nature of Comet Shoemaker-Levy/9 subnuclei from analysis of preimpact Hubble Space Telescope images. J. Geophys. Res. 101, 9271–9281 (1996)

  16. 16

    Abe, Y., Ohtani, E., Okuchi, T., Righter, K. & Drake, M. in Origin of the Earth and Moon (eds Canup, R. & Righter, K.) 413–433 (Univ. Arizona Press, Tucson, 2000)

  17. 17

    Wilson, L. Relationships between pressure, volatile content and ejecta velocity in three types of volcanic explosion. J. Volcanol. Geotherm. Res. 8, 297–313 (1980)

  18. 18

    Jeans, J. H. Problems of Cosmogony and Stellar Dynamics (Cambridge Univ. Press, Cambridge, 1919)

  19. 19

    Benz, W. in The Numerical Modeling of Nonlinear Stellar Pulsations: Problems and Prospects (ed. Buchler, J. R.) 269–288 (Kluwer Academic, Boston, 1990)

  20. 20

    Asphaug, E., Agnor, C. & Williams, Q. Tidal forces as drivers of collisional evolution. Lunar Planet. Sci. Conf. XXXVI abstr. 2393 (2005); (2005)

  21. 21

    Mizuno, H. & Boss, A. P. Tidal disruption of dissipative planetesimals. Icarus 63, 109–133 (1985)

  22. 22

    Sridhar, S. & Tremaine, S. Tidal disruption of viscous bodies. Icarus 95, 86–99 (1992)

  23. 23

    Grady, D. E. & Kipp, M. E. Continuum modeling of explosive fracture in oil shale. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 17, 147–157 (1980)

  24. 24

    Asphaug, E., Ryan, E. & Zuber, M. in Asteroids III Table I (eds Bottke, W. F. Jr, Cellino, A., Paolicchi, P. & Binzel, R. P.) 463–484 (Univ. Arizona Press, Tucson, 2002)

  25. 25

    Burbine, T. H., Meibom, A. & Binzel, R. P. Mantle material in the main belt: Battered to bits? Meteoritics 31, 607–620 (1996)

  26. 26

    Mei, S., Bai, W., Hiraga, T. & Kohlstedt, D. L. Influence of melt on the creep behaviour of olivine-basalt aggregates under hydrous conditions. Earth Planet. Sci. Lett. 201, 491–507 (2002)

  27. 27

    Walzer, U., Hendel, R. & Baumgardner, J. The effects of a variation of the radial viscosity profile on mantle evolution. Tectonophysics 384, 55–90 (2004)

  28. 28

    Tackley, P. J. Convection in Io's asthenosphere: Redistribution of nonuniform tidal heating by mean flows. J. Geophys. Res. 106, 32971–32982 (2001)

  29. 29

    Wetherill, G. W. An alternative model for the formation of the asteroids. Icarus 100, 307–325 (1992)

  30. 30

    Bottke, W. F. et al. The fossilized size distribution of the main asteroid belt. Icarus 175, 111–140 (2005)

  31. 31

    McCoy, T. J. et al. A petrologic and isotopic study of lodranites: Evidence for early formation as partial melt residues from heterogeneous precursors. Geochim. Cosmochim. Acta 61, 623–637 (1997)

  32. 32

    Haack, H., Scott, E. R. D. & Rasmussen, K. L. Thermal and shock history of mesosiderites and their large parent asteroid. Geochim. Cosmochim. Acta 60, 2609–2619 (1996)

  33. 33

    Wilson, L., Keil, K., Browning, L. B., Krot, A. N. & Bourcier, W. Early aqueous alteration, explosive disruption, and re-processing of asteroids. Meteorit. Planet. Sci. 34, 541–557 (1999)

  34. 34

    Keil, K., Stöffler, D., Love, S. G. & Scott, E. R. D. Constraints on the role of impact heating and melting in asteroids. Meteoritics 32, 349–363 (1997)

  35. 35

    Ostro, S. J. et al. Radar observations of asteroid 216 Kleopatra. Science 288, 836–839 (2000)

  36. 36

    Rivkin, A. S., Howell, E. S., Lebofsky, L. A., Clark, B. E. & Britt, D. T. The nature of M-class asteroids from 3-µm observations. Icarus 145, 351–368 (2000)

  37. 37

    Davis, D. R., Chapman, C. R., Greenberg, R. & Weidenschilling, S. J. Collisional history of asteroids: Evidence from Vesta and the Hirayama families. Icarus 62, 30–53 (1985)

  38. 38

    Dohnanyi, J. W. Collisional models of asteroids and their debris. J. Geophys. Res. 74, 2531–2554 (1969)

  39. 39

    Binzel, R. P. et al. Geologic mapping of Vesta from 1994 Hubble Space Telescope images. Icarus 128, 95–103 (1997)

  40. 40

    Greenwood, R. C., Franchi, I. A., Jambon, A. & Buchanan, P. C. Widespread magma oceans on asteroidal bodies in the early Solar System. Nature 435, 916–919 (2005)

  41. 41

    Rivers, M. L. & Carmichael, I. S. E. Ultrasonic studies of silicate melts. J. Geophys. Res. 92, 9247–9270 (1987)

  42. 42

    Ochs, F. A. III & Lange, R. A. The density of hydrous magmatic liquids. Science 283, 1314–1317 (1999)

  43. 43

    Dixon, J. E., Stolper, E. M. & Holloway, J. R. An experimental study of water and carbon dioxide solubilities in mid-ocean ridge basaltic liquids. Part I. Calibration and solubility models. J. Petrol. 36, 1607–1631 (1995)

  44. 44

    Benz, W. & Asphaug, E. Catastrophic disruptions revisited. Icarus 142, 5–20 (1999)

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This research was sponsored by NASA's Planetary Geology and Geophysics Program, “Small Bodies and Planetary Collisions”. We benefited from discussions with a number of colleagues, including W. F. Bottke and R. Canup. We particularly thank D. Stevenson and K. Zahnle for comments on the manuscript.

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Correspondence to Erik Asphaug.

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Further reading

Figure 1: Planetary embryos of comparable diameter are believed to have collided in giant impacts in the late stage of Solar System formation.
Figure 2: A Moon-sized differentiated planet ( M = 0.01 M ) grazing a Mars-sized (0.1 M ) planet, resulting in mass loss, spin-up and global pressure unloading.
Figure 3: Two typical collisions involving differentiated planetary embryos.
Figure 4: The pressures at which degassing initiates.


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