Dynamical constraints on the nature of the Late Heavy Bombardment and models of its origin

1. Dynamical constraints on the nature of the Late Heavy Bombardment and models of its origin • Morbidelli • Observatoire de la Cote d’Azur, Nice, France

2. OUTLINE • LHB: a long-lasting bombardment or a cataclysmic spike? What dynamicists have to say • Models of LHB origin • The `Nice’ model • A late formation of Uranus and Neptune • Instability of a 5th putative terrestrial planet • Needed tie-break observations

3. Nature of the LHB: a slowly declining bombardment or a cataclysmic spike? Cataclysmic LHB (Tera, Ryder, Kring, Cohen, Koeberl..) ? Slowly fading LHB (Neukum, Hartman..)

4. DECAY RATE OF POST-PLANET FORMATION POPULATION • Nectaris, Serenitatis, Imbrium and Orientale Basins: • If formed between 3.90 < t < 3.82-3.75 Gy, the total mass of the Post planet-formation population had to be ~5-8 MEARTH • If formed between 4.12 < t < 3.82-3.75 Gy, the total mass had to be at least 0.7 MEARTH Collisional erosion increases both values by a factor of 20! Declining Bombardment Model is Unrealistic for LHB! From Bottke et al., Icarus, 2006.

5. ORIGIN OF A CATACLYSMIC LHB: The `Nice Model’ (Gomes et al., Nature, 2005) J S U N

6. R. Gomes, H.F. Levison, K. Tsiganis, A. Morbidelli  2005.  Nature, 435,466  1:2 MMR crossing

7. We explain a late heavy bombardment due to comets and asteroids, which satisfies the constraints provided by lunar crater data. R. Gomes et al.   2005.  Nature, 435,466 1:2 MMR crossing

8. We also explain: I: The current orbits of the giant planets: their semi-major axes, eccentricities and inclinations, starting from circular orbits Tsiganis et al., 2005. Nature, 435, 459

9. II: The origin of the Jovian Trojans and their orbital distribution. They would be planetesimals from the tran-Neptunian disk, captured around L4 and L5 during the 1:2 MMR crossing between Jupiter and Saturn. A.Morbidelli, H.Levison, K.Tsiganis, R.Gomes  2005. Nature, 435, 462.

10. III: The existence, the orbital structure and the small mass of the Kuiper belt (Levison et al. 2007) simulated observed

11. IV: The origin of the irregular satellites of Saturn, Uranus and Neptune (Nesvorny et al., 2007, AJ.) Uranus Neptune Saturn

12. Main weakness of the `Nice’ model: The arbitrary character of the planets’ initial conditions (particularly the positioning of Saturn just next to the 1:2 MMR with Jupiter) We are currently working on this (come see my contributed talk 60.04), using as initial conditions the output of hydro dynamical simulations of the evolution of the planets in the gas disk. Main weakness of the `Nice’ model: The arbitrary character of the initial planetary positions (in particular assuming Saturn just close to the 1:2 MMR with Jupite

13. Constraints from the Lunar Cratering Record The size distribution of lunar craters is like that of the main belt. Asteroids dominated the LHB on the Moon (consistent with impact clast analysis: Kring et al.; Taegle et al.) and were chased from the main belt by a size independent process (Strom et al., 2005). Resonance sweeping due to giant planet migration could have been such a process.

14. Models alternative to the `Nice’ one: I) A late formation of Uranus and Neptune (Levison et al., 2001; Strom et al., 2005) Does not work! When having a smaller mass, Uranus and Neptune would have migrated rapidly towards the edge of the planetesimal disk. The bombardment would have occurred too early and/or the planets would have stopped on wrong positions (Levison et al., PPV book) A 5 ME Neptune in a 40 ME planetesimal disk

15. Models alternative to the `Nice’ one: II) Dynamical instability and elimination of a 5th terrestrial planet (Chambers, 2007) • 18/96 simulations produce the elimination of a 5th terrestrial planet of mass intermediate to those of the Moon and Mars, after > 200 My and without collision with another planet • What about: • Likelihood of existence of such a planet? • Final structure of the asteroid belt?

16. A key issue: did comets contributed to the LHB? If NO: a model like Chambers’ has to be preferred if YES: giant planet migration is needed (e.g. the `Nice’ model) The noble gasses budget of the Earth’s atmosphere relative to the mantle seems to require a cometary bombardment (although not as massive as that predicted in the `Nice’ model; Marty and Meibom, 2007)

17. A key issue: did comets contributed to the LHB? NO: a model like Chambers’ has to be preferred YES: giant planet migration is needed (e.g. the `Nice’ model) Iapetus suffered a Heavy Bombardment (> 100x the current bombardment integrated over the age of the solar system: Zahnle et al.) Was this bombardment late? It seems so. Ejecta blankets from basins overlap the equatorial ridgewhich should have formed at 200-800 My(Castillo et al., Icarus, 2007). Moreover, the satellite crust could not have retained basins before 100 My.

18. CONCLUSIONS • Conversely to what was previously thought, it seems more likely from a dynamical standpoint to have a cataclysmic bombardment than a long-lasting, slowly decaying one. • Although asteroids most likely dominated the LHB, two classes of models are possible • Those invoking a late migration of the giant planets (e.g. the `Nice’ model) • Those invoking an instability in the terrestrial planet system (e.g. Chambers, 2007) • Key issue: the role of comets in the LHB