THE LATE HEAVY BOMBARDMENT AND THE FORMATION OF

THE LATE HEAVY BOMBARDMENT AND THE FORMATION OF THE SOLAR SYSTEM A. Morbidelli, OCA, Nice

THE LATE HEAVY BOMBARDMENT

A few facts on the LHB: • Cataclysmic event triggered 3, 9 Gy ago, ~600 My after terrestrial planet formation • Global event: traces found on Mercury, Venus, Earth, Mars, Vesta…. , possibly on giant planets satellites • 20. 000 x the current bombardment rate: 1 km object impacting the Earth every 20 years! • Duration: 50 -150 My The LHB challanges our naive view of a Solar System gradually evolving from the primordial chaos to the current order

• A cataclysmic bombardment is possible only if a reservoir of small bodies, which remained stable for ~600 My, suddenly became `nuts’. • This is possible only if there was a sudden change in the orbital structure of the giant planets.

LATE PLANET INSTABILITIES Gomes, Levison, Tsiganis, Morbidelli, (2005) In all previous simulations, migration started immediately because planetesimals were placed in very unstable regions. However, at the end of the gas-disk phase, planetesimals should be only where the lifetime is longer than the nebula dissipation time Lifetime of planetesimals Planet positions

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

Two strengths of our model: I: We explain the current orbits of the giant planets: their semi -major axes, eccentricities and inclinations, starting from circular orbits K. Tsiganis, R. Gomes, A. Morbidelli, H. F. Levison 2005. Nature, 435, 459

II: 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

A first confirmation… The size distribution of lunar craters shows that the LHB was caused by the migration of the giant planets (Storm et al. , 2005). This migration could not be forced by the asteroids themselves (not massif enough). Thus it had to be forced by a massive trans. Neptunian disk. Was the migration really triggered by Jupiter and Saturn crossing their mutual 1: 2 MMR?

In our simulations we see that, when Jupiter and Saturn cross their mutual 1: 2 MMR, a fraction of the distant disk planetesimals is captured as Jovian Trojans. This is the first theoretical reconstruction of the observed orbital distribution of Trojans, which strengthens our triggering scenario. A. Morbidelli, H. Levison, K. Tsiganis, R. Gomes 2005. Nature, 435, 462.

A second confirmation …. The bulk density of the binary Trojan Patroclus is 0, 8 g/cm 3, smaller than those of any asteroid measured so far, but identical to those of binary Kuiper belt objects (Marchis et al. , Nature, 439, 565, 2006)

Sculpting the Kuiper belt in the LHB scenario

CONCLUSIONS (I) We have developped a model, based on planetesimal driven migration that: 1) Reproduces the current orbital architecture of the 4 giant planets 2) Explains the LHB: cataclysmic nature, mass delivered to the Moon, duration 3) Explains the origin of Jupiter Trojans: mass and orbital distribution 4) Is consistent with what we see in the Kuiper belt (Levison. Morbidelli-Gomes-Tsiganis in preparation)

IMPLICATIONS ON THE GAS-DISK PHASE If all this is right, then at the end of the gas disk phase: 1) The system of the 4 giants was very compact (5. 5 -17 AUs) 2) Jupiter and Saturn were not in the 1: 2 MMR; the ratio of their orbital periods was smaller than 2. Realistic? 3) Orbital eccentricities and inclinations were very small 4) A massive disk of planetesimals (35 ME) extended from a few AUs beyond the 4 th planet to 30 -35 AU. 5) This argue that gas-driven migration was never substantial. it possible? Is

Masset and Snellgrove, 2001

We have re-done the simulations using a new code The code (Crida, Masset and Morbidelli, in preparation), inside and outside of the boundaries of the 2 D grid, simulates the 1 D viscous evolution of the disk. The 1 D disk and the 2 D disk communicate with each other through their mutual boundaries ESSENTIAL TOOL FOR SIMULATING THE CORRECT GLOBAL EVOLUTION OF THE DISK AND GETTING TYPE II MIGRATION RIGHT.

If two giant planets are close to each other, their Type II migration is slowed down

…. or stopped 11: 7 res

… or reversed (Masset and Snellgrove 2001)

ANSWERS TO GAS-PHASE QUESTIONS 1) It is possible that Jupiter and Saturn spent the lifetime of the disk in a stable configuration with PS/PJ<2 2) This configuration is required to avoid the Type II migration of the two giants towards the Sun 3) This argues that in the giant planets region the disk was relatively inviscid and cool Crida, Morbidelli, Masset, in prep.

If Jupiter was for long time on a circular orbit, which consequences could this have on the standard model for terrestrial planet accretion and asteroid belt sculpting? Jupiter’s eccentricity seems to have a role

New simulations of terrestrial planet formation and asteroid belt sculpting. O’Brien, Morbidelli and Levison (2006)

Terrestrial planet formation: Final distributions, compared to the actual Solar System (O’Brien, Morbidelli and Levison 2006, Icarus in press. ) Synthetic planets are slightly more eccentric/inclined than the real ones Semi-major axis (AU)

The typical formation timescale is somewhat longer than indicated by the Hf-W chronometer (40 Ma) Median accretion time

The final eccentricities and inclinations and formation timescale will probably turn out to be smaller in future simulations taking into account a larger number of smaller planetesimals. On the contrary, in simulations with an initial eccentric Jupiter the final terrestrial planets are already dynamically colder than the real ones, and form faster than indicated by the Hf-W chronometer.

Origin of material incorporated into the planets Ob. ML 06 Circular JS case 15% of planetary mass accreted from beyond 2. 5 AU, 75% of which from embryos Eccentric JS case No material accreted from > 2. 5 AU

ANSWER TO TERRESTRIAL PLANETS QUESTION • A Jupiter on a circular orbit does not destroy the basic scenario of formation of the terrestrial planets and clearing of the asteroid belt. • Simulations assuming a circular Jupiter are probably more consistent with the terrestrial planets properties than those assuming an eccentric Jupiter

A new paradigm: the 3 main ages of the Solar System 1. Planet accretion age • Formation of giant planets • Formation of terrestrial planets • First excitation/depletion of the asteroid belt 2. A quiet age of ~600 My • Asteroid belt ~20 x more massive than current • Trans-Neptunian massive disk (50 -> 35 ME) LATE HEAVY BOMBARDMENT • Planet migration • Final sculpting of asteroid/Kuiper belts • Capture of Troyans and Irregular Satellites 3. The current age