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THE LATE HEAVY BOMBARDMENT AND THE FORMATION OF
THE SOLAR SYSTEM
A. Morbidelli, OCA, Nice
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THE LATE HEAVY BOMBARDMENT
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• A few facts on the LHB
• Cataclysmic event triggered 3,9 Gy ago, 600My
  after terrestrial planet formation
• Global event traces found on Mercury, Venus,
  Earth, Mars, Vesta., possibly on giant planets
  satellites
• 20.000x 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
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• 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.

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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
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R. Gomes, H.F. Levison, K. Tsiganis, A.
Morbidelli  2005.  Nature, 435,466   
12 MMR crossing
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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    
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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
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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 12 MMR?
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In our simulations we see that, when Jupiter and
Saturn cross their mutual 12 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.
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A second confirmation .
The bulk density of the binary Trojan Patroclus
is 0,8g/cm3, 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)
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Sculpting the Kuiper belt in the LHB scenario
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Sculpting the Kuiper belt in the LHB scenario
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Sculpting the Kuiper belt in the LHB scenario
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CONCLUSIONS (I)

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

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IMPLICATIONS ON THE GAS-DISK PHASE

• If all this is right, then at the end of the gas
  disk phase
• The system of the 4 giants was very compact
  (5.5-17 AUs)
• Jupiter and Saturn were not in the 12 MMR the
  ratio of their orbital periods was smaller than
  2. Realistic?
• Orbital eccentricities and inclinations
  were very small
• A massive disk of planetesimals (35 ME)
  extended from a few AUs beyond
  the 4th
  planet to 30-35 AU.
• This argue that gas-driven
  migration was never
  substantial.
  Is it possible?

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Masset and Snellgrove, 2001
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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 2D grid, simulates the 1D
viscous evolution of the disk. The 1D disk and
the 2D 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.
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If two giant planets are close to each other,
their Type II migration is slowed down
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.or stopped
117 res
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or reversed (Masset and Snellgrove 2001)
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ANSWERS TO GAS-PHASE QUESTIONS

• It is possible that Jupiter and Saturn spent the
  lifetime of the disk in a stable configuration
  with PS/PJlt2
• This configuration is required to avoid the Type
  II migration of the two giants towards the Sun
• This argues that in the giant planets region the
  disk was relatively inviscid and cool

Crida, Morbidelli, Masset, in prep.
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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? Jupiters
eccentricity seems to have a role
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New simulations of terrestrial planet formation
and asteroid belt sculpting. OBrien, Morbidelli
and Levison (2006)
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Terrestrial planet formation Final
distributions, compared to the actual Solar
System (OBrien, Morbidelli and Levison 2006,
Icarus in press.)
Synthetic planets are slightly more
eccentric/inclined than the real ones
Semi-major axis (AU)
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The typical formation timescale is somewhat
longer than indicated by the Hf-W chronometer
(40 Ma)
Median accretion time
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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.
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Origin of material incorporated into the planets
ObML06
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 gt 2.5 AU
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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

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A new paradigm the 3 main ages of the Solar
System

• Planet accretion age
• Formation of giant planets
• Formation of terrestrial planets
• First excitation/depletion of the asteroid belt
• A quiet age of 600 My
• Asteroid belt 20x more massive than current
• Trans-Neptunian massive disk (50 -gt 35 ME)
• The current age

• LATE HEAVY BOMBARDMENT
• Planet migration
• Final sculpting of asteroid/Kuiper belts
• Capture of Troyans and Irregular Satellites