Diapositive 1

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

• Morbidelli
• Observatoire de la Cote dAzur, Nice, France

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• 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

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Nature of the LHB a slowly declining bombardment
or a cataclysmic spike?
Cataclysmic LHB (Tera, Ryder, Kring, Cohen,
Koeberl..)
?
Slowly fading LHB (Neukum, Hartman..)
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DECAY RATE OF POST-PLANET FORMATION POPULATION

• Nectaris, Serenitatis, Imbrium and Orientale
  Basins
• If formed between 3.90 lt t lt 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 lt t lt 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.
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ORIGIN OF A CATACLYSMIC LHB The Nice Model
(Gomes et al., Nature, 2005)
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R. Gomes, H.F. Levison, K. Tsiganis, A.
Morbidelli  2005.  Nature, 435,466   
12 MMR crossing
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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
12 MMR crossing
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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    
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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 12 MMR crossing between
Jupiter and Saturn. A.Morbidelli,
H.Levison, K.Tsiganis, R.Gomes  2005. Nature,
435, 462.
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III The existence, the orbital structure and the
small mass of the Kuiper belt (Levison et al.
2007)
simulated
observed
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IV The origin of the irregular satellites of
Saturn, Uranus and Neptune (Nesvorny et al.,
2007, AJ.)
Uranus Neptune Saturn
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Main weakness of the Nice model The arbitrary
character of the planets initial conditions
(particularly the positioning of Saturn just next
to the 12 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 12
MMR with Jupite
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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.
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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
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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 gt 200 My and
  without collision with another planet
• What about
• Likelihood of existence of such a planet?
• Final structure of the asteroid belt?

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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 Earths 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)
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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 (gt 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 ridge
which should have formed at 200-800 My
(Castillo et al., Icarus, 2007). Moreover, the
satellite crust could not have retained basins
before 100 My.
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• 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