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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, ~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
• 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 transNeptunian 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
0,8g/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)
Sculpting the Kuiper belt in the LHB scenario
Sculpting the Kuiper belt in the LHB scenario
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 (LevisonMorbidelli-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
4th planet to 30-35 AU.
5) This argue that gas-driven
migration was never substantial.
Is it possible?
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 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.
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
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 > 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 ~20x 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