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Origin of the Cataclysmic Late Heavy
Bombardment Period of the Terrestrial
Planets
Gomes, R., Levison, H.F., Tsiganis, K., & Morbidelli, A.
Nature
(2005)
Nathan Kaib
A Cataclysmic LHB
• Preferred
over a montonic decline in impact rates
Evidence for a Cataclysmic LHB
• No lunar melt-rocks older
than 3.92 Gyrs
• 15 largest lunar basins are
3.9 - 4.0 Gyrs
• Upper lunar crust is not
enriched in siderophiles
• 4.4 Gyr Earth zircons
formed with liquid H2O
Previous Models of LHB Origin
• Breakup of large asteroid
 No observed asteroid family
• Comet shower
 Too short, requires huge Oort Cloud
• Fifth terrestrial planet
 Requires 15° inclination, Mars collision
Motivation for New Model
• Planet formation theory predicts gas giants
formed on circular coplanar orbits
• Inclinations and eccentricities deviate
significantly from zero
• We know some gas giant migration has
taken place
LHB Caused by Gas Giant
Migration
1:2 MMR
• Planetesimal scattering causes spreading of gas giant orbits
• 1:2 resonance-crossing between Jupiter and Saturn results in:
-Fast, chaotic outward migration of Uranus and Neptune
-Intense scattering of outer debris
~35 AU
Numerical Simulations
• Gas giants spaced between
5.5 and 17 AU
• Saturn just inside 1:2
MMR
• Initial orbits are circular
• Planets surrounded by 40
ME debris disk out to 35 AU
Results
t = 100 Myr
t = 879 Myr
• 9 x 1021 g are scattered
into the moon in 30 Myrs
t = 882 Myr
t = 1.1 Gyr
• Consistent with actual
LHB estimate of 6 x 1021 g
Timing of LHB
• The delay of the resonance crossing
depends on the inner edge of the debris disk
What About Asteroids?
• Resonance-crossing
causes secular
resonances to sweep
through asteroid belt
• Could produce equal
contribution over 150
Myr
Planetary Orbits
• Planets survive 2/3 of runs
• Averaged orbital elements
match well with real values
• Almost always a close
encounter between gas giants
• Final orbital configuration
sensitive to Saturn-Uranus
separation
Other Issues
Moons
-Repeated a subset of planetary encounters with
moon systems
-All satellite systems survived 50% of the time
Trojans
-Trojan populations of Neptune and Uranus would
not have survived chaotic phase
-New Trojans can be captured during chaotic
phase
-Jupiter’s Trojan population is reproduced well
Summary
• Orbital migration of gas giants almost
certainly occurred early in solar system
history
• Resonance-crossing between Jupiter and
Saturn scatters both comets and asteroids
• This can account for a delayed LHB
• Gas giant orbits and Trojan population are
replicated