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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