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R. Gomes (Observatorio Nacional/Valongo - Brazil), A. Morbidelli, K. Tsiganis (Obsevatoire de la Cote d'Azur), H. Levison (Southwest Research Institute)
The cratering record on the Moon has been interpreted as an intense spike in the impact rate around 3.8 to 4.0 Gy ago (Tera et al. 1974, Earth Planet. Sci. Lett. 22, 1) rather than to an impact rate that has been monotonically decreasing since the formation of the Solar System. Here we propose that a period giant planet migration at this time triggered the delivery of icy bodies into the inner Solar System, thus producing the Lunar Heavy Bombardment (LHB). In particular, we propose that the LHB occured when Jupiter and Saturn crossed their mutual 1:2 mean motion resonance, which destabilized the orbits of Uranus and Neptune and scattered them into a trans-Neptunian belt, thereby releasing a population of small objects. The precise time for Jupiter and Saturn to cross their 1:2 resonance is a function of the initial semimajor axes of Jupiter and Saturn as well as the mass and radial extent of the disk. However, we have one case where Jupiter and Saturn crossed their 1:2 resonance at 880 million years. Initial heliocentric distances for the planets were assumed as 5.45, 8.18, 11.5 and 14.2 AU (see Tsiganis et al. 2004, this bulletin, for justification). The planetesimal disk had 35 M\oplus, with a radial extent from 15.5 to 34 AU. All our runs produced a very marked enhancement in the delivery of bodies to the inner Solar System, at the time when Jupiter and Saturn entered the 1:2 resonance. During a period of less than 100 My after the resonance crossing, 7% of the original disk planetesimals reached 1~AU. This corresponds to a mass of about 2.5 M\oplus. About 4 \times 10-7 of this mass (around 6 \times 1021 g) would thus have striked the Moon (Levison et al. 2001, Icarus 151, 286-306). This is well in accordance with the mass of the projectiles hitting the Moon during the LHB.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.