35th Meeting of the AAS Division on Dynamical Astronomy, April 2004
Session 7 Posters II
, Thursday, April 22, 2004, 7:00-8:30pm,

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[7.02] Moon's Orbital Inclination Pumped up by a Passing Protoplanet with Mars Size

K. Atobe, J. Kominami, S. Ida (Tokyo Institute of Technology)

We have investigated the dynamical evolution of the Earth-Moon system caused by its encounter with a fly-by protoplanet during the giant impact stage of terrestrial planet formation, in order to account for the discrepancy between the Moon's orbital plane and the Earth's equator. Tracing the Moon's orbital evolution due to tidal interaction with the Earth back in time revealed that the Moon's orbital inclination relative to the Earth's equator has been larger than 10 degree throughout its history (e.g., Goldreich 1966). If the Moon have formed from a debris disk created by a giant impact, the orbital inclination should be within a few degrees (e.g., Ida et al. 1997). The origin of the large inclination was addressed by Touma and Wisdom (1998) and Ward and Canup (2000).

Here we propose an alternative model. Even after the Moon was formed, protoplanets with Mars size or more can approach the Earth within its Hill radius, until their orbital eccentricities are damped by disk-planet interaction or collisions, which occurs on timescales 107 - 108 years. Through analytical arguments and numerical calculations, we found that fly-by of a Mars-sized protoplanet at 30 - 50 earth radius changes the orbital inclination of the Moon at 15 earth radius by about 0.1 radian, while the obliquity and orbital inclination of Earth are hardly changed, which accounts for the large inclination relative to Earth's equator. We also found that such a fly-by encounter is likely to occur on timescales 107 - 108 years after the lunar-forming impact.

The author(s) of this abstract have provided an email address for comments about the abstract: atobe@geo.titech.ac.jp

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Bulletin of the American Astronomical Society, 36 #2
© 2004. The American Astronomical Soceity.