HR: 11:35h
AN: P51F-06    [PDF]
TI: Constraints on the Galilean protosatellite disk from Jupiter's obliquity
AU: * Ward, W R
EM: ward@boulder.swri.edu
AF: Southwest Research Institute, 1050 Walnut Streeet Suite 400, Boulder, CO 80302
AB: The obliquity of Jupiter is only 3 degrees. Although a small obliquity seems consistent with its gas accretion phase, there are processes that could have altered it after its formation. The current spin axis precession period of Jupiter is $\sim 4.5 $\langle$ 10^5$ years due mostly to the solar torque exerted on the Galilean satellites ({\it e.g.,} Ward 1975; Harris and Ward 1982; Tremaine 1991). However, this would have been up to $\sim O(10^2)$ shorter if a minimum mass pre-satellite disk had been present. If this disk were subsequently photoevaporated after the solar nebula itself was dissipated, Jupiter's precession frequency would have drifted through one of the mutual orbital precession frequencies of Jupiter and Saturn, {\it i.e.,} the so-called ${\nu}_{16}$ that describes the precession of their orbital nodes with a period of $P_{16} \sim 5 $\langle$ 10^4$ years. An adiabatic passage could generate an obliquity of 25.6 degrees ({\it e.g.,} Henrard and Murigande 1987). This could be avoided if passage is fast enough to be non-adiabatic, in which case the final obliquity is rate dependent ({\it i.e.,} Ward {\it et al.} 1976). If $\alpha_S$ denotes the spin axis precession parameter, which is a function of the circumplanetary disk and satellite masses in addition to the Jovian oblateness ({\it e.g.,} Ward 1975), and we define $\Omega_{\it pole} \{ \Delta_{S} / \Delta_{S}$ and calculate its value to yield an obliquity comparable to Jupiter's current obliquity, we obtain $O(10^5)$ years. But since the change in $\alpha_S$ would be due primarily to dissipation of the protosatellite disk, we conclude that a disk life much longer than this is not consistent with Jupiter's low obliquity spin state. Alternatively, the pre-satellite disk may have been of insufficient mass to cause passage through the Jupiter-Saturn resonance ({\it e.g.,} Canup \& Ward 2002).
DE: 5744 Orbital and rotational dynamics
DE: 5749 Origin and evolution
DE: 6218 Jovian satellites
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting