HR: 14:55h
AN: G53C-06 INVITED     [Abstracts]
TI: Non-chaotic Obliquity Variations of Mars
AU: * Bills, B G
EM: bbills@ucsd.edu
AF: NASA GSFC, Planetary Geodynamics Laboratory, Greenbelt, MD 20771 United States
AU: * Bills, B G
EM: bbills@ucsd.edu
AF: Scripps Institution of Oceanography, IGPP, La Jolla, CA 92093 United States
AB: In the absence of energy dissipation, obliquity variations of Mars would likely be chaotic. However, relatively small amounts of dissipation suffice to suppress the chaotic variations. Tidal dissipation within Mars, as constrained by the observed evolution of the orbit of Phobos, appears sufficient to regularize the obliquity variations. If Mars has a fluid core, viscous core-mantle coupling would also tend to damp the obliquity variations. An adequate rate of dissipative damping is a necessary, but not sufficient, condition to establish that the obliquity variations are fully damped. Evidence in support of this stronger conclusion comes from two further observations. First is that the current orientation of the spin pole of Mars (in both obliquity and azimuth) is close to that predicted for a fully damped spin state. Second is that numerical integrations of the equations of motion for the spin pole, over an interval of a few million years, when performed both with and without dissipation, differ only slightly, suggesting that the spin pole has been driven to the fully damped state. It thus appears that the free obliquity of Mars is small, and that dissipation plays an important role in the rotational dynamics of this body, not unlike the situation for Mercury and Venus.
DE: 5450 Orbital and rotational dynamics (1221)
SC: Geodesy [G]
MN: Fall Meeting 2005