Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993lpi....24.1561y&link_type=abstract
In Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 3: N-Z p 1561-1562 (SEE N94-20636 05-91
Astronomy and Astrophysics
Astronomy
Ellipticity, Friction, Obliqueness, Planetary Cores, Planetary Mantles, Planetary Rotation, Venus (Planet), Astrodynamics, Dynamic Models, Oblate Spheroids, Precession, Tides
Scientific paper
The most striking feature of Venus rotational state is its slow retrograde rotation which is apparently maintained by a balance between solid tidal friction and thermal tidal torques. Solid tides tend to drive the spin toward synchronous rotation while thermal tides drive it away. A balance is achieved at a specific rate because of the inverse frequency dependence of the thermal tide to the semi-diurnal heating. Atmospheric models have been constructed to estimate the thermal tidal torque based on ground heating. The solid friction dissipation factor Q approximately equal to 50 can be deduced assuming rotation has achieved steady state. The most perplexing feature of Venus orientation is its non-zero free obliquity epsilon approximately equal to 1.5 deg relative to its orbit. Although solid tides and perhaps atmospheric tides tend to increase the free obliquity on a time 1/Kt approximately equal to 1 x 108 yr, viscous friction (CMF) at a core-mantle boundary (CMB) resulting from the differential angular orientation Delta-epsilon of the core and mantle spin axes should have damped the free obliquity on a time scale as short as 106 yr. One means of achieving a balance similar to that controlling rotation is to introduce a comparatively large CMB ellipticity ec to reduce Delta-epsilon such that there is a balance between solid-thermal tides and CMF. The balance depends not so much on the potential frequency dependence on the tides as on the quadratic dependence of CMF on Delta-epsilon if the layer is turbulent.
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