2. Tidal effects
The unusual Venus rotation results from an equilibrium between tidal friction effects resulting from body tides, and thermal atmospheric tide (Gold and Soter, 1969). In the case of the body tides, the Sun deforms planet and creates a bulge in the direction of the Sun (and opposite direction), but if the planet rotates more rapidly on itself than its revolution around the Sun, this bulge is shifted from the subsolar point, because the nonelasticity of planet does not return immediately in the direction of the Sun. There results an offset (delta) (fig. 2) between the bulge and the direction of the Sun, which induces a restoring torque on the rotation of the planet having for effect to slow it down. This effect will last as the rotation rate of planet is higher than its revolution around the Sun and will thus tend to bring planet in a synchronous state with its orbital period (as in the Earth-Moon system). If this effect had been the only present, the prediction of Schiaparelli would have been verified.
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| Figure 2: Effect of solid tides exerted by the Sun on Venus. |
Actually, in the case of a planet with a thick atmosphere like Venus, a second tidal effect is to be taken into account: the effect of thermal atmospheric tides (fig. 3). In this case, the Sun heats the atmosphere at the subsolar point, and to balance the pressures, then occurs a redistribution of the mass of the atmosphere in the more distant areas, with an important component perpendicular to the direction of the Sun. Here still, if the rotation of planet is faster than its revolution around the Sun, there will be a shift (delta) between the orientation of this atmospheric tidal bulge and the perpendicular with the direction of the Sun. If delta < 90°, this bulge induces an accelerating torque for the rotation of planet. When this torque is sufficiently important, as it is the case for Venus, the synchronous equilibrium becomes unstable, but appear two new equilibrium configurations, one prograde, and the other one retrograde (Correia and Laskar, 2001).
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| Figure 3: Tidal thermal atmospheric friction of the Sun on Venus. |