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A.C.M. Correia, J. Laskar (ASD/IMC-CNRS)
In 1962, using radar measurements, the slow retrograde rotation of Venus was discovered (see Carpenter, 1970). Since, the understanding of this particular state becomes a challenge as many uncertainties remain in the dissipative models of Venus' rotation. Various hypothesis were proposed for its evolution, aiming to search wether Venus was born with a direct or retrograde rotation. The most favored scenario assumes that its axis was actually tilted down during its past evolution as a result of core mantle friction and atmospheric tides (Lago and Cazenave, 1979, Dobrovolski, 1980, Shen and Zhang, 1989, McCue and Dormand, 1993, Yoder, 1995, 1997). Nevertheless, this requires high values of the initial obliquity, and it was proposed that Venus was strongly hit by massive bodies which would have tilted it significantly or started its rotation backward (Dones and Tremaine, 1993). Even while considering the chaotic evolution of Venus's obliquity (Laskar and Robutel, 1993), the published scenarios still have some difficulties to tilt Venus axis towards its present position (Yoder,1997). In the present work we show that due to the dissipative effects, there are only 4 possible final states for Venus' rotation, and only 3 of them are really reachable. When the planetary perturbations are added, most of the initial conditions lead to the two states corresponding to the present configuration of Venus, one with period -243.02 days and nearly 0\circ obliquity, and the other with opposite period and nearly 180\circ obliquity.We thus demonstrate that a large impact is not necessary to have a satisfying scenario for the reverse rotation of Venus.
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