Long-term evolution of the spin of Venus I. Theory

Due to planetary perturbations, there exists a large chaotic zone for the spin of the terrestrial planets (Laskar and Robutel, 1993, Nature 361, 608 - 612). The crossing of this zone in the past can lead Venus' spin to its present retrograde configuration for most initial conditions, but through two different processes (Correia and Laskar, 2001, Nature 411, 767-770). Here, we present in full details the dissipative models used for this study of the spin evolution of Venus. The present state of Venus is an equilibrium between gravitational and thermal atmospheric tidal torques (Gold and Soter, 1969, Icarus 11, 356 - 366). We present here a revised model for the thermal atmospheric tides which does not suffer the singularity at synchronous states arising in previous studies. This new model should thus provide a more realistic description of the final stages of Venus' evolution. Assuming that the present spin of Venus is in a final state, we describe the resulting constraints on the various dissipative parameters. We show that the capture in the 1:1 spin orbit resonance during Venus' history is unlikely and becomes impossible when the dense atmosphere is present as this resonance becomes unstable. Our study is presented in a very general setting and should apply to any terrestrial planet with a dense atmosphere. (C) 2003 Elsevier Science (USA). All rights reserved.