INTRODUCTION
Venus is spinning slowly backwards or “retrograde”. This condition was discovered in 1962 using radar measurement (Smith 1963) A total true rotation about its axis would take 243 days whilst the orbit of the planet takes only 225 days. This combination results in a solar day length of ~117 Earth days. This unusual situation has lead to numerous studies investigating the spinning of Venus.
Possible scenarios include an original prograde spinning of the planet followed by some dragging mechanism as a result of spinning perturbations and atmospheric tidal drag. This could have caused the spinning to slow down and then reverse or alternately the planet to flip over, also effectively causing the spinning to reverse. (Goldreich & Peale 1970, Yoder 1995, Neron de Surgy & Laskar 1997 Correia & Laskar 2003-referred to as the CL03 model). However these scenarios lead to a rather small exchange of angular momentum. Consequently the initial spinning rate of Venus could not have exceeded 3,5 days per revolution (CL03) to reach the current state within the time constrain of the lifespan of the solar system.
Alternately the planet could have been born retrograde, assuming that the spinning direction would be at random, following a multiple impact scenario of planetesimals (Hartmann & Vail1986, Lissauer & Safronov 1991, Dones & Tremaine 1993, Lissauer et al 1995,2000). These hypothesis seem not to be completely matching the earlier Solar Nebula Hypothesis model, slowly forming planets, which spin prograde as a consequence of the conservation of momentum. This would have given Venus an initial prograde spinning rate of some 13.5 hours empirically (McDonald 1964). The discovery of protoplanetary disks around other stars (Marcy & Butler 2000) tends to weaken alternate scenarios in favour of a possible an hypothesis that can account for such an condition.
Here we propose a mechanism, that would be compatible with such an initial spinning condition, considering Venus initially having similar orbit and spinning characteristics like Earth, however without moon. It is actually a (major) modification of the CL03 model, assuming that the spinning axis of the solid inner core of Venus would not follow the spinning perturbations of the planets mantle.
A secondary consequence of such a scenario would be a high braking effect, dissipating the spinning energy into heat. We propose that this heat can account for the present enigmatic features of the planet
THE SPIN OF VENUS AND THE CORE.
Orbital Variations
The motions of the planets in the Solar system are chaotic. Evidence for chaotic behavior of the orbital motions was discovered for Pluto first of the planets. After Pluto (Sussman and Wisdom, 1988), the evidence that the motion of the whole Solar system is chaotic, was established with the averaged equations of motion (Laskar, 1989, 1990), and confirmed later on by direct numerical integration (Sussman and Wisdom, 1992). Not only orbits but with it’s gyroscopic properties, the planet spin axis also perform complicated precession and nutation cycles. The following perturbations cause the spin axis of the planet to change:
A - Precession of the equinoxes, caused by a torque force due to a difference of gravitational forces of the sun (and moon), causing the spin axis to follow a cone.
B - Precession of the perihelion which slowly changes its position in the orbit as a result of the gravity pull of other planets.
C – Inclination cycle, motored by the gravitational interaction between the planets in their different inclinations. (Muller, McDonald 1995) (can’t find the cause so far – due to gravitational interaction with the other stars in the galaxy?)
D – Resulting from interactions between the previous cycles the obliquity of the spin axis also cycles (Ward 1992) . The Earth’s obliquity varies between 22.1 and 24.5 degrees in a complicated cycle that averages about 41,000 years. Mars obliquity is assumed to cycle between about 15 degrees and 35 degrees over a 124,000-year cycle. At present Venus has a stable obliquity of about 3 degrees
Chaotic zone.
When the precession frequencies of the spin axis of the terrestrial planet coincide with the frequency of variations in its orbital inclination, spin-orbit resonance occurs where the cycles amplify each other. This causes big and erratic or chaotic variations in the obliquity of the terrestrial planets. Ward** Laskar and Robutel (1993). The area with the matching frequencies, that lead to those large excursions is called the chaotic zone.
Considering the Earth, the precession frequency of the axis (26,000 years) is far from the main orbital secular frequencies of the Earth is nowhere near a chaotic zone. In the absence of the moon, the situation would be very different. The precession frequency would be much lower, bringing it to comparable values with the obliquity cycle, and multiple resonances could then occur between the precession of the axis and the precession of the orbital plane This is also what may happen in the future as the moon continues to recede from the Earth Ward 1992** , Neron de Surgy and Laskar, 1997). This situation is very similar for all inner planets. They may have been in a chaotic zone at any specific time in the past. A prograde spinning Venus with hypothetical comparable parameters to Earth and Mars a chaotic zone is very well possible. Numerical integrations have suggested that even a 180 degrees obliquity, or a complete flip, was achievable (Laskar and Robutel, 1993, Laskar 1995, Yoder1997).
The Core
Goldreich and Peale (1970) proposed that friction at a core-mantle boundary should drive the spin pole to a fully dampened obliquity state which ends with retrograde rotation So after a chaotic obliquity behavior Venus ended in a current stable state, also CL03 works this out under the assumption that the interior of Venus resembles that of Earth and remained constant. Moreover it’s observed that:
“the core and the mantle do not have the same dynamical ellipticity because of their different shapes and densities. Since the precession torques exerted by the Sun on Venus' core and mantle are proportional to their dynamical ellipticity, the two parts tend to precess differently around an axis perpendicular to the orbital plane.”
Nevertheless it’s assumed that the hydrodynamic and magnetic core mantle coupling is adequate to preserve or restore spin axis alignment of core and mantle. At this point we deviate from the hypothesis using the following observations/assumptions:
The interior of Venus is not constant but depending on thermal properties. Internal temperatures and pressures determine the molecular viscosity of the fluid outer core and the size of the inner core. The inner core tends to grow due to cooling (Vanyo ****) transferring considerable angular momentum from the fluid outer core. Eddy viscosity in the outer core is most likely a major player in the stability of the inner core spin axis but is variable with thermal gradient and subject to chaotic flow (Glatzmaier 1997).
As has been showed for Earth, the inner core spin axis lags the mantle spin slightly axis under the current semi-stable precession and obliquity conditions (Vanyo 2004) However, considering the variable properties of the interior of the planet, it cannot be assumed that core mantle coupling remains adequate enough throughout all unstable conditions and a spin axis break out of the inner core is not unlikely.