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According to this NASA overview, the planet Venus is unique (amongst the major planets), Venus has a slow retrograde axial rotation, taking 243 Earth days to make one rotation (which is longer than its orbital revolution).
What is the current accepted theory as to why (and how) Venus developed this anomalous slow retrograde axial rotation?
There seem to be a few, and none are accepted by the whole scientific community. The main ones:
That final one seems to be the most recent, being proposed by Alexandre Correira and Jacques Laskar in 2001. Their research seems to imply that the conditions on Venus and its distance for the sun make a retrograde spin slightly more likely than a forward one.
There is also the theory which involves Mercury as an ex-moon of Venus, largely based on calculations done by Van Flandern and Harrington (A Dynamic Investigation of the Conjecture that Mercury is an Escaped Satellite of Venus. Icarus 28: 435-40 (Abstract), 1976) and goes as follows (Van Flandern, Missing Planets, Dark Matter, and New Comets, 1999):
As Mercury tidally drifted outward it necessarily produced rotational drag on Venus, and it raised even bigger tides on the Venusian atmosphere causing it to circulate in retrograde direction. After billions of years this might impart retrograde motion on the whole planet.
Tides caused on Venus by Mercury while the latter was still spinning rapidly would have caused great interior heating and outgassing, and probably a great deal of surface upheaval (mountain building), too, causing the very dense atmosphere, the massive release of carbonate in the rocks as CO2 into the atmosphere, and the very high mountains. Mercury is massive enough to have taken much of Venus's spin in the 1st half-billion years after formation and Venus's orbit is close enough to the Sun that complete escape occurs. The interchange of energy between Venus and Mercury would have been enormous, given Mercury's large mass (4 1/2 times more massive than the Moon).
Most of the iron (which eventually produces the magnetic field) in Venus would have been forced up into the crust by an excessively high spin rate, with Mercury getting most of the iron during fissioning, which would explain why Mercury has a stronger magnetic field than Venus. By contrast, the Earth's iron was not forced to the surface, perhaps because the Earth was not as hot and molten as Venus during that phase of its formation.
During its lunar phase Mercury would have acquired a prolate shape (somewhat elongated towards Venus) because of tidal forces.
Both planets would have been melted by tidal heating in the early stages following escape. If this occured before Venus differentiated, it might have caused Mercury's high density and stronger magnetic field. Subsequently, both planets would have melted from mutual tidal heating.
After escape, Mercury acquired greater tilt and eccentricity, and Venus would have lost more of its spin. Its prolate shape would have been reduced after escape but still maintained.
At the point of escape Mercury would have had a period of revolution of about 40 days, and would have retained its spin period, which would also be 40 days since it was locked with Venus. But tides raised by the Sun would slow down its spin to its present 60 days, which gives it a 3-2 spin-revolution ratio (3 spins per 2 revolutions, in other words, its rotational period is 2/3 its period of revolution, which is 88 days), because the next stable configuration for such a body (Mercury mass and diameter and degree of prolateness) is this ratio, so it is a predicted outcome of its having been a moon of Venus.
This model, then, explains all the anomalies of both Venus and Mercury. Musser (2006) says it would require too much time for Venus to lose a moon but does not provide any reference for this, and the possibility has been corroborated by Kumar (1977) and Donnison (1978). This is the abstract from Donnison:
Kumar's (1977) suggestion that the slow rotations of Mercury and Venus are in part due to natural satellites that subsequently escaped is discussed. A more useful criterion for the escape of such satellites than that previously proposed is derived, and it is shown that this distance is sufficiently small for Mercury and Venus to make the escape of satellites a likely possibility.
And this is the abstract from Kumar:
It is suggested that the slow rotations of Mercury and Venus may be connected with the absence of natural satellites around them. If Mercury or Venus possessed a satellite at the time of formation, the tidal evolution would have caused the satellite to recede. At a sufficiently large distance from the planet, the sun's gravitational influence makes the satellite orbit unstable. The natural satellites of Mercury and Venus might have escaped as a consequence of this instability.
They do not, however, specifically say that Mercury was once a moon of Venus.
This is the abstract from Van Flandern and Harrington (gizidda.altervista.org):
The possibility that Mercury might once have been a satellite of Venus, suggested by a number of anomalies, is investigated by a series of numerical computer experiments. Tidal interaction between Mercury and Venus would result in the escape of Mercury into a solar orbit. Only two escape orbits are possible, one exterior and one interior to the Venus orbit. For the interior orbit, subsequent encounters are sufficiently distant to avoid recapture or large perturbations. The perihelion distance of Mercury tends to decrease, while the orientation of perihelion librates for the first few thousand revolutions. If dynamical evolution or nonconservative forces were large enough in the early solar system, the present semimajor axes could have resulted. The theoretical minimum quadrupole moment of the inclined rotating Sun would rotate the orbital planes out of coplanarity. Secular perturbations by the other planets would evolve the eccentricity and inclination of Mercury's orbit through a range of possible configurations, including the present orbit. Thus the conjecture that Mercury is an escaped satellite of Venus remains viable, and is rendered more attractive by our failure to disprove it dynamically.
