Was Mercury a moon of Venus?

Answer 1

There is a theory by Thomas Van Flandern (1940-2009) that had little support from mainstream astronomy. The model for Mercury, which has phases and a western evening and eastern deep night version like Venus, both planets being inferior (having orbits between Earth and the Sun), as an ex-moon of Venus is largely based on calculations done by Van Flandern and Harrington (1976) and goes as follows (Van Flandern, 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.

* Venus' retrograde rotation is alternately explained by having been tilted upside down by a collision (as probably happened to Uranus, which is extremely tilted), or having had its rotation reversed by a similar massive collision at a tangent to its orbit.

M.M. Woolfson proposed Mercury was the moon of a giant planet that collided with another one.