I was thinking about this today. Mercury is slightly bigger than the moon. Venus and Earth are roughly the same size.
Do you think that it is possible that, at one time, Venus had a satelite, which would have been Mercury, and that the proximity to the sun ripped Mercury from Venus?
If so, Mercury would have stablized Venus like our moon stablizes Earth. This may have given similiar circumstances in the development of Venus and Earth.
Mercury is actually denser than Venus because of its very sizable iron core, so there is no feasible way it could have formed as a moon around Venus. Compare with the Earth-Moon system, where the Moon is made of lighter materials than the Earth because the heavier elements (the metals) were gathered up by the greater gravitational pull of the Earth.
Also the sun couldn't really have pulled away any moon of Venus and placed it in the orbit that Mercury currently occupies. While it's true the Earth's moon is moving an inch or two farther away from Earth every year, I think the process that's causing the drift isn't destined to peel the Moon away from Earth in 50 billion years. I think...
Originally posted by SoothfastTidal effects (of the moon and the Sun) is causing the Earth's rotation to slow down. This in turn will accelerate the moon to conserve angular momentum, hence causing the moon to drift further out.
Actually I forget why the Earth's moon is drifting away. It may have something to do with the sun.
Eventually the same side of the moon will be pointing to the same side of the Earth, but this will take billions of years. The moon will stop drifting apart then.
I think the Sun would have expanded into a red giant by then and engulfed the Earth and the moon.