Why doesn't Venus have any moons?  

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Earth's moon was formed from a huge collision with another planet billions of years ago when it was still extremely young. This impact threw up an enormous cloud of debris and temporarily shifted Earth's gravitational field by warping the shape of the newly formed planet. This shift in gravitational pull allowed for the debris to remain in orbit long enough to come together into a moon. The pull of the moon on our oceans creates the daily tide fluctuations that we experience. The water itself is not advancing or receding; it is actually the Earth rotating underneath the tidal bulge created by the moon. The tides and Earth itself influence the moon's orbit as well by acting back on it. Because of this gravitational interaction, the moon has slowly been receding since it formed.

One theory about Venus's lack of moon, attributed to Caltech's David Stevenson and Alex Alemi, suggests that Venus had a moon of its own at one point, formed through this same process. Rather than drifting away until it was unleashed from Venus's pull, which would have taken billions and billions of years, the moon careened back into Venus and was destroyed. The first impact that formed this moon reversed Venus's spin and the moon behaved as Earth's has and still does today. However, a second impact cancelled this counterclockwise spin and changed the gravitational interaction between Venus and its moon. As a result, the moon was drawn towards the planet, where it collided and was obliterated.

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