Why does Venus rotate backwards from the other planets?

The rotation period of Venus cannot be decided through telescopic observations of its surface markings because its featureless thick atmosphere makes this impossible. In the 1960's, radar pulses were bounced off of Venus while at its closest distance to the Earth, and it was discovered that its rotation period, its day, was 243.09 +/- 0.18 earth days long, but it rotated on its axis in a backwards or retrograde sense from the other planets. The above image was created by NASA's Magellan spacecraft whos radar imaging technique was able to detect surface feaatures as small as a few kilometers across.

If you were to look down at the plane of the solar system from its 'north pole' you would see the planets orbiting the Sun counterclockwise, and rotating on their axis counterclockwise. Except for Venus. Venus would be rotating clockwise as it orbited the Sun counterclockwise. Venus is not alone. The axis of Uranus is inclined so far towards the plane of the solar system that it almost rolls on its side as it orbits the Sun.

What accounts for the extreme inclinations of the rotation axis of Venus and Uranus? For years it was thought that in the case of Venus that Earth was the culprit. It is a curious fact that as Venus rotates three times on its axis in 729.27 days, Earth goes twice around the Sun ( 728.50 days) This has suggested to many astronomers that Earth and Venus are locked into a 3:2 tidal resonance. There are many bodies in the solar system that seem to be locked into various kinds of spin-orbit resonances, especially families of asteroids with the planet Jupiter. Mercury also seems to be gravitationally locked into some kind of resonance with the Sun since its day (58.646 days) and its year ( 87.969 days) are also in the proportion of 3:2.

Forces acting on spinning bodies result in some peculiar acrobatics. For instance, if you take a spinning top and give it a push, it will begin to wobble in a manner called precession. The axis of the Earth makes a 26,000 year wobble with an amplitude of tens of degrees. This is all due to the influence of the Moon's tidal attraction of the Earth. In the case of Venus, however, the gentle gravitational forces it may receive over billions of years to place it in a 3:2 resonance with the Earth don't seem to be strong enough to tip the entire planet over to make its rotation retrograde.

The best, current, ideas still favor some dramatic event that occurred while Venus (and Uranus for that matter) were being formed. It is known from the cratering evidence we see on a variety of planetary surfaces, that soon after the planets were formed, there were still some might large mini-planets orbiting the Sun. One of these may have collided with the Earth, dredging up material that later solidified into our Moon. The satellites of the outer planets are probably representitives of this ancient population of bodies. Venus may have experienced an encounter with one of these large bodies in which, unlike for Earth, the material didn't form a separate moon, but was absorbed into the body of Venus. In addition to mass and kinetic energy, this body would also have contributed angular momentum. The result is that the new spin direction and speed for Venus was seriously altered from its initial state, which could have been very Earth-like. Today, the result of that last, ancient collision is Venus with a retrograde rotation.

This theory may also apply to Uranus provided that the collision happened before the 15 satellites themselves were captured or formed. Their orbital planes look very uniform and show no evidence for a dramatic gravitational event such as a collision. It may be, too, that the Uranian collision event dredged up matter and flung it into orbit around Uranus, and out of this were formed the larger, coplanar, moons of Uranus.

This is, clearly, a complicated and not well understood phenomenon. The facts for Venus point towards a collision event to put its axis and rotation in the retrograde sense. The tidal action of the Earth on Venus, acting steadily over billions of years, then established the 3:2 spin-orbit resonance. Every 2 earth years, the exact same portion of the Venerian ( Cytherian) surface faces Earth. Could there be some sub- surface concentration of mass on this portion of Venus that the Earth can grab onto to create the tidal lock?

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