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The diameters of Venus (7,520.8 miles) and Earth (7,917.5 miles) are comparable, but the disparity of the length of the day for each planet - as expressed in terrestrial hours - is enormous; a day on Venus lasts 2802 hours vs. an Earth day lasting 24 hours (its 116.750 times that of Earth).

How can there be such a great difference when the size of the two planets is nearly the same? Does the fact that Venus’ rotation is retrograde to all of the other planets in our solar system affect the length of it's day?

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  • $\begingroup$ More of an astronomy question than earth science. Related question here: astronomy.stackexchange.com/questions/6183/… The size of the planet is largely irrelevant. Everything else, Gordan Strange covered. $\endgroup$ – userLTK Nov 13 '16 at 22:34
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Gordon Stanger's answer has already made perhaps the main point: There's absolutely no reason to believe that the evolution of a planet in one part of the Solar System should exactly mirror that of any other planet. We see drastic composition and size differences among the terrestrial planets; why should any other properties be similar?

I'm aware of several possible mechanisms that explain why Venus spins so slowly.

Repeated impacts and the formation (and death) of a moon

Venus has no moon - a curious fact, considering that the only other planet to not have one is Mercury. You could ascribe this to many factors, such as relatively low masses and close proximity to the Sun. However, some models have suggested that there was a moon around Venus, which is now gone.

Alemi & Stevenson (2006) proposed a complicated story of impacts. Their model suggests that Venus suffered a giant impact from a moderately sized protoplanet. As was the case with Earth, debris was thrown up, and a moon coalesced in orbit. This changed Venus' rotation, and began to transfer even more angular momentum to the moon. However, a second impact to Venus reversed its rotation, and the moon fell back to the planet, transferring its angular momentum back. This would all happen on the order of ten million years or so, starting and possibly ending during the Late Heavy Bombardment.

The upshot of all this is that Venus emerged with not just a retrograde rotation, but a much longer day. This is, of course, not the sole explanation for the retrograde rotation - exotic scenarios involving 180 degree flips have been proposed; see e.g. Correia & Laskar (2001) - but the impact hypothesis does have the added advantage of explaining the lack of a moon.

Atmospheric tides and internal friction

Interactions with the Sun have also been proposed as a mechanism affecting not just Venus, but exoplanets like it. Atmospheric tides, generated through heating and cooling from the Sun, could enact a torque on the planet, bringing it into one of several "equilibrium states" (see Auclair-Desrotour et al. (2016)). Internal friction could cause more complex interactions in the mantle, further exaggerating the effects on the solid part of the planet. The same thing could happen to any planet orbiting the Sun at roughly the same distance as Venus in the Solar System.

It was once proposed that an additional torque was applied to Venus by Earth, complementing the solar atmospheric tides, as there appears to be a coupling between the time between close approaches to Earth and the time of one Venusian day. However, it has since been determined that such a resonance does not, in fact, exist, and should be discounted as a possible reason for the slowing of Venus's rotation (see Shapiro et al. (1979)).


Why not Earth?

So, why would these effects happen to Venus but not Earth? The chaos of the early Solar System is of course the best answer, but there are other explanations for the specific proposals. The impact theory simply relies on randomness: While Venus and Earth were both likely to suffer major moon-forming collisions, they were not as likely to go through this twice. The atmospheric tidal effects, on the other hand, could have been unavoidable, as interactions with the Sun may have been more severe with Venus, for a variety of reasons including distance to the Sun and the exact properties of the atmosphere.

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There is no reason why the day length should be the same. From the time of planetary formation, some 4.6 Bn years ago there has been some degree of slowing of day length due to tidal friction, which is much higher on Earth than on Venus, so the original contrast in day length may have been even greater than today.

There are three major reasons for differencial spin rates. Firstly, the initial planetary rotation rate was a function of the net angular momentum of the planetesimals, gas, and miscellaneous debris from that part of the solar nebula from which the planet condensed (in accordance with the conservation of angular momentum). There is no reason to suppose that that the original solar nebula consisted of a nicely ordered, evenly distributed mass/velocity distribution. Rather, there is every reason to think that it was a highly chaotic heterogeneous cloud of matter undergoing gravitational mayhem - probably including large scale gravitational disruption by gas giants much closer to the sun than we see them today. From such chaos one could model almost any distribution of relative rotation speeds amongst the final planets.

Secondly, there is strong evidence that proto-Earth collided with a 'trojan' named 'Theia', about the size of Mars, about 4.3 Bn years ago. This is the 'giant impact hypothesis', which provides a good explanation for Earth having such a large moon. We don't know the relative angular momentum of either proto-planet, their relative motions, or the angle of impact (head on? glancing?), so it is probably impossible to reconstruct the event. However, it is (relatively) easy to envisage how such an impact could radically change the Earth's angular momentum. The spin could have been hugely accelerated or decelerated according to the physics of the impact.

Thirdly, Venus's relative proximity to the sun could have resulted in its undergoing rotational capture, such that eventually one face of Venus will perpetually face the sun. This gravitational locking of Venus is probably the most significant reason for its' long day. As you rightly note, the retrograde spin of Venus is curious. Therefore we may infer that Venus's spin history is more complex than we can account for by simple planetary accretion.

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