Why does Venus spin in the opposite direction? Given: Law of Conservation of Angular Momentum.


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*Reverse spinning with dense atmosphere (92 times > Earth & CO2 dominant sulphur based).

*Surface same degree of aging all over.

*Hypothetical large impact is not a sufficient answer.


Assuming any object large enough to alter a planets rotation or even orbit would likely destroy most of its shape, yet Venus has retained a spherical property with a seemingly flat, even terrain indicating no volcanoes,and few if any visible meteor impacts. It would be fragmented and dispersed for billions of years.  Even the question of what meteor, comet, asteroid composition could survive traveling that close to the sun's temperature, radiation, electromagnetic energy, solar flares, or gravity to equal a mass reactionary change as to alter it's spin.
 A: IMO there is no solid explanation, as anna said. Only clues (WP).
In this simulation (2002) Long term evolution of the spin of Venus- II, Numerical simulations we find a mix of: 'chaotic zone', instability, large impact, close encounter, tidal effects, planetary perturbations,... 
There is room for speculation:
I think that the heavy atmosphere is not a significant factor of slowing the rotation.
I discarded any violent event because it can easily change more than one parameter; in this case we have only one (the direction).
The planet's minute axial tilt (less than three degrees, compared with 23 degrees for Earth) make me think to keep only the tidal perturbation, although the present configuration Sun-Venus-Earth is not able to justify it.
As the Venusian surface rotates as low as 6.5 km/h (on Earth is about 1,670 km/h) we can think that Venus may have changed the direction of rotation not long time ago.
The solar system angular momentum problem is not solved (the planets strangely have almost all of it) and I think that the solution of this does one is not related to the present question.
The equation 35 of this paper (a new model, undiscussed) allows the slow evolution of the configuration Sun-Venus-Earth.  
A: Well, I Binged and found some references.
Seems that a collision is most probable, if it happened at a time when the whole system was malleable. But there is no solid explanation.
A: There seems a lot of conjecture in any event. Venus could have been a meteor, with an innate spin, that swung by the Sun and have been captured into our Solar systems anticlockwise orbital arrangement. Retaining her original spin momentum, clockwise relative to the others.
A: This is a very late response, but there is no accepted answer as of yet, and none of the answer quite hit the mark.

Regarding the magical collision hypothesis, that smacks of being rather non-scientific.  Scientists as well as Missourians are wont to say, "Show me!" Other than the fact that Venus's rotation is anomalous, what, exactly, is the evidence for a collision with enough oomph to create this anomalous rotation? Even more problematically, this collision hypothesis hand waves away the problem of Venus's thick atmosphere.
Part of the problem here is thinking that the current rotation rates and rotation axes of the terrestrial planets are somehow related to the initial angular momentum of protoplanetary disk from which the planets formed. That may well be the case for the two gas giants in the solar system, but it's not the case for the terrestrial planets. The primordial angular momenta of the terrestrial planets is not a conserved quantity thanks to external torques from the Sun, Jupiter, moons, and other planets. Mercury is in a 3:2 spin-orbit resonance. Mars has suffered chaotic variations in its rotational state due to perturbations from Jupiter. Whatever angular momenta those two planets had initially is long lost. The Earth's Moon has apparently stabilized the Earth's rotation axis, but has sapped its primordial rotation rate. So what about Venus?
Helder Velez's answer to me comes closest to the mark but misses some key points. Helder explicitly discounted Venus's thick atmosphere as playing a role. That Venus has a very thick atmosphere may well be a key part of the answer. Helder referenced the second of two papers published in Icarus on Venus's rotation by Correia and Laskar but did not the reference the first (or the similar Nature article by Correia and Laskar published couple of years prior to those Icarus articles), and Helder did not pay attention to the key point in the Correia and Laskar: Venus rotation is a natural consequence of Venus's thick atmosphere. No collision is needed.
A parsimonious explanation of Venus's rotational state would not need a magical gigantic impact that even more magically did not blow away Venus's primordial atmosphere. This parsimonious explanation is exactly what Correia and Laskar argue happened in their Nature paper and their two Icarus papers. Venus rotates the way it does because this is one of the four final states in which a collision-free terrestrial planet with a very thick atmosphere would rotate. Two of those final states have Venus rotating prograde, the other two, retrograde. The prograde rotational states are statistically unlikely compared to the retrograde rotational states. Venus's thick atmosphere plays a key role in determining Venus's final, stable rotational state.

References:
Correia, A. C., & Laskar, J. (2001). The four final rotation states of Venus. Nature, 411(6839), 767-770.
Correia, A., Laskar, J., & de Surgy, O. N. (2003). Long-term evolution of the spin of Venus: I. theory. Icarus, 163(1), 1-23.
Correia, A., & Laskar, J. (2003). Long-term evolution of the spin of Venus: II. numerical simulations. Icarus, 163(1), 24-45.
A: 
Why does Venus spin in the opposite direction?
Given: Law of Conservation of Angular Momentum.
Reverse spinning with dense atmosphere (92 times > Earth & CO2 dominant sulphur based).

Surface same degree of aging all over.

Hypothetical large impact is not a sufficient answer.

Assuming any object large enough to alter a planets rotation or even orbit would likely destroy most of its shape, yet Venus has retained a spherical property with a seemingly flat, even terrain indicating no volcanoes,and few if any visible meteor impacts. It would be fragmented and dispersed for billions of years. Even the question of what meteor, comet, asteroid composition could survive traveling that close to the sun's temperature, radiation, electromagnetic energy, solar flares, or gravity to equal a mass reactionary change as to alter it's spin.

Various theories have been put forth. There is the other answer about impacts, and the one from the paper: "The four final rotation states of Venus" by Correia and Laskar, (2001). Nature. 411. 767-70. 10.1038/35081000.

"... we show that independent of uncertainties in the models, terrestrial planets with dense atmosphere like Venus can evolve into one of only four possible rotation states. Moreover, we find that most initial conditions will drive the planet towards the configuration at present seen at Venus, albeit through two very different evolutionary paths.".

Other answers not yet offered here are:

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*"Spin dynamics of close-in planets exhibiting large TTVs (transit timing variations)" (12 May 2017), by Delisle, Correia, Leleu, and Robutel:

"Recently, Leconte et al. (2015) used simulations including global climate model (GCM) of the atmosphere of Earth-mass planets in the habitable zone of M−type stars to show that these planets might be in a state of asynchronous rotation (see also Correia et al. 2008). This asynchronous rotation is due to thermal tides in the atmosphere. This same effect was also invoked to explain the retrograde spin of Venus (see Correia & Laskar 2001, 2003). However, for close-in planets, the gravitational tides dominate the thermal tides, so synchronous rotation is believed to be the most likely scenario (Correia et al. 2008; Cunha et al. 2015). In this paper we investigate another effect that can drive the spin of close-in planets to asynchronous rotation, namely planetary perturbations. Correia & Robutel (2013) showed that in the case of co-orbital planets, planet-planet interactions induce orbital perturbations that can lead to asynchronous spin equilibria, and even chaotic evolution of the spin of the planets. The planets librate around the Lagrangian equilibrium and have oscillations of their mean longitude that prevent the spin synchronization.".



*"Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki" (7 Sept 2017), by Horinouchi, Murakami, Satoh, Peralta, and 14 others:

"The planet Venus rotates westward with a very low angular speed corresponding to a period of 243 days, but its atmosphere rotates to the same direction with much higher angular. This superrotation reaches its maximum near the cloud top located at around the altitude of 70
km, where the rotational periods are 3 to 5 days, several tens of times faster than the planetary rotation. Measurements by entry probes like Veneras, Pioneer Venus Multiprobe, and VEGA revealed that zonal wind speeds below the cloud top decreases quasi-linearly with depth. Despite the long history of studies, the mechanism of the superrotation remains unsolved.".



*"Atmospheric thermal tides and planetary spin I. The complex interplay between stratification and rotation" (28 Sept 2017) and "Atmospheric tides and their consequences on the rotational dynamics of terrestrial planets" (27 Sept 2017), by Auclair-Desrotour, Laskar, and Mathis:

"Atmospheric tides can have a strong impact on the rotational dynamics of planets. They are of most importance for terrestrial planets located in the habitable zone of their host star, where their competition with solid tides is likely to drive the body towards non-synchronized rotation states of equilibrium, as observed in the case of Venus.".



*"The rotation of planets hosting atmospheric tides: from Venus to habitable super-earths" (17 Nov 2016), by Auclair-Desrotour, Laskar, Mathis, and Correia:

"The competition between the torques induced by solid and thermal tides drives the rotational dynamics of Venus-like planets and super-Earths orbiting in the habitable zone of low-mass stars. The tidal responses of the atmosphere and telluric core are related to their respective physical properties and strongly depend on the tidal frequency. The resulting torque determines the possible equilibrium states of the planet's spin. We compute here an analytic expression for the total tidal torque exerted on a Venus-like planet. This expression is used to characterize the equilibrium rotation of the body. Close to the star, the solid tide dominates. Far from it, the thermal tide drives the rotational dynamics of the planet. The transition regime corresponds to the habitable zone, where prograde and retrograde equilibrium states appear. We demonstrate the strong impact of the atmospheric properties and of the rheology of the solid part on the rotational dynamics of Venus-like planets, highlighting the key role played by dissipative mechanisms in the stability of equilibrium configurations. ".



*"On the equilibrium rotation of Earth-like extra-solar planets" (7 Aug 2008), by Correia, Levrard, and Laskar:

"Here we provide a general description of the allowed final equilibrium rotation states of these planets, and apply this to already discovered cases in which the mass is lower than twelve Earth-masses. At low obliquity and moderate eccentricity, it is shown that there are at most four distinct equilibrium possibilities, one of which can be retrograde. Because most presently known "Earth-like" planets present eccentric orbits, their equilibrium rotation is unlikely to be synchronous.".



*"A Formula for the Rotation Periods of the Planets & Asteroids" (8 Dec 1998), by Subhash Kak:

"It is generally believed that the Titius-Bode relationship between the distance of the planets from the sun may have some significance regarding the formation of the solar system. If there is a similar simple pattern defining the rotation periods of the planets then that may also provide clues regarding the dynamics of the early solar system. In this note I present a simple relationship that is in good agreement with the rotation period information of the superior planets, and it indicates that Venus has retrograde rotation although it does not give an accurate value of the rotation of this planet or Mercury.".

