Unravelling the rotational puzzle of Venus

We know that every celestial body spins like a top with varying speeds and planets are no exception. A careful look at their pictures will tell you which are fast rotators and which are slow. Often, we look upon rotation and changes in it as clues on the dynamics of the planet and the influence of other planets, which in turn will help us understand our own planet. In fact, the shape of Earth with its polar axis shorter than the equatorial radius is defined by its rotation. Venus, with its almost perfect spherical shape, is a classic example of a slow rotator.

A day on Venus is longer than a year. This interpretation arises from the fact that Venus takes 243 (Earth) days to turn once on its axis, while it takes 225 (Earth) days to revolve around the Sun. Moreover, as you watch the Sun from the surface of Venus, another peculiar phenomenon attracts your attention — on Venus, the Sun rises in the west and slowly drifts to east to set after 117 days, (but whether we can make out the Sun rise or set cannot be resolved so easily here). This is referred to as retrograde rotation, which means Venus orbits the Sun in the direction opposite to the one the Sun is rotating in.

Of the many theories explaining retrograde rotation, one particular theory believes that billions of years ago, Venus was impacted by another large body, which resulted in a complete change of rotational speed and direction. Another argument favours a continuous chaotic effect caused by planetary perturbations and tidal effects. The latter makes a lot of sense as Venus has a dense atmosphere.

What lies beneath?
The surface of Venus is barely visible through the dense envelope of the atmosphere. Thus, determining its rotation was only possible after the invention of the radio telescope, which measured the small Doppler shift in the beam reflected from the surface. A very interesting phenomenon was noticed while studying the winds on Venus. On the surface, the winds were gentle enough to just scatter dust. However, at the upper layers, the speed increased by over a factor of 60. The atmospheric circulation, though similar to Earth in patterns, differs from our planet in this respect. This fetched Venus the title ‘super rotator’.

The circulation scheme in the atmosphere can be broadly explained by the wave patterns generated. Again, the large variety is not fully understood, but studies point to the slow rotation as one of the major causes. In the absence of the angular momentum available through rotation, winds take charge of the maintenance of equilibrium here.

You will be fascinated to know that on Venus, cyclonic storms have two eyes, unlike the one-eyed storms in our planet.  The two vortices appear to be a permanent feature on its South Pole as it has remained the same since the first observation in 2006. These storms are at heights of 40 km and unlike their counterparts on Earth, cannot produce rains since all water will evaporate at a height of 20 km.

One of the recent studies based on the results from Venus Express Mission, which identifies six different types of waves with different characteristics, necessitated to explain the energy balance via transportation through the globe. Oscillations of 255 days have been identified in its atmosphere. Quite interestingly, Cassini mission has detected similar oscillations on Titan, a natural satellite of Saturn.

While studying the surface features and comparing with the ones taken two decades ago by Magellan spacecraft, another interesting fact emerged: there was a displacement of nearly 20 km. The pictures could be matched only after forcing a decrease in the rotation period by 6.5 minutes. Recent measurements with radar have also confirmed the change in the rotational speed. So, why is the planet Venus slowing down so fast? The extreme weather conditions, tides associated with the winds — all have been considered as possible sources of decrease as of yet.

It is possible that close approaches to  Earth may also be contributing to the long-term changes in the rotation speed.  For example, at inferior conjunctions, (when Venus passes between Earth and the Sun), the average distance from Earth is about 41 million km, which recurs in about 584 days. Although the orbit of Venus is almost spherical, the eccentricity of the orbit of Earth causes a variation in this distance. For example, in these 5,000 years, there have been 526 approaches with separations less than 40 million km, (some as close as 38 million km). Thereafter, there are none with such small separations for about 60,200 years. 

Venus has always been offering vital clues on important concepts — right from the heliocentric concept to the green house effect. Now it can do the same for rotation.