The planet Venus rotates slowly on its axis, just once, while the Earth goes round 243 times. This time of rotation, however, has been found to have increased between recent observations. In case of the Earth there is evidence that the speed of rotation varies, generally over millennia. But the length of a day on Venus has been found to have got longer by 6.5 minutes within a span of 16 years.

Thomas Navarro, Gerald Schubert and Sébasien Lebonnois from the University of California at Los Angeles and the Sorbonne in Paris describe in the journal, Nature Geoscience, their simulation of the atmosphere of Venus, which may explain how disturbances in the dense atmosphere would affect the planet’s rate of rotation.

The simulation allows a variation of as much as two minutes within the time of a solar day. The simulation carried out is to reproduce an unusual feature, a planet-sized pattern, which may be an atmospheric wave that has been found in the upper atmosphere of Venus. This can explain the variation of about seven minutes that has been observed in the length of the day on Venus over the last 40 years, the paper says.

Spinning objects are able to change their speed by altering their internal structure without making any external contact. An object that is moving in a straight line would keep moving till it is stopped or slowed down, generally by braking. But once it has been slowed down, it will not speed up unless it is given a nudge.

That is not the case with a spinning object. If a spinning figure skater, or acrobat were to stretch her arms out, her rate of spin would slow down. And the speed would pick up as soon as she draws her arms in again. This is because the parts of a spinning object that are further from the axis of spin store more of the energy of the spin than parts that are nearer the axis. This is unlike an object that is moving in a straight line, where all parts of the object share the energy of movement only according to their mass.

Objects like stars and planets started out as vast clouds of gas or dust and gradually coalesced under the force of gravity. Any slight, initial net rotation was magnified as parts that were at great distances came nearer the centre. When the star or planet has final shape and profile, there is a final rate of spin, which usually stays constant.

In case of the Earth, although the shape and dimensions are largely stable, there have been changes over the ages. The spin, itself, exerts forces that alter the shape of the object. As the equator spins round faster than the poles, matter is thrown outwards in the form of a bulge at the equator and there’s a flattening at the poles.

That leads to slowing of the rate of spin till the bulge stabilises. During the ice ages too, the water content of the oceans gets stored as ice at the poles. The load of ice causes compression and further bulge at the equator, slowing the rotation. When the earth warms again, the ice melts, the pressure relaxes, the bulge reduces and the rotation speeds up.

Ocean currents or winds can also affect the rate of spin of solid mass. As currents and winds come about through an opposite thrust on solid mass, the rate of rotation of the mass would need to change, to keep the total energy of rotation unchanged. At the same time, on Earth, the mass of the ocean and atmosphere is so much less than the remaining parts, that the effect is scarcely perceptible. Mutual tidal effects of the Earth and Moon also bring about a very slow reduction of the speed of rotation. But that effect is extremely feeble, just 2.3 milliseconds a day per century!

As Venus has no moon, there is no tidal effect to worry about. The atmosphere, however, becomes significant. The Venusian atmosphere, which is largely carbon dioxide, and sulphuric acid at high altitudes, is at a pressure that is 92 times the pressure on Earth and the mass of the atmosphere is 93 times the mass of the Earth’s atmosphere.

We could add 20 per cent to these figures, as the mass of Venus is 80 per cent of the mass of Earth. And further, Venus’s atmosphere is highly energetic, blowing feverishly, to go round the planet in four Earth days, while the planet takes 243 days for a rotation. The component of the energy of rotation in the whirling atmosphere of Venus is hence not negligible like it is on Earth.

The definitive measurement of the rotation of Venus was considered as that by Nasa’s Magellan mission of 1990-92, which was 243.0185 ± 0.0001 days. The European Space Agency’s mission, Venus Express, of 2006, however, found an error of 12.4 miles where some features of the planet had been calculated to be. The implication was that Venus had slowed in its rotation by 6.5 minutes since the last measurement 16 years ago.

Starting late in 2015, the Japanese Venus orbiter, Akatsuki beamed back detailed features of the planet’s atmosphere. It had been observed that while the high speed atmospheric wind showed small scale features, there were also large, planet-scale features that seemed to move slower or faster than the main wind. It was thought that these would represent planet-scale waves within the atmosphere.

The orbiter, Akatsuki showed the presence of a bow-shaped structure, 10,000?km across, from the northern hemisphere to the southern hemisphere, at the cloud-top level. “Over several days of observation, the bow-shaped structure remained relatively fixed in position above the highland on the slowly rotating surface, despite the background atmospheric rotation being faster than the planet,” the 2017 report of the finding says. Four more large-scale wave features were seen over four Venus days, on the sun-ward side of the planet, in the afternoons, the present paper notes.

Thomas Navarro and his colleagues carried out computer simulations of different conditions that could exist on Venus, to be consistent with known parameters and the present structure. They found what was observed was in keeping with surface features of the planet giving rise to atmospheric waves, where the weight of the atmosphere tries to restore equilibrium that has been disturbed. Such waves, called “gravity waves”, arise in the Earth’s atmosphere too, when energy is transferred from the lowest layers, where the air cools as one goes higher up, to the higher layer, separated by an intermediate region.

This kind of interaction between the high speed atmosphere and slower moving solid part of the planet could lead to a pull that would affect the rate of rotation, the paper says. The planet would hence show variations in the speed of rotation, like the ballerina or the acrobat. While the reduction, 6.5 minutes over 243 days, is not large, it has come about in a short span of 16 years. The phenomenon reported, however, may be the reason that the rate of rotation of Venus is the slowest in the solar system.

The writer can be contacted at response@simplescience.in