New Theory Explains Superrotation on Venus

Akatsuki, the first extraterrestrial weather satellite, began its journey to Venus this morning after a successful launch from the Tanegashima Space Centre in Japan.

The spacecraft should help answer one of the great mysteries of the Solar System: why the winds on Venus blow faster than the planet itself rotates.

Venus rotates once every 243 days but it takes a mere 4 days for clouds in the Venusian atmosphere to go all the way round the planet at a whopping 200 metres per second. This phenomenon is known as superrotation.

Astrophysicists have long speculated that the difference in temperature between the day and the night side of Venus at 300K and 100K respectively, is what drives these winds. But there’s a problem with this calculation

The puzzle is that the Venusian atmosphere has a certain viscosity and so, by itself, ought to dissipate energy at a rate of 10^9 W and slow down. Something else must be injecting energy into the system at this rate. How does this happen?

Today, Héctor Javier Durand-Manterola and pals from the Universidad Nacional Autónoma de México say they have solved the puzzle. They point out that in addition to the ordinary atmospheric winds, there is another much faster flow higher above the planet. These are ionic winds in the ionosphere between 150 and 800 km above the surface and were discovered by the Pioneer Venus Orbiter in the early 80s.

Known as the transterminator flow, these winds travel at supersonic speeds of several kilometres per second, probably driven by the planet’s interaction with the solar wind.

The question that Durand-Manterola and co address is what happens when the supersonic winds in the ionosphere interact with the slower winds in the atmosphere. Their answer is that the interaction generates turbulence in the atmosphere and that dissipation of this turbulence creates sound waves in that inject a significant amount of energy into the atmosphere.

How much? Durand-Manterola and pals calculate that the process injects energy at a rate of 10^10 W, more than enough to account for the amount lost due to viscosity. In fact, one prediction they make is that the sound waves created by the energy injection process have an intensity of 84 dB. That’s a significant roar that ought to be measurable in future.

To back up the idea, the team have performed a simple experiment with water to show how the energy transfer occurs, albeit it in rather different conditions.

That’s an interesting idea but one that will need more observations of Venus itself before it can be claimed as a home run. The fact that this process could replace the dissipated energy doesn’t mean that it does.

As it happens, Akatsuki might be able to help. It will arrive at Venus in December and should start sending data back soon after that. Durand-Manterola and others will be watching.

Ref: arxiv.org/abs/1005.3488: Superrotation on Venus: Driven By Waves Generated By Dissipation of the Transterminator Flow