Technology Review - 21 May 2010
Superrotation on Venus: Driven By Waves Generated By Dissipation of the Transterminator FlowAs a Japanese weather satellite heads to Venus, a new theory tackles one of the outstanding mysteries of the planet.
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.
- arXiv.org > astro-ph > arXiv:1005.3488 > 19 May 2010
Context: The superrotation phenomenon in the atmosphere on Venus has been known since the late 60's. But until now no mechanism proposed has satisfactorily explained this phenomenon.
Objective: The aim of this research is to propose a mechanism, until now never considered, which could drive the atmosphere of Venus in its superrotation. This mechanism involves the transfer of the transterminator ionospheric flow momentum to the lower atmosphere via pressure waves generated in the cryosphere of Venus. The mechanism proposed presents a source of energy sufficiently strong to allow the transfer of energy despite dissipation. Method: The energy flow which transports the transterminator flow and the energy lost by the viscosity in the superrotating atmosphere were calculated. Both results were compared to establish if there is sufficient energy in the transterminator flow to drive the superrotation. Finally, the amplitude that the waves should have to be able to obtain the momentum necessary to induce superrotation was calculated. Also an experimental model was made presenting some similarities with the process described. Results: The calculated power for the transterminator flow is 8.48x10e10 W. The calculated viscous dissipation of the superrotating flow is 1.4x10e9 W. Therefore, there is sufficient energy in the transterminator flow to maintain superrotation. The amplitude of the waves generated in the cryosphere, necessary to deposit the power dissipated by the viscous forces, is 10e-4 m for waves of 1 Hz and 10e-8 m for waves of 10e4 Hz. These amplitudes imply that at the altitude of the clouds on the night side there must be a constant sound of 83 dB. If the superrotation of Venus were to stop, with the continuous injection of 1.4x10e9 W, the actual superrotation would appear again in 1.4x10e6 years.