Atmospheric super-rotation
Atmospheric super-rotation is a phenomenon where a planet's or moon's atmosphere rotates faster than the planet itself. This behavior is observed in the atmospheres of Venus, Titan, Jupiter, and Saturn. Venus exhibits the most extreme super-rotation, with its atmosphere circling the planet in four Earth days, much faster than the planet's own rotation of 243 Earth days. The phenomenon of atmospheric super-rotation can influence a planet's climate and atmospheric dynamics.
Dynamics of super-rotation
In understanding super-rotation, the role of atmospheric waves and instabilities is crucial. These dynamics, including Rossby waves and Kelvin waves, are integral in transferring momentum and energy within atmospheres, contributing to the maintenance of super-rotation. For instance, on Venus, the interaction of thermal tides with planetary-scale Rossby waves is thought to contribute significantly to its rapid super-rotational winds. Similarly, in Earth's atmosphere, Kelvin waves generate eastward along the equator, playing a vital role in phenomena like the El Niño-Southern Oscillation, demonstrating the broader implications of these dynamics in atmospheric science.Venus: Extreme super-rotation
The atmosphere of Venus is a prominent case of extreme super-rotation; the Venusian atmosphere circles the planet in just four Earth days, much faster than Venus' sidereal day of 243 Earth days. The initial observations of Venus' super rotation were Earth-based. Modern general circulation models and observations are often enhanced by looking at past ancient climates. In a model where Venus is assumed to have an atmospheric mass similar to Earth, subsolar-antisolar circulation could have dominated over super-rotation in an ancient thinner atmosphere.Titan
Super-rotation present in the stratosphere of Titan has been inferred by Voyager IRIS, Cassini CIRIS, stellar occultation and temperature observations, and Doppler shifts of the Huygens probe’s radio signal. Latitudinal pressure gradients established from measurements taken by Voyager IRIS were sufficient to produce super-rotation of the atmosphere. Stratospheric zonal winds on Titan were observed on the order of 100-200 m s−1, faster than the highest zonal winds on Earth at ~60-70 m s−1. Questions on the effect of obliquity in super-rotation on Titan is often compared to Venus, as they share similar centrifugal accelerations to achieve dynamic balance. Any seasonal variations effected by obliquity between Titan and Venus is much different, as the small obliquity of Venus at 2.7° negates any strong seasonal effects. Titans obliquity at 26.7° is high enough to cause seasonal variations within the stratospheric spin. Attempts to model super-rotation on the gas giants, including Titan, has been abundant. The first observations of Titan in the 1980's revealed little information about circulation within the atmosphere due to the low contrast photochemical haze covering the moon. The first general circulation model in the 1990s provided insight into the stratospheric properties that should be expected on Titan with further observation, and predicted super-rotation with winds up to 200 m/s. Super-rotation was supported by the first 3D Titan GCM created by the Laboratoire de Météorologie Dynamique, in which they used an atmosphere similar to the observations of Voyager and recently Cassini.The most recent GCM that is able to simulate super-rotation in the stratosphere successfully is TitanWRF. Modeled after the PlanetWRF, which was designed to be a global weather, research, and forecasting model, TitanWRF added planetary physics and generalized parameters to produce a successful super-rotation model. Work done with TitanWRF v2 was able to simulate gradients in latitudinal temperature, zonal wind jets and super-rotation in the stratosphere. Comparing TitanWRF v2 simulations with constant solar forcing models, showed that in the latter, a rapid buildup in rotation, attaining > 100m/s, happened in a few Titan years. The parameters in these older forcing models differ greatly in the mechanisms involved in generating the initial super-rotation compared to the more realistic TitanWRF models. After initial spin up, similarities evolve between the different models when a steady state is produced, but differ again in the final states of the model. The initial mechanism producing spin up to super-rotation is still an on going question, as correlations between models differ greatly within this regime.