Shallow jet streams

The outer ice giant planets, Uranus and Neptune, have the strongest winds in our Solar System.  Equatorial east-west zonal winds can blow at a fierce 1,000 km/h on Neptune,  while at higher latitudes the winds swap direction and become easterly jet steams. The global wind patterns on Uranus are similar but the wind speeds not as strong.  (The strongest jet streams on Earth are in the polar regions and average around 200 km/s.)

Wind speeds on Uranus (left) and Neptune (right) as a function of latitude.  Data comes from Voyager 2 (circles) and HST (squares) observations.  (Credit: Kaspi et al. 2013, Nature)

Wind speeds on Uranus (left) and Neptune (right) as a function of latitude. Data comes from Voyager 2 (circles) and HST (squares) observations. (Credit: Kaspi et al. 2013, Nature)

While these measurements were first made in the 1980’s when the Voyager 2 spacecraft fly past Uranus and Neptune, what has remained a mystery is how deep these flows penetrate into the atmospheres of the giant planets.  The zonal wind speeds are determined by following methane and ammonia clouds near the upper cloud deck of the atmosphere. Knowing the depth of these winds is vital in understanding the global dynamics as well as internal structure of the giant planets, so astronomers want to know whether the high-speed jet streams result from shallow atmospheric processes or if they extend deep into the planetary interior.

Cloud features in the upper atmosphere of Neptune are used to determine  jet stream speeds. (Credit: NASA, ESA, Hubble Heritage Team, STScI/AURA)

In this week’s edition of Nature, Kaspi et al. use gravity field data from Voyager 2, combined with atmospheric modelling, to answer this outstanding problem.  Perturbations to the gravity field of the ice giants, which were measured by small changes in the speed of Voyager 2 as it flew close to the planets, result from the oblateness of the rapidly rotating planets, as well as small changes in the density of the planet caused by the fast zonal winds. By analysing the gravity field data, Kaspi et al. show that the winds must be confined to the outer 1,000 km of both planets (which have radii of about 25,000 km).  They conclude that the dynamics controlling the zonal jet streams come from shallow processes.

What remains unknown is the energy source that drives the winds.  Being so far from the sun, solar heating is much weaker than on Earth.  Neptune has its own internal heat source, and it seems likely that the rapid rotation and internal convective motions play a part in driving the jet streams.

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