The gas giant Jupiter is known for its huge storm eddies and frenzied cloud bands. Now observations with the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile reveal that there are even faster storms over the cloud cover of Jupiter: around the poles, narrow bands of wind move at up to 1,450 kilometers per hour – that’s twice as much as fast as the winds in the Great Red Spot. Because there are no optically traceable clouds in the stratosphere of Jupiter, the astronomers used the spectral signatures of hydrogen cyanide molecules in this layer of the atmosphere to measure their speed.
Jupiter is famous for its characteristic red, white and brownish cyclones and the storm bands blowing in strips parallel to the equator. Only recently, planetary researchers used data from NASA’s Juno spacecraft to determine that these tropospheric storms extend from the top of the cloud cover to a depth of 3,000 kilometers. A considerable part of the lower lying gas masses is therefore in constant, rapid movement. Much further up, in the ionosphere of the gas giant, even faster streams of charged particles race through the thin gas envelope. These ionospheric jets are closely related to the polar lights of the planet and can travel up to two kilometers per second, as Thibault Cavalié from the University of Bordeaux and his colleagues explain.
Comet impact as a helper
But so far there has been a gap between these two windy layers: It was unknown whether and which storms were blowing in the stratosphere between the cloud cover and the ionosphere. Because there are no clouds or other visible markers in this layer whose movements could be followed, no direct measurements of wind speeds in the Jupiter stratosphere could be made. But Cavalié and his team have now exploited a lucky coincidence to determine the speed of the Jovian stratospheric winds for the first time. When Comet Shoemaker-Levy struck Jupiter in a spectacular way in 1994 and broke into numerous fragments, new molecules were formed in the planetary stratosphere, including hydrogen cyanide (HCN) and carbon monoxide. If there are storms in this layer of the atmosphere, these molecules would have to move with the winds.
The decisive factor, however, is that it is known from other planetary observations that the chemical signature of hydrogen cyanide and carbon monoxide can be detected using spectral analyzes in the millimeter wave range – and this is exactly what Cavalié and his team have now analyzed using the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile . For their study, they used 42 of the 66 high-precision ALMA antennas to measure the Doppler shift in the spectral signature of hydrogen cyanide – tiny changes in the frequency of the radiation emitted by the molecules, which is caused by the movement of the molecules. “By measuring this displacement, we determined the speed of the winds, much like determining the speed of a passing train by changing the frequency of the train whistle,” explains co-author Vincent Hue of the Southwest Research Institute in Texas.
Frenzied jets for the pole
In this way, the research team succeeded for the first time in detecting winds in the stratosphere of Jupiter and measuring their speed. “The most spectacular result is the appearance of powerful jets with speeds of up to 400 meters per second, which are located under the northern lights near the poles,” reports Cavalié. According to this, these polar jets reach a speed of 1,450 kilometers per hour and are therefore twice as fast as the winds in the Great Red Spot of Jupiter. As the astronomers report, the newly discovered winds lie several hundred kilometers below the already known ionospheric jets and move against the rotation of the planet. “Our discovery suggests that these jets could behave like a giant vortex that is around four times the diameter of the earth and about 900 kilometers high,” says Cavalié’s colleague Bilal Benmahi. A vortex of this size would be unique in the solar system.
In addition to the surprising polar winds, the scientists identified other stratospheric wind bands around the planet’s equator. These are a little less pronounced and, at around 600 kilometers per hour, only barely half as fast. Overall, however, the existence of such strong winds in the layer between the cloud cover and the ionosphere is surprising, as the researchers explain. Because according to previous models, these winds would have to lose speed in the upper atmosphere and disappear before they can reach the stratosphere. “The new ALMA data tells us the opposite,” says Cavalié. “These ALMA results open a new window, among other things, for exploring Jupiter’s northern light regions.”
Source: Thibault Cavalié (Laboratoire d’Astrophysique de Bordeaux) et al., Astronomy & Astrophysics, doi: 10.1051 / 0004-6361 / 202140330