Strong stratospheric winds first measured on Jupiter

Using the Atacama Large Millimeter / Submillimeter Array (ALMA), of which the European Southern Observatory (ESO) is a partner, a team of astronomers first directly measured winds in Jupiter’s central atmosphere. By analyzing the aftermath of a comet collision from the 1990s, the researchers uncovered incredibly powerful winds, at speeds of up to 1,450 kilometers per hour, near Jupiter’s poles. They can imagine what the team described as a ‘unique meteorological animal in our solar system’.

Jupiter is known for its distinctive red and white bands, swirling clouds with moving gas that astronomers traditionally use to detect winds in Jupiter’s lower atmosphere. Astronomers have also seen near the Jupiter poles the living glow known as aurorae, which is apparently associated with strong winds in the upper atmosphere of the planet. But so far, researchers have never been able to directly measure wind patterns between these two atmospheric layers in the stratosphere.

Measuring wind speeds in Jupiter’s stratosphere using cloud tracking techniques is impossible due to the absence of clouds in this part of the atmosphere. However, astronomers have found an alternative measurement tool in the form of the comet Shoemaker-Levy 9, which collided with the gas giant in a spectacular way in 1994. This impact has yielded new molecules in the stratosphere of Jupiter, where they have ever moved with the wind since.

A team of astronomers, led by Thibault Cavalié of the Laboratoire d’Astrophysique de Bordeaux in France, has now detected one of these molecules – hydrogen cyanide – to directly measure stratospheric rays on Jupiter. Scientists use the word “jets” to refer to narrow wind bands in the atmosphere, such as the Earth’s jets.

“The most spectacular result is the presence of strong rays at a speed of up to 400 meters per second, which is located under the aurora near the poles,” says Cavalié. This wind speed, equivalent to about 1450 kilometers per hour, is more than twice the maximum storm speed reached in Jupiter’s Great Red Spot and more than three times the wind speed measured on the strongest tornadoes on Earth.

“Our detection indicates that these jets can behave like a giant whirlpool with a diameter of up to four times the earth and about 900 kilometers high,” explains co-author Bilal Benmahi, also from the Laboratoire d’Astrophysique de Bordeaux. “A vortex of this size would be a unique meteorological animal in our solar system,” Cavalié adds.

Astronomers were aware of strong winds near Jupiter’s poles, but much higher in the atmosphere, hundreds of kilometers above the focus area of ​​the new study, which took place today in Astronomy & Astrophysics. Previous studies have predicted that these winds in the upper atmosphere will decrease in velocity and disappear well before reaching as deep as the stratosphere. “The new ALMA data tells us the opposite,” Cavalié says, adding that finding these strong stratospheric winds near Jupiter’s poles was a “real surprise.”

The team used 42 of ALMA’s 66 high-precision antennas in the Atacama Desert in northern Chile to analyze the hydrogen cyanide molecules that have been orbiting Jupiter’s stratosphere since the impact of Shoemaker-Levy 9. The ALMA data enabled them to measure the Doppler shift – small changes in the frequency of the radiation emitted by the molecules – caused by the winds in this region of the planet. “By measuring this shift, we were able to deduce the speed of the winds, just as one could deduce the speed of a passing train by changing the frequency of the train whistle,” explains co-author of the study, Vincent Hue, a planetary scientist. at the Southwest Research Institute in the USA.

In addition to the surprising polar winds, the team also used ALMA to confirm the existence of strong stratospheric winds around the equator of the planet by measuring their speed directly, also for the first time. The jets observed in this part of the planet have an average velocity of about 600 kilometers per hour.

The ALMA observations to detect stratospheric winds in both the poles and the equator of Jupiter took less than 30 minutes of telescope time. “The high levels of detail we have reached in this short time really show the power of the ALMA observations,” said Thomas Greathouse, a scientist at the Southwest Research Institute in the USA and co-author of the study. “It’s amazing to me to see the first direct measurement of these winds.”

“These ALMA results open a new window for the study of the auroral regions of Jupiter, which was really unexpected a few months ago,” says Cavalié. “They also set the stage for similar but more extensive measurements to be made by the JUICE mission and its Submillimeter Wave Instrument,” Greathouse added, referring to the European Space Agency’s JUpiter ICy-moon Explorer, which is expected will start in space next year.

This research is presented in the paper ‘First direct measurement of auroral and equatorial jets in the stratosphere of Jupiter’ published today in Astronomy & Astrophysics.