Astronomers directly measured winds in Jupiter’s middle atmosphere

Powerful stratospheric winds measured on Jupiter for the first time.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, a team of astronomers have directly measured winds in Jupiter’s middle atmosphere for the first time. By analyzing the aftermath of a comet collision from the 1990s, the researchers have revealed incredibly powerful winds, with speeds of up to 1450 kilometers an hour, near Jupiter’s poles. They could represent what the team has described as a “unique meteorological beast in our Solar System.”

Jupiter is famous for its distinctive red and white bands: swirling clouds of moving gas that astronomers traditionally use to track winds in Jupiter’s lower atmosphere. Astronomers have also seen, near Jupiter’s poles, the vivid glow known as aurorae, which appear to be associated with strong winds in the planet’s upper atmosphere. But until now, researchers had 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 because of the absence of clouds in this part of the atmosphere. However, astronomers were provided with an alternative measuring aid in comet Shoemaker-Levy 9, which collided with the gas giant spectacularly in 1994. This impact produced new molecules in Jupiter’s stratosphere, where they have been moving with the winds ever since. 

A team of astronomers, led by Thibault Cavalié of the Laboratoire d’Astrophysique de Bordeaux in France, have now tracked one of these molecules — hydrogen cyanide — to directly measure stratospheric “jets” on Jupiter. Scientists use the word “jets” to refer to narrow bands of wind in the atmosphere, like Earth‘s jet streams.

This image, taken with the MPG/ESO 2.2-metre telescope and the IRAC instrument, shows comet Shoemaker–Levy 9 impacting Jupiter in July 1994
This image, taken with the MPG/ESO 2.2-metre telescope and the IRAC instrument, shows comet Shoemaker–Levy 9 impacting Jupiter in July 1994. Credit: ESO

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

“Our detection indicates that these jets could behave like a giant vortex with a diameter of up to four times that of Earth and some 900 kilometers in height,” explains co-author Bilal Benmahi, also of the Laboratoire d’Astrophysique de Bordeaux. “A vortex of this size would be a unique meteorological beast in our Solar System,” Cavalié adds.

Astronomers were aware of strong winds near Jupiter’s poles but much higher up in the atmosphere, hundreds of kilometers above the new study’s focus area, which is published today in Astronomy & Astrophysics. Previous studies predicted that these upper-atmosphere winds would decrease velocity and disappear well before reaching as deep as the stratosphere. “The new ALMA data tell us the contrary,” says Cavalié, 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, located in the Atacama Desert in northern Chile, to analyze the hydrogen cyanide molecules moving around in Jupiter’s stratosphere since the impact of Shoemaker-Levy 9. The ALMA data allowed them to measure the Doppler shift — tiny 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 much like one could deduce the speed of a passing train by the change in the frequency of the train whistle,” explains study co-author Vincent Hue, a planetary scientist at the Southwest Research Institute in the US.

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

The ALMA observations required to track stratospheric winds in both the poles and equator of Jupiter took less than 30 minutes of telescope time. “The high levels of detail we achieved in this short time demonstrate the power of the ALMA observations,” says Thomas Greathouse, a scientist at the Southwest Research Institute in the US and co-author of the study. “It is astounding to me to see the first direct measurement of these winds.”

“These ALMA results open a new window for the study of Jupiter’s auroral regions, which was unexpected just a few months back,” says Cavalié. “They also set the stage for similar yet more extensive measurements to be made by the JUICE mission and its Submillimetre Wave Instrument,” Greathouse adds, referring to the European Space Agency’s Jupiter Icy moons Explorer, which is expected to launch into space next year.

ESO’s ground-based Extremely Large Telescope (ELT), set to see first light later this decade, will also explore Jupiter. The telescope will be capable of making highly detailed observations of the planet’s aurorae, giving us further insight into Jupiter’s atmosphere.

Journal Reference:
  1. T. Cavalié et al. First direct measurement of auroral and equatorial jets in the stratosphere of Jupiter.

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