Free Astronomy Magazine July-August 2021

19 JULY-AUGUST 2021 ASTRO PUBLISHING 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 kilome- tres in height,” explains co-author Bilal Benmahi, also of the Labora- toire 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, hun- dreds of kilometres above the focus area of the new study, published in Astronomy & Astrophysics . Previous studies predicted that these upper- atmosphere winds would decrease in velocity and disappear well be- fore reaching as deep as the strato- sphere. “The new ALMA data tell us the contrary,” says Cavalié, adding that finding these strong stratos- pheric winds near Jupiter’s poles was a “real surprise”. The team used 42 of ALMA’s 66 high-precision antennas, located in the Ata- cama Desert in northern Chile, to analyse the h y d r o g e n cyanide mole- cules that have been moving around in Jupiter’s strato- sphere since the impact of Shoe- maker–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 de- duce the speed of a passing train by the change in the frequency of the train whistle,” explains study co-au- thor Vincent Hue, a planetary scien- tist 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 meas- uring their speed, also for the first time. The jets spotted in this part of the planet have average speeds of about 600 kilometres an hour. The ALMA observations required to track stratospheric winds in both the poles and equator of Jupiter took less than 30 minutes of tele- scope time. “The high levels of de- tail we achieved in this short time really 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 really unexpected just a few months back,” says Cavalié. “They also set the stage for similar yet more ex- tensive measurements to be made by the JUICE mission and its Submil- limetre Wave Instrument,” Great- house adds, referring to the Eu- ropean 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 ex- plore Jupiter. The telescope will be capable of making highly detailed observations of the planet’s auro- rae, giving us further insight into Jupiter’s atmosphere. A n artist’s impression of winds in Jupiter’s stratosphere near the planet’s south pole, with the blue lines representing wind speeds. These lines are superimposed on a real image of Jupiter, taken by the JunoCam imager aboard NASA’s Juno spacecraft. [ESO/L. Calçada & NASA/JPL-Caltech/SwRI/MSSS] !