An international team of astronomers just measured Jupiter’s raging stratospheric winds for the very first time—and they used a 27-year-old comet to do it.
Scientists had already measured wind speeds down in Jupiter’s troposphere—where the planet’s iconic stripes lie—and way up in its ionosphere. But this new study was first to take wind speed measurements of Jupiter’s stratosphere using the incredibly sensitive Atacama Large Millimeter/submillimeter Array (ALMA). They measured wind speeds near the equator and near the poles.
Some results weren’t too surprising—they found that speeds at the equator were roughly what models had predicted. “But what was completely unexpected is what we saw near the poles,” says study-author Thibault Cavalié, a planetary scientist at the Laboratoire d’Astrophysique de Bordeaux who led the experiment. The team found 300- to 400-meter-per-second winds— roughly 700 to 900 miles per hour—whipping across the poles in unanticipated directions.
“It’s a really hard observation,” says Imke de Pater, a planetary scientist at Berkeley who has used ALMA previously but was not part of this study. She adds, “it’s a really great paper, they really show very nicely these wind profiles in the … upper atmosphere.”
Jupiter’s winds almost exclusively go eastward or westward, as we see in the planet’s trademark red and white horizontal stripes. This rule holds for the troposphere—save for vortices like Jupiter’s red eye, where winds swirl like a hurricane. In the stratospheric layer above however, the winds instead appear to follow the shape of Jupiter’s auroral rings, which, like Earth’s northern lights, result from its magnetic field steering solar winds to the poles. Those auroral rings aren’t perfectly lined up with the poles, so the wind flow doesn’t match the neat bands of the troposphere.
The unusual polar wind patterns trekking north and south instead of staying east and west are “really mind boggling,” says Glenn Orton, senior research scientist and observational astronomer at NASA’s Jet Propulsion Laboratory, who wasn’t involved in the study.
For decades, the easiest way to figure out planetary wind speeds was to simply take a snapshot of the planet, then another one some time later and see how far the clouds moved between the two frames, Cavalié says. But at higher altitudes this doesn’t work, because the winds are invisible. There are no clouds to track.
But ever since the Shoemaker-Levy 9 comet impact on Jupiter back in 1994, researchers have kept tabs on two compounds the object delivered there: hydrogen cyanide and carbon monoxide. Both chemicals are long-lived, and they’re still floating around in the jovian atmosphere. The team was able to trace the unique spectral fingerprints of the hydrogen cyanide and carbon monoxide. Since they could track winds using the movement of clouds, perhaps they could use these molecules to do the same.
To do so, the team first pinpointed both types of molecules by detecting their frequencies. Then, they made use of something called the doppler effect, which means those frequencies change depending on if the molecules are moving toward us, or away from us. So on Jupiter, as the molecules blew towards the telescope, they would produce slightly different spectral signals than those moving away. By measuring the difference—how much the frequencies got bumped—the team could measure the speed at which the molecules (and the wind) were moving.
In the future, Cavalié says, space telescopes may be able to learn more from the water deposited by the comet’s impact, because water is such a rare molecule on Jupiter. The study is also a stepping stone, he says, for the European Space Agency’s JUpiter ICy moons Explorer (JUICE) mission which plans to launch next year. That craft will get close looks at Jupiter and three of its moons and be the first to orbit Ganymede—the largest moon in the solar system.
Cavalié was 12 years old when the Shoemaker-Levy 9 comet hit Jupiter—a little young to be part of that observation. But he says the event nudged him towards a career in planetary science.
Years later, the comet is still making its mark.
