A rather different storm on Jupiter
Juno finds things have changed at the south pole.
By Richard A Lovett
NASA’s Jupiter-orbiting Juno spacecraft has discovered a new giant storm on the planet.
That in itself isn’t surprising – Jupiter has many giant storms. But this storm has shouldered its way into a ring of storms at Jupiter’s south pole, converting a tidy pentagon of evenly spaced storms into an equally tidy hexagon.
“One of the very first things we discovered at Jupiter is that the poles don’t look like the rest of the planet,” Steve Levin of NASA’s Jet Propulsion Laboratory told a meeting of the American Geophysical Union.
When Juno arrived in 2016, he adds, one of its finds was that the south pole had a giant central storm surrounded by a ring of five others. “Now we have six,” he says.
The new storm, says Levin’s colleague Alessandro Mura, from Italy’s National Institute for Astrophysics, muscled its way in from the outside, forcing the others to rearrange to accommodate it, sometime during Juno’s latest orbit, which only brings it into viewing range close to the pole once every 53 days.
“It’s like having a family where there is a mother in the center and [now] there is a new baby brother,” Mura says – a particularly apt analogy because the new storm is currently smaller than its kindred.
When the original storms, all of them about 7000 kilometres across, were first discovered (the new storm is about one-third as big), scientists were amazed by their symmetrical arrangement. But the recent change is even more amazing.
“This is the first time we have seen a symmetrical structure in the Solar System changing itself – [in this case] from a pentagon to a hexagon,” Mura says.
Furthermore, says Cheng Li, a fluid dynamicist at the California Institute of Technology, from a fluid dynamics perspective neither arrangement of storms should even exist.
Instead, the storms should all have merged into a single, larger storm. “There should be one big vortex at the pole,” he says.
What appears to be going on, he says, is that there are buffer zones between the storms, spinning in the opposite direction from the storms themselves, “like two gears that spin together, but in different directions”.
These buffer zones not only prevent the original storms from merging, they made it easier for the new one to join the group. “[When] the intruder moves in, the original five are happy to welcome it,” Li says. “It opens up a gap and the intruder becomes part of the family.”
Ironically, the polar ring’s change might not have been discovered if the Juno mission had not had an engine malfunction that stranded it in its present 53-day orbit instead of shortening it to a 14-day cycle, as originally planned.
That’s because each close approach to Jupiter brings the spacecraft into Jupiter’s intense radiation belts – something the spacecraft can only survive a limited number of times.
By accidentally stretching out the mission by increasing the time between close approaches, the engine malfunction has had the side effect of allowing Jupiter to be viewed over a longer period of time, opening the window to see changes such as the addition of the new storm to the polar ring.
“For a lot of things it’s a great gift,” Levin says. “Anything that’s changing with time we have many more years to see.”