NASA’s Juno spacecraft has found breathtaking cyclones at Jupiter’s poles and probed how deep the planet’s jet streams go.
I’ll admit, I’ve always found Jupiter a wee drab. Sure it has the four Galilean moons trotting around it and its Great Red Spot is wider than the whole planet Earth, but really, it’s just a big ball of gas.
Juno has knocked me out of my indifference. The NASA spacecraft has been zipping around the king of the planets in a big elongated loop since July 2016, at its closest approaches whizzing 67 km/s (150,000 mph) over the cloudtops as it flies from pole to pole in two hours. Juno’s data have shown us a Jupiter we’ve never seen before. It has spotted white clouds dancing on the highest crests, casting shadows on the surrounding atmosphere. Deep down, it’s found that Jupiter’s “core” of heavier elements is actually fuzzy, dissolved in the surrounding hydrogen and helium (sorry, Arthur C. Clarke). It’s also given us our first look at the planet’s poles, bizarre bluish regions (yes, they’re actually bluish — it’s not just image processing) with breathtaking cyclones that startled planetary scientists.
Four papers published in the March 7th Nature give us the latest look at Juno’s Jupiter, revealing details of the giant planet’s gravity, internal structure and motions, and its poles.
Astounding Polar Storms
By far, the results that take my breath away are the ones for the poles. Alberto Adriani (INAF Institute for Space Astrophysics and Planetology, Italy) and colleagues report the discovery of two stunning sets of cyclones, one at each pole. Cyclones are low-pressure circulation systems, which rotate counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. (The Great Red Spot, in contrast, is an anticyclone, which means its central “air” pressure is higher than its surroundings and it rotates in the opposite direction — in this case, counterclockwise.)
The complex at Jupiter’s north pole has a central, 4,000-km-wide cyclone, with a big ring of eight, regularly spaced storms of similar size around it. The ring of eight subtly splits into two alternating sets, with the cyclones that are slightly farther from the central one having distinct spiral arms and the ones slightly closer to the center looking more turbulent.
The south pole has fewer, larger storms than the north pole. The central cyclone sits in the center of an imperfect pentagon of five others, each between 5,600 to 7,000 km wide (about half of Earth’s diameter). The central cyclone and two of those in the pentagon have distinct cloud spirals; the other three are more chaotic.
“It looks like an alien whirlpool,” principal investigator Scott Bolton (Southwest Research Institute, South Antonio) said in January when previewing these images at the winter American Astronomical Society meeting in Washington, D.C.
The individual storms each rotate every 27 to 60 hours, at hundreds of kilometers per hour. Yet both patterns appear to stay put or, potentially, drift around the central cyclones, with little change over the 7 months spanned by Juno’s observations. The team doesn’t know why the patterns are so stationary, or why the cyclones don’t merge.
It also remains unclear whether the cyclones formed at the poles or migrated there from elsewhere. The east-west flows that slice lower latitudes into horizontal strips weaken at higher latitudes, replaced with turbulence that, influenced by the planet’s rotation, would give birth to cyclones. Conversely, that same rotational pseudoforce, called the Coriolis effect, tends to nudge whirling cloud systems away from the equator and might encourage Jupiter’s cyclones to migrate poleward and collect there.
Deep Down in Jupiter
The other three Juno papers deal with Jupiter’s gravity field, its interior, and motions in its atmosphere. The team found that the planet’s gravitational pull on the spacecraft is not globally symmetric but instead varies from region to region, notably between the northern and southern hemispheres. These variations are caused by jet streams in the atmosphere: the biggest ones flow around high- and low-pressure regions and push sections of atmosphere around, making some parts denser and others less so. Denser regions exert a stronger gravitational pull on the spacecraft.
We already knew that Jupiter rotates differentially, which means that the equator moves at a different speed than the poles. (This behavior shows up elsewhere in the solar system, including on Saturn.) But we didn’t know how deep the behavior goes. Different ways of studying the atmospheric motions all point to the same conclusion: the planet’s jet streams reach some 3,000 km down, far deeper than many scientists expected. This differentially rotating jet-stream layer contains about 1% of the planet’s total mass. Below this layer of atmosphere, the planet appears to rotate more like a solid ball.
“Having winds go down to 3,000 km is a big deal,” explains planetary-atmospheres expert Andrew Ingersoll (Caltech). The discovery may settle a decades-long debate between two very different models for what causes the motions in the gas giant’s atmosphere. Jupiter’s weather layer — the part where sunlight is absorbed and clouds form — is only about 100 km deep, yet the atmospheric flows below the familiar light zones and dark belts plunge 30 times deeper. That favors a longstanding theory for Jupiter’s interior in which the jet streams form a series of nested cylinders, like a roll of toilet paper that’s been carefully carved into a sphere (but with fewer layers). Each differentially rotating latitude band corresponds to a different layer in the nest, with higher latitudes corresponding to deeper cylinders. Bolton says they’re still working on the cylinder picture, though, and things don’t quite match the original idea.
So why do the jet streams die out 3,000 km down? One of the teams reporting in Nature, led by Tristan Guillot (Université Côte d’Azur, France), suggests that, at this depth, the pressure is so high that the molecular hydrogen becomes ionized and thus susceptible to electromagnetic forces. So the flows drag on one another magnetically and force everything to rotate en masse as a big, rigid ball.
The interior’s electrical conductivity depends on pressure and, in turn, the planet’s mass. Juno’s results thus suggest less-massive Saturn might transition to rigid rotation three times deeper down than Jupiter does, while brown dwarfs would have shallower differentially rotating envelopes.
There are still more results forthcoming, including widespread lightning (predominantly in the northern hemisphere), and many lingering questions such as how deep the Great Red Spot goes. Preliminary data only show that the iconic storm reaches as deep as the microwave instrument can penetrate, which is a few hundred kilometers, Bolton says. In the meantime, I’ll leave you with this simulated flyover video of Jupiter’s cloudtops. It combines real JunoCam observations with computer simulations.
A. Adriani et al. “Clusters of cyclones encircling Jupiter’s poles.” Nature. March 8, 2018.
T. Guillot et al. “A suppression of differential rotation in Jupiter’s deep interior.” Nature. March 8, 2018.
L. Iess et al. “Measurement of Jupiter’s asymmetric gravity field.” Nature. March 8, 2018.
Y. Kaspi et al. “Jupiter’s atmospheric jet streams extend thousands of kilometres deep.” Nature. March 8, 2018.