Juno’s marching orders — to pick the lock on Jupiter’s secrets — have been spectacularly fulfilled. And by the summer of 2020, it still had at least a year of science observations to go.
Juno achieved rough, global coverage of the giant planet by the end of 2018, but at a coarse resolution; it then began a new set of orbits to fill in the details. The spacecraft’s long, looping orbits are meant to keep it mostly clear of the doughnut-shaped belts of harmful radiation close to Jupiter and its moons. Every 53 days, however, Juno makes a close pass to observe Jupiter’s gigantic clouds and titanic storms, crackling with lightning. Powerful pulses of energy — super-charged particles streaming through Jupiter’s magnetic field — create auroras. They’re similar to the northern and southern lights on Earth, that is, if you can imagine scaling them up to the size of a planet that could hold 1,300 Earths.
The atmosphere: The spacecraft took deep soundings of Jupiter’s atmosphere, giving a vertical profile of clouds that can plunge nearly 2,000 miles (3,000 kilometers) into Jupiter’s depths. Before Juno, no one knew how deep Jupiter’s cloud bands went. These familiar cloud bands, or jet streams, visible from Earth as orange and white stripes, move alternately in opposing directions, west and east. Stranger still: beneath these layers, the interior of Jupiter shows a completely different pattern, rotating as a single, rigid body.
Juno also found a more watery atmosphere than might have been expected, based on results from an earlier probe. The Galileo spacecraft dropped a probe into Jupiter’s atmosphere in 1995, finding it to be extremely dry. Juno’s microwave radiometer instrument, however, found much more water in Jupiter’s equatorial regions — about 0.25 percent of the molecules in Jupiter’s atmosphere, or three times that of the Sun (as measured by water’s molecular components, not liquid water itself).
The auroras: The Juno science team expected to find powerful auroras on Jupiter, driven by strong currents of electrical energy flowing along the invisible lines of the planet's magnetic field. But they didn’t expect a strange kind of disconnect: the most intense auroras on Jupiter are not driven by the biggest of those energies. These reach as high as 400,000 electron volts, whereas on Earth, it takes only a few thousand volts to produce the writhing, shimmering light show we call the northern lights.
Electrical energy does play a role in Jupiter’s auroras — slamming electrons, or negatively charged particles, into Jupiter’s atmosphere near its poles. But for the most intense of these auroras, some other process — as yet unidentified by scientists — takes over.
The magnetic field: A kitchen magnet has a magnetic field, with invisible “field lines” — which you can make visible by holding it under a piece of paper and sprinkling iron filings on top. Planets have magnetic fields as well, and Jupiter’s is a whopper. Many aspects of these fields remain mysterious, even on Earth. And scientists have been waiting for decades to find out if Jupiter’s field possesses a dynamic quality: does it vary over time, and if so, what drives the variation? Score two more firsts for Juno.
As if conducting a giant iron-filing experiment, Juno’s magnetometer made a 3D map of Jupiter’s magnetic field over the spacecraft’s multiple science passes. Comparing the data to measurements from past probes — Pioneers 10 and 11, Voyager 1 and Ulysses — and with the help of a computer model, the science team showed that this internal magnetic field had indeed changed subtly over time. They also found a zone of especially intense variation in the field (scientists call this "secular variation") dubbed the Great Blue Spot. This, combined with Jupiter’s powerful jet streams, extending more than 1,800 miles (3,000 kilometers) deep, are believed to shear and stretch Jupiter’s magnetic field lines as they push them around the planet. These findings could help shed light on unknown aspects of planetary magnetic fields, including Earth’s.
The poles: Almost from the moment Juno arrived in polar orbit around Jupiter, it’s been sending back stunning pictures and measurements of the planet's polar regions. In its first look at the north pole, JunoCam revealed a bluish expanse dotted with raging storms as wide as Earth . It’s like nothing else in our solar system. The discoveries included a system of nine cyclones at the north pole and six in the south. The southern group looked like a stable configuration, but on later passes, Juno detected a brand-new, seventh storm that had joined the fray — even if a bit smaller than its siblings.
Juno’s Jovian Infrared Auroral Mapper (JIRAM) also examined the north and south poles in infrared light, detecting warm and hot spots, as well as the southern auroras.