NASA’s Juno probe has been orbiting Jupiter since 5 July 2016, swooping past the gas giant once every 53 days. By combining the data collected during its close passes – or ‘perijoves’ – the Juno science team is working to reveal the world beneath the planet’s cloud tops.
Previous missions have only been able to see the top few hundred kilometres of Jupiter’s atmosphere, a region striped with white zones and brown belts.
But Juno is carrying several instruments that are capable of seeing beyond this layer and peering down into the planet beneath.
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The Microwave Radiometer, for instance, observes using wavelengths of light that can make the clouds appear transparent.
“What Juno’s revealed is that what we see at the cloud tops is just the tip of the iceberg,” says Dr Leigh Fletcher from the University of Leicester and a participating scientist on Juno.
“Below the cloud tops are these enormous structures that are responsible for what we see going on higher up. The exact mechanism of how what’s going on in the atmosphere is connected to what’s going on down at depth is still to be resolved, but if there’s any spacecraft that’s can do that, it’s Juno.”
Juno also has a gravity instrument that measures how the planet tugs on the spacecraft.
If the gravity is stronger in one place, then it means there’s something massive or dense beneath the visible clouds in that region.
By looking out for subtle changes in the gravitational pull, researchers can create a density map of the planet.
Juno now has enough passes under its belt to start building up a global image of the gas giant.
The detail improves with each perijove and is beginning to reveal the planet hidden beneath the clouds.
“We’ve determined that the winds you see at the cloud tops go down about 2,000–3,000km towards the inside of the planet. ” says Dr Fletcher.
“Remember we’re talking about a planet that is over 70,000km in radius, so the winds of Jupiter are really only the outer shell of the planet.”
The spacecraft also is also equipped with a magnetometer, which is mapping the gas giant’s immense magnetic field.
From the first orbit around Jupiter, Juno detected that the planet’s field was much stronger than expected. And now that it’s gathered enough data to begin building a map, a strange feature is beginning to emerge.
“People have been referring to it as the Great Blue Spot,” explains Dr Fletcher. “It’s not a great storm, like the Great Red Spot, but a region where all these magnetic field lines seem to diverge or converge, creating a strange pattern within the deep insides of Jupiter.”
The field around the Great Blue Spot also seems to be changing over time, making Jupiter the only planet other than Earth where we’ve seen a dynamic magnetic field.
Observing how the ever-changing Jovian atmosphere evolves is one of the main goals of the Juno mission.
It’s hoped the probe’s unprecedented views of the planet will add to the decades of observations that have tracked how it changes during its long, 12-year orbit of the Sun, and inform the other changes noticed over longer timescales too.
For instance, Fletcher’s team noticed that every six to seven years, an event known as a disturbance cycle caused the white haze found at high altitude to disappear, revealing the dark brown clouds beneath.
These cycles create large streaks across the planet’s equatorial zone and remain visible for many months, and according to the six-to-seven-year pattern, the next one was due to begin in 2019.
“Earth-based Jupiter observers are getting very excited at the moment because the event is actually in full swing. So the prediction that we would see one of these equatorial disturbances turned out to be correct,” says Dr Fletcher.
The timing was perfect. Not only is Juno in prime position to get a close-up look, but the event began just as Jupiter came into opposition, the point when the planet is at its closest to Earth and the best images can be taken.
The Juno team has been collaborating with professional and amateur astronomers all over the world who are observing Jupiter.
Many of these Earth-based astronomers are supplying views of the planet in light wavelengths that Juno can’t detect.
Telescopes used by professionals, such as the Hubble Space Telescope and the Very Large Array, deal with light at far ends of the spectrum (infrared and ultraviolet), while the visible part of the spectrum is covered by amateur astronomers.
And although this latter group might not have access to the multi-metre telescopes that the professionals do, what they do have is dedication.
“The amateurs are out in their back gardens, night after night, taking images of Jupiter for us. It helps us track the various atmospheric features on the planet and allows us to target JunoCam’s observations to particular features of interest,” explains Dr Fletcher.
JunoCam, the spacecraft’s only visible light camera, was never intended as a scientific instrument.
Instead, it provides image data from its unprecedented viewpoint, which is immediately made available for anyone to process.
There is currently a thriving community of volunteers, with skills ranging from image editing to mathematical expertise, working together to turn the raw data being gathered by the JunoCam into beautiful and scientifically useful pictures.
“The JunoCam team itself is tiny. They don’t have the resources to process the images themselves, so this has been a very successful exercise in getting all these very passionate and talented people on board and feeling like one big team working together,” says Dr Fletcher.
The spectacular JunoCam images have helped to keep the mission in the public eye, despite it only being intended to last for the first few months, as the belts of extreme radiation that surround Jupiter can rapidly destroy sensitive components.
To avoid constant exposure, Juno follows a highly elliptical orbit, so the spacecraft only dips into the deadly region for a few hours every 53 days.
Even with this precaution, JunoCam was only expected to last through a few close passes. But three years later, it’s still going strong.
The unusual orbit has the added benefit of giving Juno an unusual perspective on the planet, sweeping from top to bottom and allowing it to look down on the poles.
From that perspective, Jupiter looks completely different. Gone are the familiar belts, replaced by giant storms.
In the north, eight cyclones encircle the pole, while the South Pole has only five.
“[At the poles] we enter a completely different regime that’s dominated by these small clusters of storms and enormous cyclones,” points out Dr Fletcher.
“Trying to come up with a theory of how you go from the ordered belts to these storms is going to keep atmospheric physicists scratching their heads for some time to come.”
Time, fuel and money
Juno will continue to observe the planet for as long as it’s able to. The main mission will come to an end in July 2021, but there is the chance to extend it if the spacecraft is still healthy.
“Although the radiation environment was seen as being the thing that would limit the time of the spacecraft, that doesn’t seem to be the case,” says Dr Fletcher. “The radiation environment just hasn’t been as damaging as anticipated.”
Although Juno’s engine – built by British company Moog-ISP (now part of Nammo Westcott) – uses propellant to manoeuvre, Juno’s orbit means that it’s unlikely to run out, as happened to the Saturn explorer, Cassini.
Instead, Juno will probably fall victim to the most pedestrian of ends: budget cuts.
The team behind Juno will continue to fight to keep the mission running for as long as possible.
With every pass, the spacecraft continues to refine our view of Jupiter and provide new insights into the constantly evolving atmosphere of this giant planet.
Jupiter’s shrinking Great Red Spot
The Great Red Spot has been a familiar fixture on Jupiter for centuries.
The swirling maelstrom is caught between the South Equatorial Belt to its north, and a white zone to its south, and spans a distance 1.3 times Earth’s diameter.
Juno has spent several of its passes looking at the Spot. The team hopes its gravitational maps will eventually reveal how far down the storm extends, giving an insight into how the storm has been raging for so long.
But could the storm be about to blow itself out?
The Spot has been getting smaller for many years, but this shrinking sped up dramatically in May 2019.
By mid-June, analysis of amateur images showed the Spot’s size had reduced by 3,000km – around 20 per cent of its overall size.
Fragments of the storm, referred to as blades or flakes, are breaking off the main storm and melting away into the surrounding belts and zones.
Rivers of dark material also appear to be flowing from the Spot, making it appear as though the storm is unravelling.
This is the first time such activity has been seen around the Spot and astronomers, both amateur and professional, are watching with avid interest.
The next look at Jupiter
Though Juno is expected to survive beyond the end of its nominal mission in 2021, the spacecraft can’t last forever.
Fortunately, the European Space Agency (ESA) is already designing the follow up mission – the Jupiter Icy Moons Explorer (JUICE).
As the name suggests, this mission’s priority is to look at Jupiter’s satellites, but it will still examine the gas giant.
“They are two very different missions,” says Dr Leigh Fletcher, a member of the Juno team. “But JUICE is designed to be complementary to Juno. JUICE will be looking at the cloud tops and upwards, whereas Juno focuses on the cloud tops downwards.”
It does mean that if Juno does make a discovery, there’s already a mission in development that should be able to take over from Juno when it reaches the planet in just over a decade’s time.
“A great example is the cyclones at the North and South Pole,” says Dr Fletcher. “We’re only going to be able to look at those for a handful of years with the Juno spacecraft.
“Fast forward to a decade from now and JUICE will be there and able to tell us whether those cyclones still exist. It will reveal if these are long-lived features of the Jovian atmosphere, or whether they’re something that comes and goes with time.”