A JunoCam’s-eye view of Jupiter’s south pole, in enhanced colour NASA/JPL-Caltech/SwRI/MSSS/John Landino

Big planets come with big surprises. Last week, delegates at the annual European Geosciences Union meeting got the first glimpse of data from the Juno spacecraft now in orbit around Jupiter, and the findings are already challenging assumptions about everything from the planet’s atmosphere to its interior.

“The whole inside of Jupiter is just working differently than our models expected,” said mission principal investigator Scott Bolton of the Southwest Research Institute in Texas.

Launched on 5 August 2011, Juno reached Jupiter and began its first orbit on 4 July last year. Since then, it has performed four more circuits. There are 33 planned pole-to-pole circuits in all, encircling the entire planet bit by bit.


The findings presented in Vienna come from these first few circuits, which each last 53 Earth days and include a 6-hour scan of the planet from north to south. Although the information is preliminary, the researchers involved are thrilled.

Ammonia weather, fuzzy core

Much of the excitement centres on the discovery of a dense zone of ammonia gas around Jupiter’s equator, plus other regions where ammonia is depleted, which together suggest an ammonia-based weather system. We have long known that Jupiter is completely shrouded in ammonia clouds, but the existence of such a deep “belt” is surprising.

“We’ve known there’s a spike at the equator, but the new microwave data is showing that the spike goes way, way down into the abyss, 300 kilometres below the cloud,” says Leigh Fletcher of the University of Leicester, UK, who was not involved in the work. “It suggests ammonia is being distributed by a weather system that penetrates much deeper than anyone expected.”

The findings are also challenging models of what’s inside the planet. We had assumed Jupiter has a uniform interior, with a shallow “crust” of liquid hydrogen overlying a thin layer where helium rains down. Under that is a much deeper layer of metallic hydrogen, with a smaller solid core around 70,000 kilometres down. Those assumptions were based on mapping the planet’s gravity.

But initial gravity measurements from Juno challenge the idea that the internal layers inside are completely regular in their make-up. “Jupiter’s molecular envelope is not uniform,” said Tristan Guillot of the University of the Cote d’Azur in France. “We assumed we could treat the envelope as global, but now, with the finer data, it appears less regular.”

Fletcher says it points to a core that is not solid like Earth’s, but “fuzzy” and dilutely mingled with the overlying metallic hydrogen layer.

Massive magnetism

Another shock is that Jupiter’s huge magnetic field is even stronger and much more irregular than expected. The irregularity of the field so far is a sign that the dynamo driving it may originate higher up in Jupiter’s interior, perhaps from a layer of metallic hydrogen.

“I didn’t expect all the theories to be wrong, but there’s motion going on in the planet we did not anticipate,” Bolton said.

Jupiter’s magnetic field also dwarfs the 0.25 to 0.65 gauss at Earth’s surface by an even bigger margin than we expected. Juno readings on its closest approaches so far, presented by Jack Connerney of the NASA Goddard Space Flight Center in Maryland, suggest it could be 8 to 9 gauss rather than the 5 gauss predicted.

More tantalisingly, Juno’s magnetometers found that the field dipped in other regions, a telltale sign that the dynamo driving the field is close to the surface over the entire planet, not buried deep within it like Earth’s core.

“Jupiter’s magnetic field is spatially complex, and there were deficits of up to 2 gauss elsewhere,” said Connerney. “We may need many more orbits to resolve this.”

Earth-sized cyclones

The first orbits have also produced several new insights into the planet’s atmosphere. The probe’s JunoCam camera has already sent back amazing pictures of hitherto unknown cyclones over the poles.

Glenn Orton of the Jet Propulsion Laboratory in Pasadena, California, who helps manage the JunoCam website, showed stunning composite videos of the cyclones swirling. “They’re the size of Earth, or maybe half an Earth,” Orton told New Scientist. “They’re probably composed of condensed ammonia.”

Strange white ovals have been spotted, too, in belts south of Jupiter’s equator. They could be clouds containing ammonia and hydrazine, a substance used as rocket fuel on Earth, according to an analysis of Juno infrared radiation readings presented by Alberto Adriani of the Institute for Space Astrophysics and Planetology in Rome.

Adriani also presented stunning infrared images of the auroras which occur daily at the poles. His analyses revealed that the areas where they glow are composed mainly of methane and an ion containing three hydrogen atoms (H 3 +), at temperatures ranging from 500 to 950 kelvin. Adriani’s composite movies of the auroras – not released to the public yet – were equalled by others showing similar features imaged with ultraviolet spectrometers, presented by Bertrand Bonfond of the University of Liège in Belgium.

The camera is proving tougher than expected, too. Fears that it would last just a dozen circuits because of the battering from Jupiter’s intense radiation have turned out to be misplaced. “The good news is radiation damage so far is almost negligible, so it will operate for many years,” Orton said.

And more data will arrive after the next closest approach on 19 May. Eventually, Juno will fly over Jupiter’s famous Great Red Spot, and Fletcher is excited about the data that will generate. “It means that for the first time, we can go down deep and find out what’s going on underneath,” he says.