Juno peers below Jupiter's clouds

By Jonathan Amos
BBC Science Correspondent

image copyrightNASA/JPL-Caltech/SwRI/MSSS/BA.Hall/G.Robles
image captionSouth Pole: The polar cloud structure at Jupiter is very different from that at Saturn

Scientists working on the American space agency's new Juno mission say its initial observations at Jupiter have taken their breath away.

In particular, they have been amazed by the storms seen at the planet's poles.

"Think of a bunch of hurricanes, every one the size of the Earth, all packed so close together that each hurricane touches the other," said Mike Janssen.

"Even in rooms of hardened researchers, these images of swirling clouds have drawn gasps," the Nasa man added.

image copyrightNASA/SWRI/MSSS/G.Eichstädt/S.Doran
image captionMany of the best images are being processed by citizen scientists

The Juno probe arrived at the fifth planet from the Sun on 4 July last year. Since then, it has been making a close pass over the gas giant every 53 days.

The first data to come out of these observations are now being reported in two papers (here and here) in the journal Science, and in more than 40 others in a special collection for Geophysical Research Letters.

The mission team says that in nearly all instances, previously cherished theories about how Jupiter works are being challenged.

"We're getting the first really close up and personal look at Jupiter and we're seeing that a lot of our ideas were incorrect and maybe naive," said Juno principal investigator Scott Bolton from the Southwest Research Institute in San Antonio, Texas.

Those big cyclones that cover the highest latitudes of the planet are only now being seen in detail because previous missions to the planet never really got to look from above and below like Juno - certainly not at such a high resolution. Features down to 50km across can be discerned.

The structures are very different from those seen at Saturn's poles, for example, and the team will have to explain why. It is also not clear at this stage how long-lived they are. Will they dissipate much faster than the storms at lower latitudes, which in some cases - as with the famous Great Red Spot on Jupiter - have persisted for centuries?

image copyrightNASA/SWRI/MSSS/G.Eichstädt/S.Doran
image captionThe white spots in this image are ice clouds about 50km across

Another surprise comes from Juno's Microwave Radiometer (MWR), which senses behaviour below the visible cloud surface. Its data indicates the presence of a broad band of ammonia around the equator that goes from the top of the atmosphere to as deep as it is possible to detect, at least 350km down. It could be part of a major circulation system.

But the MWR records the ammonia at higher latitudes to be much more variable.

"What this is telling us is that Jupiter is not very well mixed on the inside," said Dr Bolton. "The idea that once you drop below the sunlight everything would be uniform and boring was completely wrong. It's actually very different dependent on where you look."

Mission team-members picked out a number of highlights in the new results.

One concerned the magnetic field of Jupiter. It was known to be strong but it has now been determined to be even stronger than expected - a doubling of the assumed strength where the probe makes its closest approach to the planet (the field is about 10 times the strength of Earth's magnetic field).

But the signal is quite lumpy, which tells the scientists that the dynamo system - the electrically conducting region generating the field - is probably not that deep into the planet.

"When we see small spatial-scale variation, it indicates to us that we may be very close to the source and so that might mean the dynamo is above the metallic hydrogen (layers) and it may operate in the molecular hydrogen envelope above. That's very significant," said Jack Connerney, Juno's deputy principal investigator and the lead for the mission's magnetic field investigation.

image copyrightNASA/JPL-Caltech/SWRI
image captionThese bright lights in the ultraviolet are the auroras over Jupiter's southern pole

It is the magnetic field investigation that is also at the heart of trying to understand Jupiter's very bright auroras - its northern and south lights. And again, what Juno is finding is not what everyone was expecting.

The auroras should result from electrons running down field lines and then striking the atmosphere. But the current carried by the electrons should have its own magnetic signature, and Juno has not at this stage been able to detect it.

"It's got us all scratching our heads, I have to say," said UK scientist Dr Jonathan Nichols from the University of Leicester.

"We see the auroras, we have a good idea we think of how they're generated but when it comes down it we're not seeing the signature of supposedly millions of amps of current."

image copyrightNASA/JPL-Caltech/SwRI

One very smart picture taken by Juno and released on Thursday showed the ring of dust that surrounds Jupiter. It is not well known that Jupiter has a ring, but it does. What was clever was getting the familiar stars of the Orion constellation to be in on the shot as well.

"This is the first image of Jupiter's ring that has ever been collected from the inside of it looking out," said mission scientist Heidi Becker from Nasa.

"Juno is 3,000 miles from the planet when we took this picture. So, what you're looking at is a ring of dust that's 40,000 miles away and stars that are hundreds of light-years away, all in the same picture."

image copyrightNASA/SWRI/MSSS/Gerald Eichstädt/Seán Doran

It is early days in the mission still (it is likely to run for several years yet), but the first gravity sensing data is pointing to some weirdness in respect of Jupiter's centre. Theories had suggested it either had a relatively small rocky core or no core at all (one suggestion was that the planet's gases went all the way down to the centre in an ever more compressed state).

Scientists are now considering something in between - a diffuse core. "It really looks fuzzy," said Dr Bolton. "There may be a core there but it's very big and it may be partially dissolved, and we're studying that."

Dr Bolton flagged up Juno's next pass, on 11 July. This will be dedicated to investigating the Great Red Spot.

  • Jupiter is 11 times wider than Earth and 300 times more massive
  • It takes 12 Earth years to orbit the Sun; a 'day' is 10 hours long
  • In composition it resembles a star; it's mostly hydrogen and helium
  • Under pressure, the hydrogen assumes a state similar to a metal
  • This 'metallic hydrogen' could be the source of the magnetic field
  • Most of the visible cloud tops contain ammonia and hydrogen sulphide
  • Jupiter's low-latitude 'bands' play host to very strong east-west winds
  • The Great Red Spot is a giant storm vortex twice as wide as Earth

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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