It's hard to imagine that the Solar System's biggest planet, which provided Galileo with his first key astronomic observations, has a lot of secrets left 450 years later. Yet, despite countless hours spent peering through telescopes and numerous robotic visitors, there's a lot we still don't know about Jupiter. Most notably, we didn't even have a decent picture of the planet's poles, and we have little idea of what its interior might look like.

Thanks to the arrival of the Juno probe, however, that's starting to change. After just a few orbits, Juno has imaged both poles, tracked some of the dynamics of its atmosphere, and started providing evidence of what may lie at the crushing depths of the planet's interior.

Staring at the clouds

Earlier this year, Juno performed the closest approach to Jupiter ever made by human hardware, passing within 5,000km of Jupiter's cloud tops. Juno's highly elliptical orbit also takes it over both poles, allowing them to be imaged in greater detail than ever before. And every instrument on the probe managed to capture some data.

The easiest data to interpret comes from its camera, the JunoCam, which captures images in RGBM—the usual RGB plus a wavelength sensitive to methane. Expectations were that we might see something like Saturn's pole, which has a vortex sitting right at the pole and an odd hexagonal shape in its clouds. Instead, the poles of Jupiter are studies in chaos. The neat banding pattern of the mid-latitudes gives way to a darker surface littered with cyclones that we can observe rotating in time-lapse imagery. There are also more diffuse, light-colored areas up to 10,000km across that don't have a defined rotation. And there's no hint of a hexagon.

Microwave imaging hardware also provided the opportunity to track the temperature of different cloud bands at Jupiter's mid-latitudes. Past work has focused on how these bands move so rapidly to the east or west of the planet. But the Juno data shows that there's a band near the equator where warm material from the planet's interior wells up and reaches the cloud tops, while a cooler band to the north allows material to return to the depths. The scientists involved in analyzing the data suggest this may be an analog of the Hadley cell found in Earth's atmosphere, concluding "The structure is a Hadley cell without rain."

An infrared device (the Jovian Infrared Auroral Mapper) has tracked much smaller hot and cold regions spread through the bands of clouds. The hot regions are thought to be areas that have dried out, cooled off, and are now descending into Jupiter's interior. They look hotter simply because the lack of water and ammonia allows more of Jupiter's internal heat to reach the surface instead of being absorbed.

Looking inward

One of the key goals of the Juno mission is to provide some perspective on Jupiter's interior. There's ongoing debate about whether Jupiter's formation required a solid core to start gathering the gas that made it a giant. And we suspect, but don't know, that hydrogen turns metallic at the pressures within the planet, producing a magnetic field that can extend out over 100 times the planet's radius.

Juno doesn't need any special instruments to track Jupiter's gravitational field; instead, it can be sensed as it accelerates the spacecraft, a change that shows up as a Doppler shift in the communications signals. The results of just a couple of orbits are inconsistent with some of our models of the giant planet's interior, but still consistent with several others. So we'll probably have to wait for several more orbits to refine the Juno measurements before we hear much more about Jupiter's internals.

The magnetic field, by contrast, has already yielded up a bit of a surprise to Juno. The probe's orbit takes it through the area where Jupiter's magnetosphere interacts with the solar wind, and it often crosses this area several times with each orbit, due to fluctuations in the magnetosphere's strength and the solar wind. But, more significantly, Juno is the first craft to measure the magnetic field from inside the orbit of the moon Io since 1974 (when Pioneer 11 visited).

Unexpectedly, the magnetic field got much stronger during the closest approach, reaching strengths over 1.5 times those predicted by models (and an order of magnitude stronger than Earth's). It's also more variable than expected. This, the team concludes, "portends a dynamo generation region not far beneath the surface." At those levels, hydrogen isn't supposed to be metallic; instead, it should just be standard H 2 molecules. So it's not clear what's providing the conductive medium needed to produce magnetism.

Planetary particle accelerator

With Juno, we sent a JEDI to Jupiter. That's the Jupiter Energetic Particle Detector Instrument, which is sensitive to high-energy electrons and ions. JEDI tracked energetic electrons falling into Jupiter's polar regions, where they created aurorae picked up by Juno's UV and visible-imaging systems. At its most intense, the authors estimate that each square meter of Jupiter's atmosphere is receiving 200 milliWatts of energy from charged particles. But the planet isn't only a victim of bombardment; the instrument also detected streams of electrons being sent upward, channeled along Jupiter's magnetic field lines.

Electrons aren't the only thing zipping around near the giant planet. The detector hardware separated ions based on mass and charge, and it picked up protons, ionized oxygen, and sulfur. The sulfur almost certainly originated in the volcanoes of Io. In fact, imaging of the aurorae showed clear signatures of the orbits of Jupiter's four largest moons.

Unfortunately, this intense radiation is going to help cut Juno's life short. And it's not the only thing; the science teams report that dust grains "impact the spacecraft with a relative velocity of >60km/s, which provides sufficient kinetic energy to vaporize the grain and a portion of target material." But, in just a few orbits, we've already gotten a lot of high-resolution data on our Solar System's giant. As long as it survives a few more orbits, it will almost certainly provide planetary scientists with enough data to keep them occupied for years.

Science, 2017. DOI: 10.1126/science.aal2108, 10.1126/science.aam5928 (About DOIs).