Juno has arrived! Simon Schneider magnetic fields as well as to peek through the upper cloud layers into the inner structure of the planet. The highly elliptical polar orbit with a period of 53 days and minimal altitude of only about half an earth radius above the cloud tops enables Juno to get data from Jupiter's polar region that could not be easily observed from Earth or by previous spacecraft. The carefully chosen orbit makes it possible to do all this while still avoiding the extremely strong radiation belts around the planet that could damage Juno's instruments and electronics. During its very first orbit over the poles, Juno was already able to surprise scientists with detailed images showing that the weather and cloud

structure at high altitudes are very different from the Figure 1 Juno arriving at Jupiter, artist’s rendering. iconic band structure that can be seen near the image credit: NASA/JPL-Caltech equator. Instead, the polar regions are a chaotic After its launch in August 2011, and a nearly five scene of ammonia gas dwelling up from deep in the year long journey through the solar system, NASA's interior amidst massive storm systems irregularly Juno spacecraft1 (see figure 1) finally arrived at the scattered across these latitudes. The poles gas giant Jupiter in July 2016. Juno – the second themselves, however, show a stunning regularity: spacecraft of NASA's New Frontiers Program after both poles feature a central cyclone circled by the New Horizons mission2 to Pluto – is equipped evenly spaced storms forming a regular polygon, an with a series of exciting instruments that allow octagon in the north and a pentagon in the south measurements and observations far more detailed (see figure 2). How this pattern is maintained and precise than previous missions to Jupiter such instead of the storms merging together is an as the Galileo mission3. These instruments give ongoing puzzle.4 Juno the ability to measure gravitational and

1https://www.missionjuno.swri.edu/#/mission 3https://solarsystem.nasa.gov/missions/galileo/overview/ 2https://www.nasa.gov/mission_pages/newhorizons/main/in 4https://www.nature.com/articles/nature25491 dex.html Juno was also able to use both its infrared and Besides the purely scientific objectives, Juno also ultraviolet cameras to observe the extensive auroras included the visible-light camera “JunoCam”8, generated by highly energetic particles directed onto intended to enable the public to participate in the the poles by Jupiter's enormous magnetic field. mission. For each orbit, people can vote on the Together with direct magnetic measurements, this targets to be pictured. The raw images are then will allow scientists to draw conclusions about the uploaded onto the web page for anybody to inner structure that generates the fields. download and process. This already resulted in a

gallery of stunning images of this fascinating planet generated by interested people all over the world.9 The project also calls all amateur astronomers to upload their telescopic images and data of Jupiter to help with the planning of future observations.10

While the results of this spectacular mission are still coming in we can only imagine what further mysteries lie hidden beneath the clouds of Jupiter. Figure 2 This computer-generated image shows the structure of the cyclonic pattern observed over Jupiter’s south pole. Readers might also be interested in our pre-arrival coverage of Like in the North, Jupiter’s south pole also contains a central cyclone, but it is surrounded by five cyclones with the Juno mission: diameters ranging from 3,500 to 4,300 miles (5,600 to http://depts.washington.edu/astron/old_port/TJO/newsletter/sp 7000 kilometers) in diameter. r-2016/Matttjo.pdf image credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

Meanwhile, Juno's measurements of the gravitational field show an unexpected asymmetry between the northern and southern hemisphere which indicates that the jet streams – observed as the colored bands at the cloud tops – extend far down into the planet, possibly up to about 2000 miles (3000 kilometers).5,6 These observations allowed the conclusion that the deep interior of the planet, unlike the upper layer, rotates very much like a rigid body.7

5https://www.nature.com/articles/nature25793 8https://www.missionjuno.swri.edu/junocam 6https://www.nature.com/articles/nature25776 9https://www.missionjuno.swri.edu/junocam/processing 7https://www.nature.com/articles/nature25775 10https://www.missionjuno.swri.edu/junocam/planning