On 27 August 2016, the Juno spacecraft made its first close pass around our solar system's largest planet, Jupiter, obtaining insights into its atmosphere and interior that challenge previous assumptions. The Juno mission, which launched in 2011 and began its first orbit last year, allows scientists to view Jupiter in new ways because of its highly elliptical orbit; it passes over the planet's poles and dives within 5,000 kilometers of its cloud tops. Now two new studies in Science report the results of its initial Jupiter encounters. In one study, Scott Bolton et al. present results from Juno's flight just above the cloud tops. Images of Jupiter's previously-unseen poles show a chaotic scene of bright oval features, very different from Saturn's polar regions. A time-lapse of Juno images reveals that the ovals are cyclones, some of which reach diameters up to 1,400 kilometers across. Juno measured the thermal structure of Jupiter's deep atmosphere as it passed over the cloud tops. These data show unexpected structures, which the authors interpret as signs of ammonia welling up from the deep atmosphere and forming giant weather systems. Measurements of Jupiter's gravitational field were made, which will help understand the structure of the planet's atmosphere and whether it has a solid core, as models have predicted. Analysis of the gas giant's magnetic field reveals that close to the planet, the field greatly exceeded expectations - it is substantially stronger than models predicted, at 7.766 Gauss, or roughly ten times Earth's magnetic field.
In a second study, John Connerney et al. present data on Jupiter's aurorae and magnetosphere, the region where the planet's magnetic field dominates over the solar wind. Juno encountered the giant planet's bow shock, essentially a stationary shockwave, as it entered the magnetosphere on 24 June 2016. Since the spacecraft only encountered one bow shock as it approached the planet, compared to multiple encounters on subsequent orbits, this suggests that the magnetosphere was expanding in size at the time, the authors say. Taking advantage of its unique perspective when positioned above the poles, Juno detected downward-traveling electron beams that shower energy into Jupiter's upper atmosphere, potentially powering the huge aurorae that Juno saw in ultraviolet and infrared images. Intriguingly these electron showers appear to have a different distribution from those that occur on Earth, suggesting a radically different conceptual model of Jupiter's interaction with its space environment, the authors say.