Jupiter is the largest planet in the Solar System, more than 300 times larger than the Earth in mass. Surrounded by a system of dust rings and more than 60 moons, we already know something about the characteristics of the gas giant planet itself. Its atmosphere is composed of about 75% hydrogen and 24% helium by mass, with trace amounts of ammonia and other compounds including methane and water vapor. The planet is bulged around the equator because of its rapid rotation, and it is perpetually covered with clouds of ammonia, with its outer atmosphere segregated into several bands at different latitudes and featuring a persistent anti-cyclonic storm called the Great Red Spot south of the equator.
But there is still a lot more to discover about Jupiter’s interior, magnetic field, aurora, radio emissions, and history. NASA’s Juno mission carried to Jupiter a suite of nine scientific instruments designed to gather data on Jupiter’s atmosphere, gravity, magnetic field, energetic particle and radiation environment, aurora, and radio emissions. Entering the planet’s orbit on July 4, 2016, Juno has been collecting a vast amount of new information in a series of close flybys to within 4200 kilometers of the planet (1/17 of the planet’s radius)!
The first results of the Juno mission are the subject of a special issue of Geophysical Research Letters, and they reveal several interesting new findings. For example, Juno has found ammonia upwelling near the equator that exhibits significant variability in its abundance at depths corresponding to 30 bars (30 times Earth’s atmospheric surface pressure). The measured gravity hints at a more gradual “differential rotation” (difference in rotation speed between the pole and the equator), and the measured magnetic field is both stronger and more structured than current models expect.
Contrary to theoretical predictions, this measured magnetic field does not exhibit any perturbations associated with the electrical currents in the high-latitude regions. Likewise, contrary to expectations, the observed intensity variations of the ultraviolet aurora do not quite correlate with the fluctuations of the solar wind dynamic pressure.
Another big surprise is the occurrence of protons originating from the planet energized to hundreds of kilo-electron volts and moving away from the planet. At the same time, there are downward beams of electrons in the polar region that are possibly the source for Jupiter’s intense radio bursts, which have long been detected from Earth.
These new insights offer important clues into how Jupiter may have evolved over history into what it is and where it is today. By illuminating the underlying physics, they contribute to our understanding of the other planets, and provide many more questions than answers.
—Andrew Yau, Andrew Dombard, W. K. Peterson, and Paul D. Williams, Editors, Geophysical Research Letters; email: [email protected]