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Uranus and Neptune are the least-explored planets in our Solar System. This paper summarizes mysteries about these incredibly intriguing planets and their environments spurred by our limited observations fromVoyager 2and Earth-based systems. Several of these observations are either inconsistent with our current understanding built from exploring other planetary systems, or indicate such unique characteristics of these Ice Giants that they leave us with more questions than answers. This paper specifically focuses on the value of all aspects of magnetospheric measurements, from the radiation belt structure to plasma dynamics to coupling to the solar wind, through a future mission to either of these planets. Such measurements have large interdisciplinary value, as demonstrated by the large number of mysteries discussed in this paper that cover other non-magnetospheric disciplines, including planetary interiors, atmospheres, rings, and moons.

The process that generates Earth’s most intense aurora is found to occur at Jupiter, but is of only secondary importance in generating Jupiter’s much more powerful aurora. The most intense aurora on Earth are generated by a 'discrete' process whereby electrons are accelerated coherently. Weaker aurora arise from wave scattering of magnetically trapped electrons. As Jupiter's aurora is orders of magnitude more powerful than Earth's, it was naturally assumed that the former process was responsible, yet early in situ observations by the Juno spacecraft found no evidence of the discrete process. Barry Mauk and collaborators report discrete downward accelerations of electrons on some auroral crossings, but the energy flux is much less than that caused by broadband processes, with broadband characteristics that are very different from those at Earth. The most intense auroral emissions from Earth’s polar regions, called discrete for their sharply defined spatial configurations, are generated by a process involving coherent acceleration of electrons by slowly evolving, powerful electric fields directed along the magnetic field lines that connect Earth’s space environment to its polar regions1,2. In contrast, Earth’s less intense auroras are generally caused by wave scattering of magnetically trapped populations of hot electrons (in the case of diffuse aurora) or by the turbulent or stochastic downward acceleration of electrons along magnetic field lines by waves during transitory periods (in the case of broadband or Alfvenic aurora)3,4. Jupiter’s relatively steady main aurora has a power density that is so much larger than Earth’s that it has been taken for granted that it must be generated primarily by the discrete auroral process5,6,7. However, preliminary in situ measurements of Jupiter’s auroral regions yielded no evidence of such a process8,9,10. Here we report observations of distinct, high-energy, downward, discrete electron acceleration in Jupiter’s auroral polar regions. We also infer upward magnetic-field-aligned electric potentials of up to 400 kiloelectronvolts, an order of magnitude larger than the largest potentials observed at Earth11. Despite the magnitude of these upward electric potentials and the expectations from observations at Earth, the downward energy flux from discrete acceleration is less at Jupiter than that caused by broadband or stochastic processes, with broadband and stochastic characteristics that are substantially different from those at Earth.

The formation of Titan’s induced magnetosphere is a unique and important example in the solar system of a plasma-moon interaction where the moon has a substantial atmosphere. The field and particle conditions upstream of Titan are important in controlling the interaction and also play a strong role in modulating the chemistry of the ionosphere. In this paper we review Titan’s plasma interaction to identify important upstream parameters and review the physics of Saturn’s magnetosphere near Titan’s orbit to highlight how these upstream parameters may vary. We discuss the conditions upstream of Saturn in the solar wind and the conditions found in Saturn’s magnetosheath. Statistical work on Titan’s upstream magnetospheric fields and particles are discussed. Finally, various classification schemes are presented and combined into a single list of Cassini Titan encounter classes which is also used to highlight differences between these classification schemes.