We review the key observations and theories relevant to the origin and evolution of the Galilean satellites. Key observations include: the potentially undifferentiated nature of Callisto; the increasing ice fraction with semi-major axis; the present-day existence of the Laplace resonance; the potential resurfacing of Ganymede mid-way through its evolution; and the metal-enriched nature of Jupiter’s envelope. The most widely accepted theory for the formation of the satellites is the so-called “starved disk” model, although newer alternatives including decretion disks and pebble accretion have also been proposed. Models that allow slow satellite formation in a cold disk are preferred, based on the density progression and Callisto’s apparent differentiation state. Major model uncertainties include the angular momentum distribution of the material infalling to the circumplanetary disk, the source of the solids, and the thermal and viscosity structure of the disk. We identify six outstanding questions, some of which will be answered by JUICE, Europa Clipper and Tianwen-4. A major difficulty in answering some questions is overprinting of primordial characteristics by later events.
Abstract Surveys and observations of lightning on Jupiter prior to the NASA Juno mission used night‐side imaging approaches, and a common conclusion was that the optical energy was similar to the highest energy terrestrial lightning flashes, or superbolts. We use data from the Juno Microwave Radiometer (MWR) to measure the first radio pulse power distribution of Jovian lightning. The power distribution measurement was enabled by unique meteorological conditions in Jupiter's North Equatorial Belt (NEB) in 2021–2022, as the belt transitioned from an anomalously quiescent (non‐convective) state to its more typical configuration with small moist convective plumes scattered in longitude. During this transition, convective plumes in the NEB occurred only in isolated storms we label “stealth superstorms.” The isolated nature of these storms (as lightning sources) resolved the degeneracy between pulse location and pulse strength, allowing measurement of a pulse power distribution with statistical median values ranging from 27 to 214 W over the MWR bandpass, well within the observational sensitivity range. The MWR thus measures typical pulse power in the storms, rather than high‐power outliers. Pulse power in the stealth superstorms may be comparable to terrestrial lightning radio emission, or up to a million times more powerful, depending on uncertainties in unresolved pulse duration and lightning spectral energy distributions. Future studies may determine whether the lightning pulse power in stealth superstorm is typical or anomalous of Jupiter's lightning in general.
The IMAP-Hi Energetic Neutral Atom (ENA) Imager on NASA’s Interstellar Mapping and Acceleration Probe (IMAP) mission (McComas et al. 2018a, 2025) is designed to measure ENAs from the global interaction between the heliosphere and the local interstellar medium (LISM). These ENAs are initially plasma ions of solar wind origin that are neutralized by charge exchange with the cold neutral atoms of LISM that freely flow through the heliosphere-LISM interaction region. IMAP-Hi consists of two identical single-pixel sensors, each covering the ENA spectral range from 0.44 keV to 15.6 keV over nine contiguous energy passbands and having an approximately conical field-of-view (FOV) of 4.1o full width at half maximum (FWHM). The Hi-45 sensor points 45o relative to the spacecraft spin axis from the antisunward direction; each spacecraft spin, it measures ENA intensity over a circular swath with half-cone angle 45o centered on the ecliptic plane. The Hi-90 sensor points 90o relative to the spin axis; each spacecraft spin, it measures ENA intensity over a great circle in the sky, sampling both the north and south ecliptic poles. As the IMAP spin vector is re-pointed daily toward the Sun, the ecliptic longitude of the swaths moves daily by ∼1o such that a full sky map is acquired by Hi-90 every six months and a complete low latitude (−45o to +45o) map is acquired by Hi-45 annually. The IMAP-Hi sensor design has direct heritage from the IBEX-Hi imager on the Interstellar Boundary Explorer (IBEX) mission, with substantial improvements in energy range, energy resolution, angular resolution, signal-to-noise ratio, and, for ecliptic latitudes within ±45o, temporal resolution and exposure time. The global ENA maps acquired by IMAP-Hi partially overlap in energy and viewing with the ENA maps acquired by the IMAP-Lo and IMAP-Ultra ENA imagers, which we combine to answer fundamental questions about the structure and dynamics of the interaction of the heliosphere and the LISM.
The Solar Wind Electron (SWE) instrument of the Interstellar Mapping and Acceleration Probe (IMAP) mission is designed to measure the in situ solar wind thermal and suprathermal electrons at the spacecraft. SWE contributes to the IMAP science objective to understand particle injection and acceleration processes near the Sun and in the heliosphere and heliosheath, and provides context for the IMAP measurements of energetic neutral atoms from the outer heliosphere. SWE measures the solar wind thermal and suprathermal electron distribution from 1–5000 eV in 24 log-spaced steps with energy resolution Δ E/E of 14
The Interstellar Mapping and Acceleration Probe (IMAP) mission is poised to revolutionize our understanding of two of the most central issues in heliophysics today: the nature of the interaction of the solar wind with the local interstellar medium (LISM), and the acceleration of energetic particles throughout the heliosphere. Here we focus on the anticipated contributions to this endeavor by measurement of hydrogen energetic neutral atoms (ENAs) that emanate from the solar wind–LISM interface region. ENA imaging allows us to remotely sense the structure and plasma characteristics of the heliosheath and the LISM just beyond the heliopause, and how these regions evolve in response to solar activity. These instruments, IMAP-Lo, IMAP-Hi, and IMAP-Ultra, cover energies from 10 eV to 300 keV with unprecedented angular resolution, energy resolution, and detection sensitivity. In this paper, we present the IMAP mission science specific to the study of the heliosphere/LISM interaction. We review what we have learned about the heliosphere interaction based on prior observations and theoretical studies. We then present the outstanding science questions that have consequently arisen and how IMAP plans to resolve them.
