Science Priorities for Future Exploration of Jupiter’s moon Io

Science priorities for exploration of Io have been defined in the US Decadal surveys, but recent results may suggest reconsideration of those priorities.  Recent Io results highlighted here have come from telescopic observations and from Io flybys by the Juno mission.  The decadal objectives for a New Frontiers 5 mission to Io are:

• Study Io’s active volcanic processes;
• Determine melt fraction of Io’s mantle;
• Constrain tidal heating mechanisms;
• Study tectonic processes;
• Investigate interrelated volcanic, atmospheric, plasma-torus, and magnetospheric mass- and energy-exchange processes;
• Constrain the state of Io’s core via improved constraints on whether Io generates a magnetic field; and
• Investigate endogenic and exogenic processes controlling surface composition.

Isotopes and the long-term history of Io.  De Kleer et al. (2024) used telescopic submillimeter observations of Io's atmosphere to measure sulfur isotopes and found ³⁴S/³²S to be highly elevated compared to average Solar System values. They interpret this to mean that Io has lost 94-99% of its available sulfur and has been volcanically active for most of its history.  However, this result depends on a model for how sulfur is recycled inside Io (Hughes et al., 2024), which remains poorly understood, so a better understanding of the sulfur cycle is a priority.  Isotopic data is the best way to understand the long-term history of Io and should be a priority for future exploration.  A spacecraft making repeated close (<200 km) flybys of Io should carry both gas and dust mass spectrometers.  The Io Volcano Observer (IVO) mission (Phase A study for a Discovery mission; Hamilton et al., in press) would carry a neutral mass spectrometer for plume gasses and Io’s atmosphere (Vorburger et al., 2021).  Addition of a dust mass spectrometer would provide information about the composition of solid surface materials ejected by fast micrometeoroid  impacts (Kempf et al., 2025).  Analysis of isotopes from both instruments provides information about different reservoirs and how volatiles are cycled.  A dust instrument also provides key information about the hazards of low-altitude flybys, and adding such close encounters can enhance the science in multiple ways.  Sample return is another way to obtain isotope data from a plume (Ogliore et al., 2024).

Total heat flow and global patterns.  Juno observations show new eruptions (Ravine et al., 2024; Perry et al., 2025) and revealed that many hot spots consist of hot rings suggestive of lava lakes (Mura et al., 2024).  The Juno and other data shows the polar regions may be less active than equatorial regions (Davies et al., 2024), except when there are short-lived events that make it more active (Mura et al., 2025).   Polar versus equatorial heat flow is key to understanding the depth of tidal heating in Io’s mantle, but near-IR observations (e.g., Juno JIRAM) cannot detect ~50% of Io’s endogenic heat flow, which is released at low temperatures (<200 K).  The low-temperature heat flow is indicative of longer time periods, less dominated by currently active eruptions. Longer-wavelength (>10 microns) observations are needed with good time of day coverage for thermal inertia and broad phase-angle coverage from ~0.4 to 3 microns to map Bond albedo.  Europa Clipper can do this (McEwen et al., 2025), but at ~75 km scale.  IVO would map heat flow at higher resolution (~1 km), clearly separating high-temperature sources from low-temperature anomalies.  An extended mission of at least 2 years would be needed to complete global thermal-IR mapping at ~1 km in both sunlight and darkness for thermal inertia.  

What is the distribution of melt in Io’s mantle?  Juno acquired gravity data to measure tidal k₂ as a test for a magma ocean with a detached lithosphere, and concluded that “a shallow global magma ocean in Io does not exist” (Park et al., 2025).  Aygün and Čadek (2025, Icarus 436) concluded that the Park et al. k₂ result is compatible with a deep (>320 km) global magma ocean, or with a thin (<10 km), shallow (<250 km) global magma layer.  An exciting new contribution to this debate (Mura et al., 2025) came from Juno’s observation of extensive and powerful synchronized eruptions over a vast region (~65,000 sq. km) near Io’s south pole, which “supports models of massive, interconnected magma reservoirs.”  Such interconnected magma reservoirs (a type of magma ocean?) may explain Io’s induced magnetic signature (Khurana et al., 2011). Clearly the interrelated decadal objectives to determine the melt fraction of Io’s mantle and to constrain tidal heating mechanisms remain high priority.  IVO would greatly advance knowledge of Io’s interior through measurement of tidal gravity, libration, magnetic induction, heat-flow patterns, and lava composition.  The tidal gravity measurement would be from pairs of encounters that are identical except one near periapsis and one near apoapsis, to ensure measurement of tidal k₂ with no bias from static gravity anomalies.  A New Frontiers Io mission can support 20 rather than 10 close encounters, including some at lower altitudes (~50 km) to measure gravity anomalies such as over Loki Patera. 

The IVO mission completed a Phase A study for Discovery in 2021 and the concept was updated in 2023 for a New Frontiers 5 opportunity (Hamilton et al., 2025).  We will further discuss how a future Io-dedicated mission could address the science questions raised by recent results.  Such results are relevant to understanding the coupled Io-Europe-Ganymede system and exoplanetary systems with tidal heating. 

Figure: Candidate groundtracks when below 1000 km for Discovery (top) and New Frontiers (bottom) Io concepts (from Hamilton et al., 2024).

References

Aygün and Čadek, 2025, Icarus 436.

Davies et al., 2024, PSJ 5.

De Kleer et al., 2024, Science 384.

Hamilton et al., PSJ in press

Hughes, E. et al., 2024, JGR Planets 129.

Kempf et al., 2025, SSR 221.

Khurana et al., 2011, Science 332.

McEwen et al., 2025, LPSC.

Mura et al., 2024, Nature Comm.

Mura et al., 2025, preprint.

Ogliore, R. C. et al., 2024, 86th Annual Meeting of the Meteoritical Society 2024.

Park et al., 2025, Nature 638.

Perry, J.E. et al., 2025, PSJ 6. 

Ravine et al., 2024, EPSC2024-731.

Vorburger, A. et al., 2021, 43^rd COSPAR scientific assembly.