Io, the world with the greatest tidal flexing, volcanic and tectonic activity, and mass-loss in our solar system, begs for dedicated exploration. Missions such as Juno and JUICE, along with Earth-based telescopes such as JWST and ALMA, will acquire important Io observations over the next 15 years, as could Europa Clipper. However, a mission designed for Io science is necessary to accomplish key science goals that have been consistently prioritized in the National Academy of Sciences Decadal Surveys and the ESA Voyage 2050 (Thomas, 2021, Experimental Astronomy online), including understanding the early evolution of terrestrial planets, tidally heated exoplanets and ocean worlds, and magnetospheric physics across the galaxy. The NASA Discovery-class Io Volcano Observer (IVO; McEwen et al., 2021, LPSC 1352) completed Phase A in 2021 and was deemed selectable, but was not chosen for programmatic (i.e., non-science/engineering) reasons. The IVO concept study demonstrated how a total of ten carefully designed, close Io flybys could determine the melt distribution in Io’s interior to confirm or refute the presence of a magma ocean, constrain Io’s global average lithospheric structure, identify where and how Io is losing heat, and determine processes and rates for Io’s volatile loss. Such encounters would also measure Io’s rate of orbital migration, key to determining the stability of the LaPlace resonance that heats Europa and Ganymede, as well as Io. The ambitious (for Discovery) science payload included a magnetometer, plasma instrument, narrow-angle camera, thermal mapper, neutral mass spectrometer, plus a telecom system for gravity science and options for a student-collaboration wide-angle camera (WAC), and a technology demonstration UV spectrometer. The next Discovery mission proposal opportunity is expected in 2025 or later, but an opportunity to propose an Io mission in NASA’s New Frontiers (NF) program is anticipated in 2023. How might that differ from a Discovery-class mission? An Io orbiter to provide better geophysical measurements has been suggested in the past, but would be very challenging deep inside Jupiter’s gravity well and high radiation zone. An orbiter might be feasible if new, more capable launch vehicles become available. On the other hand, the IVO Discovery concept would accomplish all of the science objectives of NF, and could be augmented in several key ways using the additional resources available in NF. A radiation design to support more than 10 encounters is a relatively straightforward enhancement. Ka-band would improve the data downlink capability and gravity science. With twice as many Io encounters, it would be possible to more completely map Io’s surface at multiple wavelengths; encounter Io over additional values of orbital true anomaly for improved gravity science, magnetic induction, and measuring Io’s libration; and sample more longitudes and times of day to understand Io’s atmosphere, plumes, magnetospheric interactions, and mass loss. The WAC, which is especially important for mapping Io’s topography, could be a required Baseline experiment. There are many science instruments that would be valuable additions, including altimetry, ultraviolet and near-IR spectroscopy, a dust mass spectrometer, passive radar sounding (using Jupiter radiation), active radar sounding, and additional fields and particle instruments. Greater emphasis may be placed on tracing the mass and energy flows in the Io-Jupiter system, especially because the new US Planetary Science and Astrobiology Decadal Survey places much emphasis on understanding planetary systems relevant to exoplanets. Interferometric synthetic aperture radar (InSAR) would be challenging but could transform our understanding of active processes on Io. An even more daring idea is to deliver a penetrator that could measure Io’s seismicity and conducted heat flow, perhaps also with a laser retroreflector or radio transponder to measure Io’s rotational and tidal deformation. However, getting substantial data back to the main spacecraft for transmittal to Earth is challenging in the very fast flybys that are preferred to keep the total ionizing radiation dose low. In terms of international collaboration, we expect a thermal mapper from DLR and a neutral mass spectrometer from UBE, plus science co-investigators; additional contributions are possible. In summary, a highly capable mission to one of the most exciting objects in the Solar System is overdue. Figure: Io is a spectacular target to observe both in daytime and at night (simulated hot spots).

Io is a priority destination for solar system exploration, as it is the best natural laboratory to study the intertwined processes of tidal heating, extreme volcanism, and atmospheremagnetosphere interactions.Io exploration is relevant to understanding terrestrial planets and moons (including the early Earth), ocean worlds, and exoplanets across the cosmos.The scope and importance of science questions at Io necessitates a broad portfolio of research and analysis, telescopic observations, and planetary missions-including a dedicated New Frontiers class Io mission.Recommendation 1: We strongly recommend a dedicated New Frontiers class Io mission for the next decade.As outlined in a companion white paper, The Science Case for Io, Io is a priority destination for future exploration.There is much that can only be learned through detailed in situ measurements and observations from a dedicated Io mission.Even as nextgeneration telescopes come online, we are unlikely to achieve spatial resolutions better than tens of kilometers, and we cannot make observations of Io's poles or night hemisphere.There are also entire suites of in situ measurements which are simply not possible without visiting Io-like measuring Io's gravity and magnetic fields, or sampling its plumes and atmosphere.Furthermore, a dedicated mission to Io in the coming decade is timely, as it would enhance the science return from current and future Jupiter-system missions (Juno, Europa Clipper, JUICE), enabling true system science and contemporaneous investigations of the tidal and orbital evolution of the entire Jovian system.A mission to Io could also inform and guide forthcoming exoplanet observations with next-generation telescopes, and inform our understanding of the origins of life through implications for the early Earth and tidally heated ocean worlds like Europa, Enceladus, and Titan.Box 1 outlines a notional mission concept for a dedicated Io mission that could plausibly address all of the Priority Science Questions outlined in The Science Case for Io.This "Io Observer" concept is an amalgam of multiple different concepts, ideas, and mission proposals [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] .While we highlight one particular concept we encourage the decadal survey to allow flexibility in implementation.The past decade is characterized by mission selections that exemplify how new, innovative, and bold ideas have the power to transform the field (e.g., Dragonfly 17 ).A Jupiterorbiting, multiple-flyby, Io Observer may be the prevailing approach at this time, but out-of-thebox concepts may be capable of addressing a preponderance of Priority Science Questions, including orbiters, landers, impactors, and distant observers.The cost of the notional Io Observer mission concept outlined in Box 1 requires a detailed study by the decadal survey (see Recommendation 2).Io mission concepts have previously targeted either the Discovery program 1,3,7,10,12,15 or the New Frontiers program 2,6,8-9,13-14 , with sporadic discussion of Flagship class Io missions 4-5 .Io exploration is possible in all cost regimes; the question is: what is the best trade between science and cost?Discovery class Io missions tend to focus on addressing a subset of possible Io Priority Science Questions (consistent with the scope of the Discovery program, which generally address targeted science investigations).New Frontiers class Io missions have potential to address all of the Priority Science Questions, and fully realize the cross-cutting planetary science potential of Io.Flagship class Io missions could completely address all Priority Science Questions, but may be unrealistic given other priorities in planetary science.Based on the scope of the Io science and the implementation challenges, we recommend the decadal survey consider Io as a priority for the New Frontiers program-consistent with past decadal surveys, midterm reports, reports from the Committee on Astrobiology and Planetary Science [18][19][20] .Put simply, Io offers New Frontiers caliber science.At the time of writing this report, NASA is evaluating a proposed Discovery class mission to Io: Io Volcano Observer (IVO) 15 .Owing to the ongoing competition, we refrain from discussing

Io is a priority destination for solar system exploration, as it is the best natural laboratory to study the intertwined processes of tidal heating, extreme volcanism, and atmospheremagnetosphere interactions.Io exploration is relevant to understanding terrestrial planets and moons (including the early Earth), ocean worlds, and exoplanets across the cosmos.1. Io is a priority destination for future exploration.Jupiter's innermost large moon, Io, is the most geologically active world in the solar system (Fig. 1).Io's surface is marked by hundreds of active volcanoes, erupting lava fountains, evolving sulfurous ices, enormous mountains, and deposits from towering volcanic plumes that pollute the Jovian system and feed its enormous magnetosphere.This unparalleled activity is powered by rampant tidal heating, where the gravitational interactions between Io and its neighboring moons result in time-varying tides from Jupiter that deform and heat Io's interior.Io is the best natural laboratory to study these intertwined processes, and it is a vitally important destination for addressing high priority, cross-cutting science investigations relevant to broad swaths of planetary science-from the Hadean Earth-Moon system when life emerged, to present-day potentially habitable ocean worlds, and distant exoplanets where conditions are even more extreme.Characterization of Io will guide future observations of both ocean worlds and exoplanetary targets.In a sense, Io is the uninhabitable world that teaches us how habitable worlds form and work.2. What has changed since the last decadal survey?Since Vision and Voyages 1 , there have been many paradigm-changing advances in our understanding of Io, including (but not limited to): extremely highresolution imaging of active volcanoes 2 ; new insights into tidal evolution, motivated by results from Cassini 3-9 ; observations of Io's poles by Juno [10][11][12][13] ; new analysis of Galileo magnetometer data suggesting the presence of a long-hypothesized global subsurface magma ocean on Io 14 , although controversial [15][16] ; and the discovery of exoplanet analogs, including resonant "super-Ios" in TRAPPIST-1 17 and "lava worlds" like 55 Cnc e 18 .Despite these advances, there are still critical knowledge gaps requiring in situ geophysical and geochemical measurements, and high-resolution imaging-all beyond the capabilities of forthcoming missions to the Jupiter system-necessitating a dedicated mission to Io.3. The science rationale for exploring Io.Io is a unique solar system world-lying at the nexus of a variety of high priority, cross-cutting scientific questions in planetary science.We frame the science case for Io exploration around five Cross-Cutting Themes: (1) tidal heating, (2) heat flow, (3) volcanism; (4) atmospheres, and (5) magnetospheric interactions.Table 1 lists Priority Science Questions for each Cross-Cutting Theme.3.1.Cross-Cutting Theme 1: Io is the best place to study tidal heating.Tidal heating is a fundamental process for shaping planetary bodies and creating potentially habitable environments across the cosmos (see ref. 19 for a review).Tidal heating drives the orbital and

A.S. McEwen1, K. de Kleer2, R.S. Park3, C.J. Bierson4, A.G. Davies3, D. DellaGiustina1, A.I. Ermakov3, J. Fuller2, C. Hamilton1, C. Harris5, H. Hay1, J. Keane2, L. Kestay6, K. Khurana7, K. Kirby8, V. Lainey9, I. Matsuyama3, K.E. Mandt8, C. McCarthy10, F. Nimmo4, M. Panning3, A. Pommier11, J. Rathbun12, G. Steinbrügge13, D. Stevenson2, V.C. Tsai2, and E. Turtle8, 1University of Arizona, Tucson, AZ 85721; 2California Institute of Technology, Pasadena, CA 91125; 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109; 4University of California – Santa Cruz, Santa Cruz, CA 95064; 5University of Michigan, Ann Arbor, MI 48109; 6US Geological Survey, Flagstaff, AZ 86001; 7University of California – Los Angeles, CA 90095; 8Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723; 9IMCCE, F-75014 Paris, France; 10Columbia University, Palisades, NY 10964; 11University of California – San Diego, La Jolla, CA 92093; 12Planetary Science Institute; 13University of Texas at Austin, Austin, TX 78705.

Ingibjörg Jónsdóttir (1), Þorvaldur Þórðarson (1), Ármann Höskuldsson (1), Ashley Davis (2), David Schneider (3), Robert Wright (4), Laszlo Kestay (5), Christopher Hamilton (6), Andrew Harris (7), Diego Coppola (8), Magnús Tumi Guðmundsson (1), Tobias Durig (1), Gro Pedersen (1), Vincent Drouin (1), Friðrik Höskuldsson (9), Hreggviður Símonarson (9), Gunnar Örn Arnarson (9), Magnús Örn Einarsson (9), Morten Riishuus (1), and the Volcanology and Natural Hazard Group, University of Iceland Team