What’s Next for Jupiter’s Fiery Moon, Io?

Table of Links

Abstract

1. Introduction

   1.1. Io as the main source of mass for the magnetosphere

   1.2. Stability and variability of the Io torus system

   1.3. Hypothesized volcanic mass supply events

   1.4. Objective of this review

2. Review of the relevant components of the Io-Jupiter system

   2.1. Volcanic activity: hot spots and plumes

   2.2 Io’s bound atmosphere

   2.3 Exosphere and atmospheric escape

   2.4 Electrodynamic interaction, plasma-neutral collisions, and the related atmospheric loss processes

   2.5. Neutrals from Io in Jupiter’s magnetosphere

   2.6. Plasma torus and sheet, energetic particles

   2.7 Jupiter’s aurora and connections to the Io torus

   2.8 Dust from Io

3. Summary: What we know and what we do not know and 3.1 Current understanding for normal (stable) conditions

   3.2 Canonical number for mass supply

   3.3 Transient events in the plasma torus, neutral clouds and nebula, and aurora

   3.4 Gaps in understanding, contradictions, and inconsistencies

4. Future observations and methods and 4.1 Spacecraft measurements

   4.2 Remote Earth-based observations

   4.3 Modeling efforts

Appendix, Acknowledgements, and References

4. Future observations and methods

The previous sections have shown that there are still many unknowns in the Io-Jupiter system and specifically several open questions about specific aspects on the supply of mass from Io to the magnetosphere. For advances in understanding the complete system, it will require many advances on these individual aspects and questions which likely can be achieved through a variety of remote observations, in-situ measurements and theoretical or modeling efforts.

4.1 Spacecraft measurements

There are three planetary missions targeting the Jupiter system that might provide measurements relevant to the topic. The NASA Juno spacecraft carried out close flybys at Io in its extended mission which will end in 2026. Later in the 2030ies, both NASA’s Europa Clipper mission and the recently-launched Jupiter Icy Moon Explorer (JUICE) of the European Space Agency (ESA) will orbit Jupiter for several years targeting primarily the planet’s large icy moons. Finally, a mission dedicated to Io would potentially allow a major leap forward.

4.1.1 Juno

The NASA Juno spacecraft went into orbit around Jupiter on 4 July 2016 and the ~5-year primary mission was designed for 35 perijove passes. The spacecraft’s polar elliptical orbit precesses such that the orbital distance Juno crosses the equatorial plane evolves inwards.

In the extended mission’s additional 43 orbits, these crossing points reached the orbital distances of the Galilean satellites and opportunities became available to observe the moons up close, including Io. The recent observations by Juno have resulted in visible and thermal images (from JunoCam and JIRAM respectively) that show a large number of higher temperature areas on the surface (e.g., Rathbun et al., 2020). Furthermore, Juno is conducting 15 flybys within 150.000 km of Io between April 2022 and May 2025, as part of this extended mission. Of those flybys, the two closest occurred at an altitude of slightly below 1500 km:

● PJ57 Io: 2023-12-30 08:36

● PJ58 Io: 2024-02-03 17:48.

During the Io flyby of PJ (perijove) 57 the spacecraft passed above Io’s north pole near close approach, and at PJ58’s Juno transited south of Io’s near-wake environment.

Juno’s plasma and particle instrumentation was designed to observe in Jupiter’s auroral regions and not in the high density and high radiation environment of Io’s orbit. However, Juno can still contribute to improving our understanding of the spatial and energy distribution of the ion species near Io. The Jovian Auroral Distributions Experiment (JADE) (McComas et al., 2017), a plasma analyzer with Time-of-Flight (TOF) mass spectrometry, will enable the first mass-resolved plasma composition observations in the vicinity of Io. Furthermore, the Jupiter Energetic-Particle Detector Instrument (JEDI) onboard Juno could determine the extent to which there are energetic particle dropouts, which could provide constraints on its extended atmosphere’s spatial extent and variability (e.g., Huybrighs et al., 2024). In addition to the close flybys, Juno transits the Io plasma torus multiple times. While the plasma and particle measurements in this region are significantly different than JADE and JEDI were designed to make, they could still provide an important set of measurements with which to improve our understanding of the plasma-neutral interactions, plasma chemistry, and mass transport from Io and the Io torus. Future plans for observations of Io are also elaborated on in Keane et al., 2022 and McEwen et al., 2023.

4.1.2 Jupiter Icy Moon Explorer

After orbit insertion in 2031, JUICE will orbit Jupiter for over three years before going into orbit around Ganymede at the end of 2034. During this Jupiter orbiting phase, observations of Io and its environment will be mostly from a distance of ≥850,000 km. However, there will be several opportunities during this phase, to remotely observe Io at around 400,000 km distance. Several instruments might take observations relevant to the topic of mass loss and we briefly mention such possible studies (Williams, Denk, et al., SSR in prep, 2024).

The visible camera JANUS (covering wavelengths between 350 and 1064 nm, Tubiana et al., 2021) aims to study different aspects with remote high-resolution images: (1) Changes in Io’s surface through repeated coverage; (2) plume detections using high phase angle and eclipse observations; (3) monitoring Io’s sodium extended clouds with its sodium filter; and (4) imaging Io’s aurora in eclipse as diagnostic for the plasma interaction and gaseous plumes. The submillimetre wave instrument (SWI) has the capabilities to measure sub-mm wave emissions from SO2 as well as other less abundant molecules in Io’s atmosphere like KaCl, NaCl, SO and O2 . The SWI measurements might allow the extraction of vertical profiles and atmospheric dynamics through Doppler shifts, line shapes and ratios. JUICE’s Ultraviolet Spectrograph (UVS) will monitor Io’s torus and neutral clouds remotely through S and O atom and ion emissions and determine the plasma production rates (Masters et al., JUICE WG3 SSR, in review). In addition, it can take remote observations of the Io local aurora and footprint to probe the plasma interaction state. These Io aurora observations obtained during eclipse ingress and egress periods, like the JANUS eclipse observations, can inform our understanding of variability in the relative plume to sublimation source contributions over the three year tour period. UV surface reflectance measurements, while only available at hemispherical-scale spatial resolutions, will be monitored as a function of orbital phase, with Lyman-α variations potentially constraining to Io’s SO2 atmosphere asymmetries. Several stellar occultations are planned, and could provide important new constraints to its nightside atmospheric density especially (not viewable from Earth). At least one JUICE-UVS Jupiter transit observation of Io’s atmosphere is also planned, possibly informing plume influences on Io’s hemisphere-scale atmospheric asymmetries (e.g., Retherford et al., 2019). The Moons And Jupiter Imaging Spectrometer (MAJIS, a visible and near-infrared imaging spectrometer covering wavelengths 0.5 to 5.54 μm) will map Io’s surface with spatial resolutions below 100 km at the closest distances with the potential monitor, e.g., SO2 frost abundances and changes. In addition to remote studies, the Jovian Neutrals Analyser (JNA) part of the Particle Environment Package (PEP) onboard JUICE could monitor S and O Energetic Neutral Atoms (ENA) of 10 eV-3 KeV from the torus (Futaana et al., 2015). The ratio of S/O obtained from such measurements could reveal that the plasma torus originates from volcanic Io materials (SO2). JMAG, RPWI, and the other PEP package plasma instruments will broadly study Jupiter magnetospheric variability, and potential correlations of Io-based volcanic or atmospheric-escape events with plasma injections and potentially other magnetospheric processes related to Io’s plasma interaction.

4.1.3 Europa Clipper

The science objectives of the NASA Europa Clipper mission focus exclusively on Europa and its habitability (Pappalardo et al., 2024, in review). The launch is planned for October 2024 and arrival at Jupiter would be in April 2030 – about one year before JUICE. Similar to JUICE, the trajectory of Clipper avoids the inner magnetosphere and the spacecraft will not be closer than 250 000 km to Io. The spacecraft has partly similar instrumentation with a near identical UVS instrument, a visible camera (Europa Imaging System – EIS), a near-infrared spectrograph (Mapping Imaging Spectrometer for Europa – MISE), and an ion and neutral mass spectrometer (Mass Spectrometer for Planetary Exploration – MASPEX). Potentially, the instruments provide capabilities to take similar measurements mentioned for JUICE above. Europa-UVS has dedicated neutral cloud and torus stare observations obtained ~1-2 days from closest approach that point at Europa and its extended, escaping atmosphere but are intended to help assess the state of the plasma environment. Likewise, Clipper’s pair of plasma sensors (Plasma Instrument for Magnetic Sounding – PIMS) assess the ion composition and thermal electron densities while its magnetometer (Europa Clipper Magnetometer - ECM) measures fields continually throughout the magnetosphere to provide context for its Europa ionosphere and induced-field measurements near closest approach. In addition, The SUrface Dust Analyzer (SUDA) on Clipper has capabilities to constrain Io-genic dust streams with much higher precision and improved mass resolution. If dust ejections are correlated to volcanic activity (Section 2.8) and loss of the bulk gaseous material from Io, SUDA measurements could provide a valuable observatory platform to monitor the activity of Io throughout Europa Clipper’s mission. Also, the E-THEMIS experiment has the ability to measure Io’s heat flow, most of which occurs at longer wavelengths and cannot be measured by a near-IR instrument.

Although Europa science has driven the development of Europa Clipper, a joint working group with JUICE is studying how Europa Clipper can contribute to Jupiter system science, including Io and the plasma torus. Post-launch the Clipper team is expecting to continue discussions of expanded observations of Jupiter system targets for calibrations, operations exercises, and eventually added value science (pending availability of future funds).

4.1.4 Dedicated Io mission

A mission with Io as the main target could potentially address many questions. Despite difficulties to realize an Io mission due to the harsh radiation environment, interesting concepts were put forward in the past. The Io Volcano Observer (IVO) concept completed a Phase A study as a NASA Discovery mission in 2021, but was not selected to proceed (McEwen et al., 2023). The mission could provide much better monitoring of active volcanism and the links between hot spots and plumes. High-resolution visible and thermal observations of vent regions would provide constraints on eruption processes. Magnetometer and plasma instruments could provide monitoring of the atmosphere-plasma interaction, Jupiter’s magnetosphere as well as the plasma torus and its variability relative to volcanic activity. Plasma composition measurements with mass spectrometry capability would be critical to improving our understanding of the chemistry and interaction between the atmosphere and plasma environment. Perhaps most important for understanding the atmosphere would be the first neutral mass spectrometer to operate close to Io, to understand what neutral species and abundances are erupting and present in the atmosphere. For these reasons, NASA’s New Frontiers program includes an Io mission as one of several predetermined targets allowed for the next two proposal opportunities, as recommended through both the Vision and Voyages 2013 and OWL 2023 Decadal Surveys.