Unraveling Io’s Atmospheric Mysteries: Gaps, Contradictions, and Open Questions

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

3.4 Gaps in understanding, contradictions, and inconsistencies

3.4.1 Significant increases in atmospheric loss inconsistent with current understanding

It can be assumed with some confidence that the volatiles from Io that supply the torus must first populate the moon’s atmosphere. As summarized in Sections 2.3 and 2.4, the loss from the atmosphere to neutral clouds and local ionization and pick-up into the torus is primarily due to collisions of the magnetospheric plasma with the atmosphere or neutral clouds. All other processes are likely insufficient to maintain a supply rate to neutral clouds and ultimately (or directly) to the plasma torus on the order of the canonical value of 1 tons/s. Importantly, direct escape from outgassing plume neutral gases is far too low (Table 1) for causing an enhancement of several tons per second.

Plasma collisions at or near the exobase (which possibly might be at the surface at some locations like on the night side) most effectively provide momentum to the molecules or atoms to escape from Io’s gravity. The effectiveness of these losses largely depends on the mass and energy flow of the corotating plasma that interacts with the atmosphere near the exobase. The characteristics of this exobase, like its altitude and variability, might thus play a key role for the atmospheric loss processes to supply the neutral and plasma environment.

Assuming the collisions between plasma and atmosphere happen at exactly the exobase, the surface area of this “exobase sphere” is proportional to the radius squared. In other words, the higher the exobase, the larger the body of plasma that is intersected by the neutral atmosphere cross section. In addition, the higher the exobase is, the lower is Io’s gravity at this altitude and thus the easier particles escape. The dependence of the decreasing escape velocity gives the third power in the proportionality.

We can also assume a fixed surface density and increase the scale height H and thereby the atmospheric column density, again for an isothermal, exponential atmosphere. The 8 times higher exobase would imply a ~5-fold temperature increase and thus a 5-fold increase in column density, as we had assumed a fixed surface density.

A transient, strong increase of the upper atmosphere temperature might potentially be caused by a period of significantly enhanced Joule heating (Section 2.3.3). The available power for Joule heating in the corotional electric field at Io is likely not fully used under standard interaction conditions (Sections 2.3.3 and Section 2.4) and thus a change of atmospheric conditions has the potential to lead to an increase of Joule heating.

Overall the strong changes (>3 orders of magnitude in density, or factor ~5 in temperature) are in contrast to the observational findings of a stable dayside atmosphere, which we summarize in the following section (Figure 22).

3.4.2 Lack of observational evidence for transient changes in the atmosphere

Io’s atmosphere reveals clear lateral (from equator to poles), longitudinal and temporal (day-night, eclipse passages, seasons) variability (Section 2.2). However, even a small change in the global atmospheric abundance not related to these systematic variabilities has never been measured with certainty, i.e., there is no observational evidence that the dayside atmosphere density undergoes changes and even less significant changes. The longest observational coverage of the dayside SO2 abundance came from mid-infrared observations (22 years) and revealed only seasonal variability on the order of factor ~2 due to the changing sublimation with changing heliocentric distance of Io (Tsang et al., 2013; Section 2.2, Figure 8).

Various atmospheric temperatures between 110 and 600 K were inferred from different methods. However, when the same method is used very similar temperatures are found in different observations (Section 2.2). Hence, there is no observational evidence for significant temperature changes in the atmosphere so far. We note, however, that the temperature of the upper atmosphere cannot be probed by the bulk atmosphere observations and thus a transient change in only the uppermost atmospheric layers, due to e.g., Joule heating, would remain undetected. Jeans escape varies exponentially with the Jeans λ parameter. Therefore, during a significant increase in the upper atmosphere temperature, might lead to significant escape of the atomic species (Section 2.3.3).

Recent observational results suggest that volcanic outgassing is a relevant source for the atmosphere in addition to sublimation of surface frost (e.g., de Pater et al., 2021a), but massive gas plumes that produce densities much higher than the average equatorial dayside atmosphere density were never seen. There is some evidence for SO2 and other gases in volcanic plumes (Section 2.1) but the abundances above plume locations are overall similar to generally inferred abundances in the equatorial dayside atmosphere. Thus there is no evidence (yet) for events of extreme outgassing or any other transient change in the atmosphere that would suggest an Io-triggered change of the atmospheric loss by a factor of 3-4 that is derived from the change in the neutral source rate.

3.4.3 Commonly assumed but unconfirmed correlations and connections

The lack of understanding on the role of the atmosphere as well as of observations of atmospheric events is a key missing part to understanding the system as a whole. This missing link raises some doubts about the connections of other parts in the system and we want to point out some weak or not yet substantiated points around often made arguments on the connection of Io’s volcanic activity to the torus and magnetosphere:

A. Global state of Io’s volcanic activity undefined. Despite the much higher candance in monitoring of thermal emissions from Earth in the last decade and new results from the Juno mission, the existing observations of thermal emissions from Io do not provide evidence for generally and globally different states of volcanic activity at Io at different times (Section 2.1). Hence, the often cited “volcanically active” and “volcanically quiet” periods can not be defined or derived from actual observations of volcanic activity. This is purely a concept that was invented for explaining the concept of different supply rates to the neutral clouds and plasma torus or of changes in the magnetosphere otherwise. There are strong increases of thermal emissions observed at volcanic sites, dubbed outbursts, which could however so far not be correlated with changes in the magnetosphere.

B. Large plumes are seen in most close-up spacecraft images, but never from Earth. Often, imaging observations of large plumes like Pele or Tvashtar taken during spacecraft flybys like Cassini or New Horizons are considered as evidence for a particular volcanic event. However, (large) plumes are seen in almost all spacecraft images (mostly taken at high phase angles) of Io, but remote observations from Earth at low phase angle are difficult and only allow faint detections of large (known) plumes (Jessup and Spencer, 2012). Hence, cadence or activity cycles of such large plumes are not well known but instead the cadence of plume detections is determined by the availability of spacecraft imaging data suitable for plume detections.

C. Complex and unclear connection between hot spots and outgassing. Hot spot activity is not necessarily connected to outgassing and thus not a diagnostic for volcanic gas input to the atmosphere (and even less to the neutral clouds and torus). This applies even for the presence of pure volcanic gases like NaCl. In addition, Galileo data showed and recent Juno data confirmed (e.g., Zambon et al., 2023) that the hot spots detectable from Earth are only the brightest and there are many more small sites with enhanced thermal emissions undetectable from Earth. Furthermore, the correlation of thermal hot spots and sodium trace gas suggested in the study of Mendillo et al. (2004) has been questioned based on new observational insights (Section 2.1).

D. Unclear connection between sodium and bulk gases. The pathways of alkali compounds including sodium through the system are likely quite different from the bulk gases. The alkali are sourced to the atmosphere only (or primarily) from volcanic outgassing while SO2 gas in the atmosphere is sublimated to at least 50% from surface frosts. The escape processes for alkali compounds and their daughter species might be different from the escape processes for the bulk SO2 and daughter species. In addition, the high velocity (larger or near Jupiter escape velocity of 25 km/s) particles that source the nebula (sodium or any other) likely originate from different processes than those sourcing the neutral clouds and ultimately plasma torus. Therefore, the variation observed in the sodium nebula might not be coupled to the neutral cloud and torus variation.

E. Unclear connection between dust in the Jovian system and volcanic eruptions. Dust streams measured in and beyond the Jovian magnetosphere have been associated with dust in Io’s plumes and thus volcanic activity. The dust particle trajectories, the flux variability and composition of the dust stream identify Io as the source and suggest volcanic origin of the particles. However, like for the gaseous trace species, the connection of abundance and variation of dust and of the bulk gases (SO2 in the atmosphere, S and O neutrals and ions in the magnetosphere) in the system is unclear. There seems to be a wide range of dust to gas ratio in plumes including dust-free “stealth” plumes (Section 2.1) and the escape processes of the dust from Io are not well understood yet. The mass rate of dust lost from Io is 3-4 orders of magnitude lower than the neutral source rate for the torus. The Galileo dust measurements did not provide evidence for a temporal connection of magnetospheric dust streams intensity to volcanic hot spot detections and the putative dust increase in 2001 suggested to be connected to a torus change has a large observational uncertainty.

F. Aurora features connected to injections possibly triggered by Io are frequent. Jupiter’s aurora is shaped and affected by various magnetospheric and external processes and the connection to the mass output from Io is rather indirect (Section 2.7). The morphological features in the main emission possibility connected to Io mass output enhancements appear relatively frequently, more often than other transient events. Unfortunately, there are no aurora imaging observations from the year 2015 during the strong and well monitored transient torus and neutral cloud enhancement.

3.4.4 Conclusion and open questions

Thus, while there is evidence that the neutral gas in the magnetosphere and the plasma in the Io torus occasionally undergo transient changes, it is not known if and how they are triggered or caused. The idea that volcanic activity at Io causes large scale changes in the magnetosphere is therefore a hypothesis with many unknown elements that yet needs to be substantiated.

While there are many open questions about the details of each of the parts in the system reviewed in Section 2, we provide here a list of overarching questions either on the workings of the system or on the diagnostics commonly used:

1. How do thermal eruptions relate to volcanic outgassing? In particular, what types of volcanoes or styles of activity directly produce gas (and dust) and how much? And how does gas output from Io’s volcanoes evolve before, during, and after a thermal eruption phase?

   
   

2. Is it possible that local outgassing at a volcanic site significantly changes the overall loss of neutral gases (or dust) from Io? If so, what effect, if any, does latitude, longitude, or time of day have on whether outgassing products are lost from Io?

   
   

3. Does the global atmosphere undergo significant transient changes, possibly preceding and triggering the transient events in the torus and magnetosphere? If so, what causes these events?

   
   

4. Can Io’s mass loss to the neutral clouds and plasma torus be enhanced significantly without significant changes in the bound atmosphere?

   
   

5. What is the composition of neutral and ionized gases lost to the environment? In particular, how much is lost in molecular vs atomic form? What is the fraction and composition of the ions directly supplied from Io (Io’s ionosphere) to the torus?

   
   

6. Is every brightening of the sodium nebula accompanied by changes in the neutral clouds and plasma torus?

   
   

7. Is the dust input from Io to the magnetosphere correlated with the gas supply?

   
   

8. What physical processes trigger and affect auroral phenomena during transient torus enhancements? Specifically, is the location shift of aurora solely achieved by variation of mass loss outflow, or do other quantities (e.g., large scale magnetospheric flow variabilities, electron temperatures or the Pedersen conductivity also contribute? And how does the morphology and brightness of the main emission evolve over a transient torus enhancement event like the one observed in 2015?