How Jupiter’s Fiery Moon Feeds the Jovian Magnetosphere

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

2.8 Dust from Io

Io is a persistent source of dust in the Jovian magnetosphere. Grains released from Io, either ejected via impact bombardment from interplanetary dust grains or volcanic activity, become charged and experience the force of Jupiter’s gravity and electromagnetic fields. Since the discovery of Io’s strong volcanism in 1979 (Smith et al., 1979; Morabito et al., 1979), it was proposed that dust grains from volcanic plumes were injected continuously into Jupiter’s magnetosphere through electromagnetic forces (Johnson et al., 1980; Morfill et al., 1980). The first observational evidence for the dust particles was provided when the Ulysses spacecraft flew by Jupiter in 1992 and the onboard dust detector measured periodic bursts of sub-micrometer dust particles within 1 AU from Jupiter. These dust particles were measured in dust streams radiating from the direction of Jupiter, indicating that the periodic bursts of dust come from the Jovian system (Grün et al., 1993a; 1993b). Somewhat similar to the readily observable trace species like sodium, dust is used to probe for variability in the Io-Jupiter system and was sometimes even connected to volcanic activity. We focus here on the smaller dust grains (~0.01 μm), which were found to trace back to Io (Graps et al., 2000), while the larger grains are found to originate from a variety of sources in the Jovian system (e.g., Liu and Schmidt, 2019). A general review of the Jovian dust environment can be found in Krüger et al. (2004).

2.8.1. Galileo dust measurements and possible connections to volcanic activity

More detailed insight into the Jovian sub-micrometer dust environment was given by long-term in situ dust measurements of the Galileo spacecraft mission. The frequency analysis of the Galileo dust detector (DDS) data by Graps et al. (2000) led to the direct evidence of Io’s volcanoes being the main dust source in the Jovian magnetosphere. Impact ejecta from Io was ruled out as a dominant source of dust for the dust streams (Krüger at al., 1999). Furthermore, it was shown that the stream particles are strongly coupled to Jupiter’s magnetic field (Grün et al., 1998).

Using Galileo’s measurements of the Jovian dust streams as a monitor for its volcanic activity, Krüger et al. (2003a) conducted a study to examine the orbit-to-orbit variability of the dust emission and link it to the volcanic activity on Io. The eruptions of large Pele-type plumes are expected to contribute most to the dust escape on Io (Krüger et al. 2003b) as only they might be able to accelerate the dust grains to high altitudes so they can escape Io’s gravity (Johnson et al. 1980, Ip 1996). The temporal coverage of direct sightings of plume activity during the Galileo mission is very limited (McEwen et al., 1998; Keszthelyi et al., 2001; Geissler and McMillan, 2008) and therefore makes it complicated to correlate the sightings to dust observations. A better time coverage of plume activity is provided by observations of surface changes due to eruptions (Geissler, 2003), but these surface changes do not provide a precise date of the eruption.

Figure 21 (left) shows the derived minimum and maximum emission rates in the distance range of 13–30 RJ by crosses and triangles, respectively. Horizontal bars represent periods when large-area surface changes were observed (Geissler, 2003). Arrows show the time of individual volcanic plume sightings, note that the length of the eruptions is not known. After ejection from Io, the escape of the dust particles from the torus is influenced by the dawn-to-dusk asymmetry of the plasma torus as grains are charged and experience electromagnetic forces. Due to the different charging conditions between dawn and dusk, grains on the dusk side preferentially escape with timescales ≾1 hour, while grains on the dawn side reside longer in the torus, escaping with timescales of ~1 day (Horányi et al., 1997). After grains leave the torus, they take several hours to travel to a distance of 30 RJ (Krüger et al., 2003b). Therefore, the particles arrive within 1-2 days at the Galileo spacecraft for the derived dust emissions shown in Figure 21 (left). Krüger at al. (2003b) derived a typical average dust emission rate of 0.1 to 1 kg/s for distances between 13 RJ and 30 RJ from Jupiter. This results in about 0.01 to 0.1% of the total mass (assuming the canonical number of 1 ton/s for the loss of mass from Io) ejected from Io into the magnetosphere (Krüger et al., 2004).

In many cases, the time of the giant plume eruptions match the time periods when increased dust emissions were detected suggesting that dust measurements may provide an effective monitor of Io’s volcanic activity (Krüger at al,. 2003b). However, the total duration of the eruptions is not known and the lack of a plume detection does not mean there is no ongoing eruption, both of these complicate this interpretation.

Converting the local dust fluxes measured by Galileo to estimates of total dust output from Io requires assumptions on their outward radial transport. Therefore, measurements farther from Io have more uncertainty with respect to estimating total Io dust emission. Hence, the large dust emission rate of about 100 kg/s (dashed line between G28 and G29 labels in Figure 21) should be considered with caution because during orbit G28 Galileo was located far away (about 280 RJ) from Jupiter (Krüger et al., 2003b). Despite this uncertainty, Delamere et al. (2004) used the dust count rate enhancement in September 2000 as supporting evidence for a proposed enhanced mass loading of the torus, as derived from Cassini UVIS measurements of enhanced torus UV emissions in this period.

2.8.2. Cassini dust measurements: Composition of the dust particles

While the Galileo measurements provided a long duration of dust measurements, it lacked the ability of further characterizing the dust particles’ composition. The measurements by the Cosmic Dust Analyser (CDA) onboard the Cassini spacecraft taken during the Jupiter flyby in 2000 provided first constraints on the dust particle makeup (Postberg et al., 2006). Sodium and chlorine ions were the most detected species from the dust and their correlation (Figure 21, right) suggested sodium chloride (NaCl) to be the primary dust particle constituent. In addition, sulfurous as well as potassium bearing components were identified. Postberg et al. (2006) interpret the primarily alkali composition of the dust as an indication that >95% of the measured particles originate from Io and its volcanoes.

The Cassini measurements started on September 4 in 2000, potentially capturing the end of the putative enhancement in the Galileo data around September (Figure 22). However, Cassini was at a large distance to Jupiter (>1 AU, on approach) at this time and an anomaly in the Cassini dust counts for this time is not mentioned in Postberg et al. (2006).

Finally, based on observations of a particular feature in sodium gas emissions Grava et al. (2021) showed sodium atoms might be sputtered from charged dust grains escaping from Io. This result supports the hypothesis that dust particles might be an important carrier of alkalis that ultimately populate the neutral clouds and extended neutral nebulae (Section 2.5).