Environments of outer planet moons: particles, plasma, fields and dust
This session focuses on outstanding questions on the environments of the moons of outer planets: including the neutral particles (e.g. torii, atmospheres, exospheres, and plumes), electromagnetic fields, dust and plasma, and the interaction of the moons with their environments. Abstracts on all outer planet moons are welcome, including those of Jupiter and Saturn (e.g. Europa, Ganymede, Enceladus, Rhea, and Titan) and the not-well explored moons of Neptune and Uranus.
Prior developments in this field have lead to significant discoveries (e.g. subsurface oceans) and have given rise to new open questions (e.g. active plumes on Europa). Considering the unprecedented opportunity to study these subjects with the upcoming ESA’s JUICE and NASA’s Europa Clipper missions it is essential to bring together the space community on this topic. This session is important to maximize the scientific output of past and current missions, in support of the future missions.
The different topics include (but are not limited to): active processes (e.g. plumes and volcanoes), moon-magnetosphere interaction, magnetic field studies to characterize sub-surface oceans, surface weathering of the moons, neutral exosphere and ionosphere, preparatory studies for future missions, supporting laboratory studies, simulation studies and observations (ground-based/remote/in-situ). Missions of particular relevance include Galileo, Voyager, Cassini-Huygens, Hisaki, Juno, JUICE and Europa Clipper.
Bertrand Bonfond, Emmunuel Jehin, and Amandine Despiegeleire
Io is one of the most active bodies of the solar system, with an intense volcanism powered by tidal forces. Directly or indirectly, the outgassing from the volcanoes is the source of particles populating both the tenuous and patchy atmosphere of Io and the magnetosphere of Jupiter. We report here on the observations of Io with a narrow-band NaI filter on the TRAPPIST-south telescope (ESO La Silla, Chile) during 15 nights between December 4, 2014 and March 31, 2015. The images are strongly contaminated by Jupiter in the field of view. However, we developed and tested several image processing techniques to remove the background and reveil spectacular sodium jets. We detected these jets on 6 nights out of 15. We will discuss the main properties of these jets, such as their length, which can be up to 7 Jovian radii long.
How to cite:
Bonfond, B., Jehin, E., and Despiegeleire, A.: Io sodium jets observed by the TRAPPIST telescopes, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-323, https://doi.org/10.5194/epsc2020-323, 2020.
The Io Torus plasma is mostly composed of singly and multi-charged S and O ions. These ions interact with the neutrals of Io’s atmosphere (S, O, SO2 and SO) through symmetrical (i.e. O+ + O => O + O+) and asymmetrical (i.e. S++ + O => S + O++) charge-exchanges. Charge-exchange cross-sections were estimated in Johnson & Strobel, 1982 and McGrath & Johnson, 1989 at 60 km/s (the plasma corotation velocity in Io’s frame), and are used in numerical simulations of the torus/neutral cloud interaction (i.e. Delamere and Bagenal, 2003).
Dols et al., 2008 proposed numerical simulations of the multi-species chemistry interaction at Io using these cross-sections at 60 km/s. The plasma/atmosphere interaction at Io is strong and the flow velocity and ion temperature are drastically reduced close to Io (v < 10 km/s). Thus, velocity-dependent charge-exchange cross-sections are critical for such numerical simulations and their effect on the local plasma and neutral supply at Io should be explored.
We propose to revisit the calculation of ion/neutral charge-exchange cross-sections following Johnson & Strobel, 1982’s approach for plasma velocities relevant to the local interaction at Io (V=10-120 km/s). More sophisticated calculations were proposed in McGrath & Johnson, 1989 but both publications offered very few details about their procedure.
We will illustrate the effect of using velocity-depend charge-exchange cross-sections in numerical simulations of the multi-species plasma/atmosphere interaction at Io.
More generally, this presentation aimed at providing an incentive for the community to expand the work of McGrath & Johnson, 1989.
Johnson & Strobel, Charge-exchange in the Io torus and exosphere, JGR, 87,1982
McGrath & Johnson, Charge exchange cross sections for the Io plasma torus, JGR, 94, 1989
Delamere & Bagenal, Modeling variability of plasma conditions in the Io torus, JGR, 108, 2003
Dols, Delamere, Bagenal, Kurth, Paterson, A multi-species chemistry model of Io’s local interaction with the plasma torus, JGR, 113, 2008
How to cite:
Dols, V., Johnson, R., and Bagenal, F.: The plasma/atmosphere interaction at Io: revisiting the ion/neutral charge-exchange cross-sections and their ion-velocity dependency, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-496, https://doi.org/10.5194/epsc2020-496, 2020.
The electromagnetic interaction between Jupiter and its innermost Galilean moon Io is a prime example for moon-planet and star-planet interaction. Very striking features are the Io Foot Prints (IFP) in Jupiter’s upper atmosphere. With the Juno spacecraft orbiting Jupiter, new insights about the complex structure of the IFP and the associated tail have been achieved which can not be fully explained by existing models. A deeper understanding is necessary to explain these Juno observations [Mura et al. 2018]. For that purpose a simulation of the system with the single fluid MHD-Code Pluto is set up to study the Alfvén wing generated by Io in detail. In our study, we use a model similar to Jacobsen et al. 2007 with a constant magnetic field and spatially varying density. Then we increase the complexity of this model by including a more realistic wave generator, i.e. Io, and a more complex model of the Jovian inner magnetosphere. Here, we focus on the reflection pattern of the Alfvén wings at the torus boundary and the Jovian ionosphere and how it affects the morphology and the properties of the Alfvén wings.
How to cite:
Schlegel, S. and Saur, J.: Io's auroral footprints: MHD simulations of the interaction between Io and Jupiter, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-646, https://doi.org/10.5194/epsc2020-646, 2020.
Mikhail Sharov, Carl Schmidt, Candace Gray, Nick Schneider, and Paul Withers
Sublimation support of Io’s sulfur dioxide atmosphere is very sensitive to small variations in its surface temperature. As Io passes into Jupiter’s shadow each orbit, its sublimation-supported atmosphere rapidly collapses, leaving volcanic outgassing as the primary mechanism sustaining Io’s thin atmosphere. Using an optical echelle spectrograph at the Apache Point Observatory (APO) 3.5m telescope, we observe Io’s atomic emissions excited solely by electron impact or ion recombination while Io is in umbra. Red line oxygen 6300Å and the sodium D doublet are Io’s brightest emissions at several kiloRayleighs (Bouchez et al. 2000). We observed these emissions falling quickly in just several minutes following Io’s ingress, with sodium airglow declining more rapidly than oxygen. Although the atomic atmosphere is not in vapor pressure equilibrium like SO2, Na and O airglow drops with similar timescales to a decline in SO2 column density that Tsang et al. (2016) reported post-ingress. Interpretation of this behavior warrants care since airglow emissions depend both on photochemical pathways producing neutral atoms and on the plasma conditions: electron density and temperature.
While no new species are identified, several previously unreported emissions in atomic neutrals are observed in the far-red and near-infrared spectral range. Emissions here range from 0.5 up 4 kiloRayleighs when averaged over Io’s disk, and our measurements are insufficiently sensitive to record their temporal response to ingress. Co-added exposures reveal the potassium D doublet and a second doublet of neutral sodium near 8189Å. Laboratory experiments of electrons impacting SO2 produce O I triplets at 7774Å and 8446Å, as well as S I at 9225Å at energy thresholds of 25eV or more (Ajello et al., 2008). Since this threshold is more energetic than the bulk ~5eV population within Io’s plasma torus, detection of these three triplets offers a new tracer for the superthermal plasma population local to the Io–torus interaction region. These spectra can now identify the emissions seen in the Cassini ISS near-infrared filters as it the passed near Io (Geissler et al. 2004), and bright spots near Io’s equator that ISS imaged reaffirm our assertion that Io’s near-IR emissions can be a tracer for energetic electrons. To the best of our knowledge, this is the first time that several of these emissions have been reported in a planetary atmosphere other than the Earth’s.
Ground-based observations of Io in eclipse require precise timing and non-sidereal blind tracking with high accuracy. Successful blind tracking can be confirmed by evaluating pointing errors in offsets to adjacent satellites. Quadrature geometries far from opposition optimize the geometry required to see Io ingress or egress, whilst the moon is well separated from Jupiter’s bright limb. Optical spectra are still strongly contaminated by Jupiter’s scattered light even under optimal geometry. To mitigate this issue, Jupiter spectra are smoothed, aligned, fit, and subtracted from the eclipsed data. A residual airglow spectrum from the Earth and Io remain, which can be separated by Doppler shift. In regions where telluric absorption interferes with the data, such as O I 6300Å, telluric features are characterized and removed using blue fast rotator (BFR) and A0V stars. A Gaussian is then fit to Io’s line spread functions and integrated to give total brightness. Jupiter’s reflectance spectrum is used as a “standard candle” to calibrate absolute flux, and a correction is made for the fact that Io does not fill the slit aperture, so brightness levels reported here are effectively disk-averages.
2. Temporal response of the brightest emissions
Fig. 1 and 2 show the changes Jovian scatter, which is subtracted to isolate Io’s airglow. Fig. 3 shows the temporal evolution of Io’s emission Rayleighs, where the two sodium D lines are summed. It is clear that immediately after Io enters the Jovian umbra both Na D and O6300 lines sharply decrease, with Na falling more rapidly. On the timescales permitted by this observation it remains unclear if these emissions have reached a new steady state associated with an atmosphere solely supported by volcanism.
3. Detection of far-red and near infrared O and S airglow
Io’s eclipse behind Jupiter presents an opportunity to study fainter emissions that are usually swamped by bright surface reflectance. Large cross-sections for several near-IR emissions are known from laboratory spectra of electrons smashing sulfur dioxide (Ajello et al. 2008). To search for these faint emissions, we repeated the above procedure, but co-added all the residuals, thereby averaging any temporal behavior. This indeed reveals oxygen transitions in highly excited states near 11 eV. A sulfur triplet near ~9225Å also indicates transitions from 7.9 eV to 6.5 eV which are somewhat more intense by comparison, but still require a co-addition of exposures to readily identify.