OPS5

Water-group gas continuously escapes from Jupiter’s icy moons to form co-orbiting populations of particles, or neutral toroidal clouds. We report the first observations of H₂⁺ pickup ions in Jupiter’s magnetosphere from 13-18 Jovian radii, confirming the presence of a neutral H₂ toroidal cloud. Pickup ion densities are consistent with an advecting Europa-genic cloud source. As the observed H₂⁺ ions are originally produced from H₂ lost from Europa, the abundance of detected ions allows us to determine that ~1 kg/s of neutral H₂ is lost from Europa. Hence, these observations presented here for the first time directly measure ions from a neutral H₂ toroidal cloud at Jupiter, prove the cloud provides an additional plasma source in Jupiter’s magnetosphere, and provide the most direct constraints on Europa’s loss of neutral H₂ via observations of the neutral toroidal cloud’s primary loss process – pickup ions.

How to cite: Szalay, J., Smith, T., Zirnstein, E., McComas, D., Begley, L., Bagenal, F., Delamere, P., Wilson, R., Valek, P., Poppe, A., Nenon, Q., Allegrini, F., Ebert, R., and Bolton, S.: Water-group pickup ions from Europa-genic neutrals orbiting Jupiter, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-702, https://doi.org/10.5194/epsc2022-702, 2022.

Pitch angle distribution of energetic particles can be used to understand the global topology of field lines (connected on both ends to the planet, connected on both ends to the moon or connected on one end to the moon and the other end to the planet etc.). The pitch angle distributions on field lines connected on one end to the moon can also be used to gauge the speed at which the field lines are convecting past a moon.

The Galileo spacecraft flew by the Galilean moons multiple times and explored the charged particle distributions in their vicinities. In this paper we will revisit the data of the Energetic Particles Detector EPD onboard Galileo in terms of pitch angle distributions for various energy channels  for different ion species and electrons in the energy range from 15 keV up to several tens of MeV.

We analyzed the Europa flybys E11, E12, E14, E15, E26.; Ganymede flybys G2, G7, G8, G28, G29; Callisto flybys C3, C9, C10.

We will show how different the energy-dependent pitch angle distributions are upstream and downstream of the moons and how different those distributions are between electrons and the various ions.

Electron PADs near Europa are trapped for energies of several 100 keV while for lower energies the PAD shapes are uncertain. Dropout signatures indicate that charged particles are lost in the moons' tenuous exospheres and onto their surface.

PADs near Ganymede showed bi-directional electron distributions for low energies upstream and trapped for higher energies while PADs of protons and heavy ions are more isotropic. Inside Ganymede’s magnetosphere trapped distributions have been observed when the S/C was connected to closed field lines.

The signal-to-noise ratio of energetic electron and ion fluxes near Callisto is a factor of 10 lower than for Europa. PADs near the moon are less clear compared to the other Galilean satellites. Bi-directional ion PADs close to the moon have been observed.

How to cite: Krupp, N., Roussos, E., Kollmann, P., Paranicas, C., Clark, G., and Khurana, K.: Pitch angle distributions (PADs) near the Galilean moons of Jupiter: Galileo flybys revisited, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-290, https://doi.org/10.5194/epsc2022-290, 2022.

Europa, the smallest of the Galilean moons, is embedded within Jupiter’s magnetosphere where a rapidly flowing plasma interacts electromagnetically with the moon’s atmosphere and its surface. The magnetic field in the plasma is also affected by Europa’s induced magnetic field in a subsurface conducting layer. Between 1996 and 2000, the Galileo spacecraft executed eight close flybys of Europa for which magnetic field measurements were obtained. Since then, several numerical magnetohydrodynamic (MHD) models have been developed to examine the interaction between the rapidly flowing Jovian magnetospheric plasma and Europa and its atmosphere (e.g. Kabin et al. [1999], Schilling et al. [2007], Rubin et al. [2015], Harris et al. [2021]).

In this study, we investigate the large scale interaction of Jupiter’s magnetospheric plasma with Europa and its atmosphere with a single fluid MHD model. We apply the 3D multiphysics model ZEUS-MP, based on Duling et al. [2014], and also employed by Blöcker at al. [2016] to describe Europa’s interaction. Our model accounts for ion-neutral collisions, electron impact ionization, dissociative recombination, and electromagnetic induction in a subsurface water ocean. In particular, we prescribe Europa’s molecular oxygen atmosphere with a number of analytical models in which we consider several degrees of asymmetry of the atmosphere and the location of Europa along its orbit around Jupiter, and we constrain its composition with Hubble Space Telescope (HST) spectral images.

The resulting magnetic field is compared with measurements performed by the Galileo magnetometer. We present an analysis of the observations from the Galileo flybys taking into consideration the variable geometry of the trajectories. Our comparative study is used to further constrain properties of the moon’s atmosphere and to quantify the effects of their variability on the plasma interaction.

How to cite: Cervantes Villa, J. S. and Saur, J.: Europa’s interaction with the Jovian magnetosphere: insights from the Galileo flybys, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-212, https://doi.org/10.5194/epsc2022-212, 2022.

For the entire ion energy range observed at Europa, we calculate spatially resolved maps of the surface sputtering rates of H₂O, O₂, and H₂ from impacts by magnetospheric ions. We use the perturbed electromagnetic fields from a hybrid model of Europa’s plasma interaction, along with a particle-tracing tool, to calculate the trajectories of magnetospheric ions impinging onto the surface and their resultant sputtering yields. We examine how the distribution of the sputtering rates depends on the electromagnetic field perturbations, the angle between the solar radiation and the corotating plasma flow, and the thickness of the oxygen-bearing layer within Europa’s surface. Our major findings are: (a) Magnetic field-line draping partially diverts the impinging ions around Europa, reducing the sputtering rates on the upstream hemisphere, but allowing for substantial sputtering from the downstream hemisphere. In contrast, zero sputtering occurs in much of the downstream hemisphere with uniform electromagnetic fields. (b) If the oxygen-bearing surface layer is thin compared to the penetration depth of magnetospheric ions, thermal ions dominate the O₂ sputtering rates, and the region of strongest sputtering is persistently located near the upstream apex. However, if the oxygen-bearing layer is thick compared to the penetration depth, energetic ions sputter the most O₂, and the location of maximum sputtering follows the sub-solar point as Europa orbits Jupiter. (c) The global production rate of O₂ from Europa’s surface varies by a factor of 3 depending upon the moon’s orbital position, with the maximum particle release occurring when Europa’s Sun-lit and upstream hemispheres coincide.

How to cite: Addison, P., Liuzzo, L., and Simon, S.: Effect of the Magnetospheric Plasma Interaction and Solar Illumination on Ion Sputtering of Europa’s Surface Ice, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-2, https://doi.org/10.5194/epsc2022-2, 2022.

Jupiter’s moon Callisto is exposed to a highly dynamic magnetospheric environment. During a full synodic period, properties of the local magnetospheric field and thermal plasma environment change by an order of magnitude, and Callisto’s resulting interaction with the ambient plasma displays a strong variability.

In this study, we combine results from the AIKEF hybrid and GENTOo test-particle models to constrain the variability of energetic particle dynamics and quantify their flux onto the top of Callisto’s atmosphere during a synodic period. For three positions of Callisto with respect to the center of the Jovian current sheet (at maximum distance above, maximum distance below, and embedded within), we model the interaction between Callisto’s atmosphere/ionosphere, its induced field, and ambient magnetospheric plasma environment, and we trace energetic ions (hydrogen, oxygen, and sulfur) and electrons through the perturbed electromagnetic fields. Our findings highlight the important role that Callisto's interaction with the low energy magnetospheric plasma and signatures associated with the moon’s induced field have on shaping the dynamics and flux patterns of the high-energy particles, which may play a role in the asymmetric ionization of, and energy deposition into, Callisto's neutral atmosphere.

How to cite: Liuzzo, L., Poppe, A., Addison, P., Simon, S., Nenon, Q., and Paranicas, C.: Energetic particle fluxes onto Callisto's atmosphere, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-282, https://doi.org/10.5194/epsc2022-282, 2022.

We present the results of the implantation of sulfur ions in ice samples (H₂O:C₃H₈) in conditions relevant to Europa and other jovian satellites. The samples were prepared and irradiated at 80 K, and were thick enough to ensure the projectiles were implanted in the ice, allowing the sulfur projectiles to become part of the ensuing chemistry. The evolution of the samples was followed with Fourier Transform Infra-Red (FT-IR) spectroscopy, and the organic residues were analyzed off-site through Ultra High Resolution Mass Spectrometry (UHRMS). The UHRMS shows formation of a complex, diverse refractory organic matter, however no evidence of organosulfur formation has been found.

Introduction:

Energetic particles from Jupiter’s magnetosphere (electrons, protons, oxygen and sulfur ions) are likely to be a key driver of the chemical evolution of the icy satellite’s surface, especially Europa; they can alter organic matter deposited on the surface from the internal ocean. In the case of impinging ions, in addition to the stopping power and total energy of the projectile [1], their reactivity is a possible factor in the chemistry. Sulfur is abundant in Jupiter’s magnetosphere [3] due to Io’s volcanic activity, and highly reactive. Experiments with sulfur projectiles have been performed and have shown the efficient creation of sulfuric acid in water ice [4, 5]; previous experiments by our group with interstellar analogs showed it can become part of the organic chemistry triggered by irradiation and form organosulfurs [6]. Here, in temperature conditions relevant to Europa (80K), we investigate the composition of the organic residue generated by the irradiation of a water:propane ice by sulfur ions.

Experiments

We performed the experiments at the ARIBE low-energy line at the Grand Accélérateur National d’Ions Lourds (GANIL) in Caen, France. The projectiles were 105 keV  S⁶⁺ and 105 keV Ar⁷⁺ (the latter to produce a reference sample with a non-reactive projectile). The samples were prepared at 80K on a ZnSe window from a 2:1 H2O:C3H8 gas mixture and irradiated at a fluence of approximately 10¹⁴ ions. To ensure the products would be abundant enough to be detected, cycles of deposition-irradiation were repeated up to 15 times. The finally obtained samples were slowly warmed up to 300K to sublimate the volatiles and leave the refractory organic residue. This residue was analyzed off-site using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). This FT-ICR-MS technique represents the highest mass resolving power (R > 10⁶) and highest mass accuracy (<200 ppb) among all mass spectrometric instruments [7]. This technique allows for detection and identification of species well beyond what is accessible with FT-IR. Two ionization techniques were used: Laser Desorption Ionization and Electro-Spray Ionization.

Results

Infrared spectroscopy shows little difference to previous work on irradiation with electrons of similar ices [8], except for the early production of CO₂ and modest abundance of CH₄. No qualitative difference was seen between Ar-irradiated and S-irradiated samples in the IR.  

The results from the ESI or LDI FT-ICR-MS analysis show a very diverse, (3000 + annotations) refractory (average Double Bond Equivalent=14) organic matter (Figure 1). The residue features an insoluble phase. Heavy molecules (m/z>200) are common, showing that a fairly low dose is enough to form large molecules. No organosulfur could be identified, in stark contrast with our previous experiments [6]. Analysis of the soluble phase by ESI-FT-ICR shows that the oxygen-bearing products tend to be lighter. We discuss how our results compare to experiments using electrons ([8], and our own work).

Figure 1: Annotations (spot size is a function of intensity) obtained with the LDI-FT-ICR analysis, showing number of carbon atoms vs double bond equivalent (DBE) and number of oxygen atoms. A large amount of the residue is oxygen poor and highly aromatic.

Acknowledgements

A.B. and G.D. acknowledge the CNRS program "Programme National de Planétologie" (P.N.P, INSU), G.D. acknowledges the “Programme de Physique et Chimie du Milieu Interstellaire” (P.C.M.I, INSU) and the “Centre National d’Etudes Spatiales” (C.N.E.S) (exobiology program). H.R., P.B. and C.P.dC. acknowledge funding from Région Normandie through RIN EMERGENT SCHINOBI.

References

[1] Teolis et al. JGR: Planets 122.10 (2017): 1996-2012.
[2] von Steiger, R., Schwadron, N., Fisk, L., et al. 2000, JGR: Space Physics, 105, 27217
[3] Paranicas, C., Cooper, J., Garrett, H., Johnson, R., & Sturner, S. 2009, Europa. University of Arizona Press, Tucson, 529
[4] Ding et al. Icarus 226.1 (2013): 860-864.
[5] Strazzulla, et al. Icarus 192.2 (2007): 623-628.
[6] Ruf, et al. ApJL, 885(2), L40 (2019).
[7] Ruf et al.(2017) PNAS 114(11), 2819-2824.
[8] Hand, K. P., & Carlson, R. W. JGR: Planets, 117(E3). (2012)

How to cite: Bouquet, A., Danger, G., Tenelanda-osorio, L., Boduch, P., Rothard, H., Maillard, J., Schmitz-Afonso, I., Afonso, C., Schmitt-Kopplin, P., Duvernay, F., Pires da Costa, C., and Honold, L.: Organic chemistry on the surface of jovian icy satellites: formation of complex refractory organic matter by implantation of sulfur ions into water-alkanes ices, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-812, https://doi.org/10.5194/epsc2022-812, 2022.

JUICE - JUpiter ICy moons Explorer - is the first large mission in the ESA Cosmic Vision 2015-2025 programme. The mission was selected in May 2012 and is currently in its final testing phase. Due to launch in April 2023 and to arrive at Jupiter in July 2031, it will spend at least three ½ years making detailed observations of Jupiter and three of its largest moons, Ganymede, Callisto and Europa.  The status of the project and the main milestones until launch are presented.

The focus of JUICE is to characterise the conditions that might have led to the emergence of habitable environments among the Jovian icy satellites, with special emphasis on the three ocean worlds Ganymede, Europa, and Callisto. Ganymede, the largest moon in the Solar System, is identified as a high-priority target because it provides a unique and natural laboratory for analysis of the nature, evolution and potential habitability of icy worlds and waterworlds in general, but also because of the role it plays within the system of Galilean satellites, and its special magnetic and plasma interactions with the surrounding Jovian environment.

JUICE will also perform a multidisciplinary investigation of the Jupiter system as an archetype for gas giants. The Jovian atmosphere will be studied from the cloud top to the thermosphere. Concerning Jupiter’s magnetosphere, investigations of the three dimensional properties of the magnetodisc and of the coupling processes within the magnetosphere, ionosphere and thermosphere will be carried out. JUICE will study the moons’ interactions with the magnetosphere, gravitational coupling and long-term tidal evolution of the Galilean satellites.

The JUICE payload consists of 10 state-of-the-art instruments plus one experiment that uses the spacecraft telecommunication system with ground-based instruments. A remote sensing package includes imaging (JANUS) and spectral-imaging capabilities from the ultraviolet to the sub-millimetre wavelengths (MAJIS, UVS, SWI). A geophysical package consists of a laser altimeter (GALA) and a radar sounder (RIME) for exploring the surface and subsurface of the moons, and a radio science experiment (3GM) to probe the atmospheres of Jupiter and its satellites and to perform measurements of the gravity fields. An in situ package comprises a powerful suite to study plasma and neutral gas environments (PEP) with remote sensing capabilities of energetic neutrals, a magnetometer (J-MAG) and a radio and plasma wave instrument (RPWI), including electric fields sensors and a Langmuir probe. An experiment (PRIDE) using ground-based Very Long Baseline Interferometry (VLBI) will support precise determination of the spacecraft state vector with the focus at improving the ephemeris of the Jovian system.

The key milestones until launch are:

• Spacecraft flight model environmental acceptance test campaign (mechanical, thermal) and deployment tests
• Spacecraft flight model end-to-end communication tests with the flight control team
• Readiness review of the ground segment
• Mission qualification and acceptance review
• Shipment to Kourou (French Guyana) and launch campaign

How to cite: Witasse, O. and the the JUICE Teams: JUICE (Jupiter Icy Moon Explorer): September 2022 status report, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-645, https://doi.org/10.5194/epsc2022-645, 2022.

1. PEP objectives

The PEP suite explores the particle populations in the Jovian system to answer three overarching science questions:

(1) How does the corotating magnetosphere of Jupiter interact with Ganymede, Callisto, Europa, and Io?

(2) How do internal and solar wind drivers cause such energetic, time variable and multi-scale phenomena in the steadily rotating giant magnetosphere of Jupiter?

(3) What are the structure and composition of the icy moons exospheres and how do they response to the external conditions?

2. PEP suite

PEP measures positive and negative ions, electrons, exospheric neutral gas, thermal plasma and energetic neutral atoms present in all domains of the Jupiter system over nine decades of energy from < 0.001 eV to > 1 MeV with full angular coverage. The six PEP sensors are:

• Jovian plasma Dynamics and Composition analyzer (JDC);
• Jovian Electrons and Ions analyzer (JEI);
• Jovian Energetic Electrons (JoEE);
• Jovian Energetic Neutrals and Ions sensors (JENI);
• Jovian Neutrals Analyzer (JNA);
• Neutral gas and Ion Mass spectrometer (NIM).

For the first time at Jupiter PEP combines global imaging via remote sensing using ENAs with in-situ measurements and performs global imaging of Europa/Io tori and magnetosphere combined with energetic ion measurements. Using low energy ENAs originating from the particle – surface interaction PEP investigate space weathering of the icy moons by precipitation particles. PEP will first-ever directly sample the exospheres of Europa, Ganymede, and Callisto with high mass resolution.

Figure 1: PEP suite and PEP sensors

3. PEP ready to go

All flight models of the PEP sensors and electronics as well as central processing unit and common power supply system have been tested, qualified, and calibrated. The complete model was delivered to ESA and ready for launch in April 2023. All sensors performance fulfilles the mission requirements. PEP will provide the most comprehensive simultaneous measurements of the plasma and neutral gas environment in the Jovian system and at the Galilean moon.

How to cite: Barabash, S., Brandt, P., and Wurz, P. and the PEP Team: The Particle Environment Package (PEP) for the JUICE mission: Ready to go!, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-681, https://doi.org/10.5194/epsc2022-681, 2022.

The Radio & Plasma Wave Investigation (RPWI) onboard the ESA JUpiter ICy moons Explorer (JUICE) is here described in detail. The RPWI provides an elaborate set of state-of-the-art electromagnetic fields and cold plasma instrumentation, including active sounding with the mutual impedance and Langmuir probe sweep techniques, where several different types of sensors will sample the thermal plasma properties, including electron and ion densities, electron temperature, plasma drift speed, the near DC electric fields, and electric and magnetic signals from various types of phenomena like, e.g., electromagnetic and plasma waves, electrostatic acceleration structures, induction fields etc. A full wave vector, waveform, polarization, and Poynting flux determination is aimed for. In addition, RPWI will enable characterization of Jovian radio emissions (including goniopolarimetry) up to 45 MHz, has the capability to carry out passive radio sounding of the ionospheric densities of icy moons and employ passive sub-surface radar measurements. RPWI can also detect micrometeorite impacts, estimate dust charging, monitoring the spacecraft potential as well as the integrated EUV flux. The sensors consist of four 10 cm diameter Langmuir probes each mounted at the tip of 3 m long booms, a triaxial search coil magnetometer and a triaxial radio antenna system both mounted on the 10.5 m long MAG boom, each with radiation resistant pre-amplifiers near the sensors. There are three receiver boards, 2x Digital Processing Units (DPU) and 2x Low Voltage Power Supply (LVPS) boards in a central box in a radiation vault at the centre of the JUICE spacecraft. Together, the RPWI system can carry out a powerful and ambitious planetary science investigation in and around the Galilean icy moons and the Jovian space environment. Some of the more important science objectives & capabilities will be described here. RPWI focuses, apart from cold plasma studies, on the understanding of how, through electro-dynamic and electromagnetic coupling, the momentum and energy transfer occur in the surrounding space environments and with the icy Galilean moons, their surfaces and salty conductive sub-surface oceans. The RPWI instrument is planned to be operational during most of the JUICE mission, during the cruise phase, in the Jovian magnetosphere, during the icy moon flybys, and in particular Ganymede orbit, and will hopefully deliver data from the near surface during the final crash orbit.

How to cite: Bergman, J. and Wahlund, J.-E.: The Radio and Plasma Wave Investigation (RPWI) for the JUpiter ICy moons Explorer (JUICE), Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-838, https://doi.org/10.5194/epsc2022-838, 2022.

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).

How to cite: McEwen, A., Carter, L., DellaGiustina, D., Kestay, L., Denevi, B., Haapala, A., Westlake, J., Park, R., Khurana, K., Thomas, N., Wurz, P., and Helbert, J.: A NASA New Frontiers Mission Concept to Io, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-37, https://doi.org/10.5194/epsc2022-37, 2022.

Abstract. Bierson and Nimmo [1] have proposed that, under certain circumstances, as Galilean-scale satellites accrete they may lose substantial (if not all) of their accreted water ice. This is not, however, a get out of jail free card for satellite accretion models or scenarios. Each satellite accretion model (of which there are many) must be evaluated against the parameters chosen in [1], and the modest ice content of Europa and the apparent anhydrous nature of Io accounted for. Here we evaluate published models in this light, and focus especially on the roles of background protojovian subnebula temperatures and pressures.

Introduction. The formation of Jupiter in the core accretion–gas capture model inevitably ends with a circumplanetary accretion and/or decretion disk (CPD) around the planet. Numerous satellite formation models have been proposed [2-11], with different parameters for disk structure (surface density, temperature, opacity, viscosity). All rely on inferences and assumptions of mass and angular momentum inflow to Jupiter. Numerical gap opening calculations [e.g., 12] provided the basis for the gas-starved model [2-4]. Later 3D radiative-hydrodynamic inflow calculations prompted consideration of heliocentric planetesimal capture as the dominant source of satellite-building solids [13]. Hypotheses for dust and small pebble deposition range from distant infall [e.g., 14] to close-in infall and formation of an outflowing, decretion disk [15]. Multiple generations of satellites may form and be lost [3], Or maybe not, if a magnetospheric cavity opens up close to Jupiter and the innermost satellite, destined to become Io, stalls there [e.g., 16,17].

The thermal environment of Io and Europa’s formation could have been hot enough to largely devolatilize any small, accreting satellitesimals, including the ablated fragments of heliocentric planetesimals that encounter the CPD [8]. Some models call for accretion of Io and Europa in the cold outer disk followed by inward type I migration [e.g., 15,18]. Ultimate removal of major amounts of ice is a non-trivial requirement in such scenarios.

Hydrodynamic Wind. We consider a water vapor atmosphere at the saturation vapor pressure (SVP) of a water ocean formed during accretion [1]. If the outflow wind speed u, which increases away from the surface, becomes transonic at the critical radius r_c = GM/2c² (where G is the gravitational constant, M the satellite mass, and c the isothermal sound speed), then the outflow can continue unimpeded [19,20]. For example, If Europa accreted with a 300 K water surface, the SVP would be 3.35 kPa, with a c = 372 m/s. The isothermal atmosphere is a favorable case for hydrodynamic escape, and because it admits simple analytic solutions for u(r), it is well-suited to this discussion. The critical distance r_c = 7.4R_e, where R_e is Europa’s present radius. At the surface r_s [20]:

For the 300 K Europa case above, u_s ≈ 0.04 m/s, and for steady-state (mass flux conserved) outflow, both the thermodynamic and ram pressure at r_c are ≈6 mPa. Such pressures are low compared with the CPD pressures in even gas-starved models [1,2], even far from Jupiter, which suggests that CPD details as well as accretional mass, radius and surface T, determine if blowoff is possible (e.g., Fig. 1).