Europa's Orbit Research Articles

Changes in the Plasma Sheet Conditions at Europa's Orbit Retrieved From Lead Angle of the Satellite Auroral Footprints

AbstractThe electromagnetic interaction between Europa and the plasma sheet in the Jovian magnetosphere generates Alfvén waves, ultimately generating auroral footprints in Jupiter's atmosphere. The position of Europa's auroral footprint is a proxy for travel time of the Alfvén waves. We measured Europa's footprint position using the far‐ultraviolet images of Jupiter obtained by the Hubble Space Telescope in two observing campaigns in 2014 and 2022. The measured footprint position indicates a longer Alfvén travel time in the 2022 campaign. We retrieved the plasma sheet parameters at Europa's orbit from the footprint position by tracing the Alfvén waves launched at Europa and found an increase of both mass density and temperature in the plasma sheet in 2022. The Poynting flux generated at Europa is calculated with the retrieved plasma sheet parameters, which suggests the total energy transfer from Europa to its auroral footprint is similar to the case of Io.

Survey and Analysis of Whistler‐ and Z‐Mode Emission in the Juno Extended Mission

AbstractWhistler‐mode and Z‐mode waves may have significant impact on electron scattering and acceleration at Jupiter as recent stochastic models indicate. New observations of these waves in the Juno extended mission expand the orbital coverage from previous surveys, extending especially to smaller radial distances near the magnetic equator, and to extended ranges of dusk‐side magnetic local time. M‐shells transiting the Io torus and the orbit of Europa are recognized as a significant source region for whistler‐mode lower and upper band chorus emission with intensity peaks near 9 RJ. Ganymede is a significant source of intense whistler‐mode chorus confined to the local moon environment. New Z‐mode observations now extend from a few kHz to over 80 kHz. Inferred source regions for Z‐mode are consistent with the outer edge of the Io torus and with auroral field lines that may also support broadband kilometric or decametric emission.

Constraints on Europa’s Water Group Torus from HST/COS Observations

In situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter provide indirect evidence that an enhancement of neutral gas is present near the orbit of the moon Europa. Simulations suggest that such a neutral gas torus can be sustained by escape from Europa’s atmosphere and consists primarily of molecular hydrogen, but the neutral gas torus has not yet been measured directly through emissions or in situ. Here we present observations by the Cosmic Origins Spectrograph (COS) of the Hubble Space Telescope (HST) from 2020 to 2021, which scanned the equatorial plane between 8 and 10 planetary radii west of Jupiter. No neutral gas emissions are detected. We derive upper limits on the emissions and compare these to modeled emissions from electron impact and resonant scattering using a Europa torus Monte Carlo model for the neutral gases. The comparison supports the previous findings that the torus is dilute and primarily consists of molecular hydrogen. A detection of sulfur ion emissions radially inward of the Europa orbit is consistent with emissions from the extended Io torus and with sulfur ion fractional abundances as previously detected.

Influence of Low Europa Orbit design on gravity field recovery

The characterization of Europa's surface and interior ocean and ice shell is key to explore the habitability of the fourth largest moon of Jupiter. Europa Clipper will largely contribute to our knowledge of Europa with multiple dedicated flybys. However, global mapping can only be significantly improved by means of an orbiter with high inclination and low altitude, as it will be done in the case of Ganymede by JUICE. This was the strategy of several mission proposals (Joint Europa Mission, Jupiter Europa Orbiter, HADES). A specific difficulty in designing suitable orbits in the Jovian system is related to the orbit stability, which is highly impacted by the influence of Jupiter as a third body exerting strong perturbations on a Europa orbiter. This results in constraints for potential science orbits which would allow, e.g., for the determination of Europa's gravity field using radio tracking to the probe. In this presentation, we will analyze a set of different low Europa orbits and compare their scientific value for gravity field recovery. We will explore different orbital characteristics, with the possibility to use repetitive ground track orbits. The analysis will be performed based on closed-loop simulations using the planetary extension of the Bernese GNSS Software. We simulate realistic Doppler tracking data (2-way X-band Doppler range rate) from the Deep Space Network. These observations are then used to reconstruct the orbit as well as geodetic parameters, such as Europa gravity field parameters and the tidal Love number k2. We will discuss the quality of the retrieved solutions.

Jupiter's Ion Radiation Belts Inward of Europa's Orbit

AbstractJupiter is surrounded by intense and energetic radiation belts, yet most of the available in‐situ data, in volume and quality, were taken outward of Europa's orbit, where radiation conditions are not as extreme. Here, we study measurements of ions of tens of keV to tens of MeV at <10 Jupiter radii (RJ) distance to Jupiter, therefore inward of the orbit of Europa. Ion intensities drop around 6 RJ, near Io's orbit. Previous missions reported on radiation belts of tens and hundreds of MeV ions located between 2 and 4 RJ. Measurements of lower energies were not conclusive because high energy particles often contaminate the measurement of lower energy particles. Here, we show for the first time that ions in the hundreds of keV range are present and suggest that ions may extend even into the GeV range. The observation of charged particles yields information on the entire field line, not just the local field. We find that there is a region close to Jupiter where no magnetic trapping is possible. Jupiter's innermost radiation belt is located at <2 RJ, inward of the main ring. Previous work suggested that this belt is sourced by re‐ionized energetic neutral atoms coming steadily inward from distant regions. Here, we perform a phase space density analysis that shows consistency with such a local source. However, an alternative explanation is that the radiation belt is populated by occasional strong radial transport and then decays on the timescale of years.

First Report of Electron Measurements During a Europa Footprint Tail Crossing by Juno

AbstractWe report the first in situ observations of electron measurements at a Europa footprint tail (FPT) crossing in the auroral region. During its 12th science perijove pass, Juno crossed magnetic field lines connected to Europa's FPT. We find that electrons in the range ~0.4 to ~25 keV, with a characteristic energy of 3.6 ± 0.5 keV, precipitate into Jupiter's atmosphere to create the footprint aurora. The energy flux peaks at ~36 mW/m2, while the peak ultraviolet (UV) brightness is estimated at 37 kR. We estimate the peak electron density and temperature to be 17.3 cm−3 and 1.8 ± 0.1 keV, respectively. Using magnetic flux shell mapping, we estimate that the radial width of the interaction at Europa's orbit spans roughly 3.6 ± 1.0 Europa radii. In contrast to typical Io FPT crossings, the instrument background caused by penetrating energetic radiation (> ~5–10 MeV electrons) increased during the Europa FPT crossing.

Juno Energetic Neutral Atom (ENA) Remote Measurements of Magnetospheric Injection Dynamics in Jupiter's Io Torus Regions

AbstractIn planetary magnetospheres, singly charged energetic particles, trapped by the planet's magnetic field, can steal electrons from cold gas atoms and become neutralized. These now energetic neutral atoms (ENAs), no longer confined by the magnetic field, can travel out of the system similar to photons leaving a hot oven. ENAs have been used to image magnetospheric processes at Earth, Jupiter, and Saturn. At Jupiter, the opportunities to image the magnetosphere have been limited and always from the perspective of the near‐equatorial plane at distance >139 RJ. The polar‐orbiting Juno mission carries the Jupiter Energetic particle Detector Instrument that is serendipitously sensitive to ENAs with energies >50 keV, provided that there are no charged particles in the environment to mask their presence. Here we report on the first ENA observations of Jupiter's magnetosphere from a nonequatorial perspective. In this brief report we concentrate on emissions seen during Perijove 22 (PJ22) during very active conditions and compare them with emissions during the inactive Perijove 23 (PJ23). We observe, and discriminate between, distinct ENA signatures from the neutral gases occupying the orbit of Io (away from Io itself), the orbit of Europa (away from Europa), and from Jupiter itself. Strong ENA emissions from Io's orbit during PJ22 are associated with energetic particle injections observed near Io's orbit several hours earlier. Some injections occurred planetward of Io's L‐shell (magnetic position), somewhat of a surprise given that injections are thought to be driven by outward transport of plasmas generated by Io.

A Physical Model of the Proton Radiation Belts of Jupiter inside Europa's Orbit

AbstractA physical model of the Jovian trapped protons with kinetic energies higher than 1 MeV inward of the orbit of the icy moon Europa is presented. The model, named Salammbô, takes into account the radial diffusion process, the absorption effect of the Jovian moons, and the Coulomb collisions and charge exchanges with the cold plasma and neutral populations of the inner Jovian magnetosphere. Preliminary modeling of the wave‐particle interaction with electromagnetic ion cyclotron waves near the moon Io is also performed. Salammbô is validated against in situ proton measurements of Pioneer 10, Pioneer 11, Voyager 1, Galileo Probe, and Galileo Orbiter. A prominent feature of the MeV proton intensity distribution in the modeled area is the 2 orders of magnitude flux depletion observed in MeV measurements near the orbit of Io. Our simulations reveal that this is not due to direct interactions with the moon or its neutral environment but results from scattering of the protons by electromagnetic ion cyclotron waves.

A new physical model of the electron radiation belts of Jupiter inside Europa's orbit

AbstractA new global physical model of the electron radiation belts of Jupiter inside the orbit of Europa is presented. It is an update of the Office National d'Etudes et de Recherches Aérospatiales model Salammbô proposed by Sicard and Bourdarie () that now integrates a contribution of the wave‐particle interaction above the orbit of Io. In addition, a revisited and more realistic boundary condition near Europa is implemented. The new Salammbô model is validated against the in situ flux measurements obtained by the Pioneer 10, Pioneer 11, and Voyager 1 missions. The role of the wave‐particle interaction is discussed by investigating its effect on the predicted in situ fluxes. In particular, the interaction appears in our simulations as a strong pitch angle scattering process explaining the flux depletions intensities near the orbit of Io as well as the flux intensities near the spacecraft perijoves.

The vertical thickness of Jupiter's Europa gas torus from charged particle measurements

AbstractMeasurements and modeling suggest the presence of a neutral gas torus collocated with the orbit of Jupiter's moon Europa. Here we use data from the CMS instrument that is part of the Energetic Particles Detector (EPD) on board the Galileo spacecraft to characterize the distribution of 130 keV protons. Near the orbit of Europa this distribution has a minimum around 70° in equatorial pitch angle. We reproduce this with a model assuming that the protons are lost via charge exchange with a gas torus. Since the pitch angle characterizes whether the protons remain mostly in the dense center of the torus or continuously bounce through it, we can determine the latitudinal extent of the torus. We find that the full thickness where its density falls to 1/e of its maximum has to be and is closer to ≈1RJ.

Charge states of energetic oxygen and sulfur ions in Jupiter's magnetosphere

AbstractPitch angle distributions of proton and energetic heavy ion fluxes near Europa's orbit have been measured by the Galileo Energetic Particles Detector (EPD). At similar energies, these distributions have important differences. If their source and transport processes are similar, as we hypothesize here, then it is difficult to reconcile their different pitch angle distributions. By looking at the same question, other researchers have proposed that the heavies are multiply charged, leading to differences in how the particles are lost. This could not be confirmed directly with EPD because that detector does not separate heavy ion measurements by charge state. However, indirect analyses of the data have extracted the charge state of a few sulfur events. We present here a complete list of ion injections observed with EPD over the whole mission. Energetic sulfur and oxygen charge states can be inferred through a dispersion analysis of dynamic injections that makes use of the charge‐dependent nature of the gradient‐curvature azimuthal drift. We find that sulfur is predominantly multiply charged, whereas oxygen is more evenly distributed between singly and doubly charged states. In addition to current theories on energetic heavy ion transport near the Europa region, we propose that charge gain for the oxygen ions (electron stripping) may play an important role in the character of energetic particles in that region.

Searching for less perturbed elliptical orbits around Europa

Space missions intending to visit Europa, one of the famous Galilean's moons of Jupiter, are among the most important topics in space activities today. There is an increasing interest in the scientific community to send spacecrafts to be inserted into Europa's orbit, with goals like mapping its surface and gravitational field. From the quality of the observations until the orbital maneuvers, the required aspects for the success of the mission will depend on the orbits used by the spacecraft. The present work searches for less perturbed elliptical orbits around Europa, because they are very important, since these orbits are expected to be more stable to place the spacecraft. The development of the study is based on the net effects of the perturbing forces over the time, evaluated by the integral of those forces with respect to the time. The value of this integral depends both on the dynamical model and the orbit of the spacecraft. Jupiter's third-body perturbation and the J2 and J3 terms of the gravitational potential of Europa are the perturbing forces considered. The results presented here are obtained by performing numerical integrations of the perturbing forces, and they show the locations of the less perturbed orbits.

A NEW UNDERSTANDING OF THE EUROPA ATMOSPHERE AND LIMITS ON GEOPHYSICAL ACTIVITY

Deep extreme ultraviolet spectrograph exposures of the plasma sheet at the orbit of Europa, obtained in 2001 using the Cassini Ultraviolet Imaging Spectrograph experiment, have been analyzed to determine the state of the gas. The results are in basic agreement with earlier results, in particular with Voyager encounter measurements of electron density and temperature. Mass loading rates and lack of detectable neutrals in the plasma sheet, however, are in conflict with earlier determinations of atmospheric composition and density at Europa. A substantial fraction of the plasma species at the Europa orbit are long-lived sulfur ions originating at Io, with ~25% derived from Europa. During the outward radial diffusion process to the Europa orbit, heat deposition forces a significant rise in plasma electron temperature and latitudinal size accompanied with conversion to higher order ions, a clear indication that mass loading from Europa is very low. Analysis of far ultraviolet spectra from exposures on Europa leads to the conclusion that earlier reported atmospheric measurements have been misinterpreted. The results in the present work are also in conflict with a report that energetic neutral particles imaged by the Cassini ion and neutral camera experiment originate at the Europa orbit. An interpretation of persistent energetic proton pitch angle distributions near the Europa orbit as an effect of a significant population of neutral gas is also in conflict with the results of the present work. The general conclusion drawn here is that Europa is geophysically far less active than inferred in previous research, with mass loading of the plasma sheet ≤4.5 x 10^(25) atoms s^(-1) two orders of magnitude below earlier published calculations. Temporal variability in the region joining the Io and Europa orbits, based on the accumulated evidence, is forced by the response of the system to geophysical activity at Io. No evidence for the direct injection of H_2O into the Europa atmosphere or from Europa into the magnetosphere system, as has been observed at Enceladus in the Saturn system, is obtained in the present investigation.

Near Ballistic Halo-to-Halo Transfers between Planetary Moons

Intermoon transfers are important components of planetary tour missions. However, these transfers are challenging to design due in part to the chaotic environment created by the multi-body dynamics. The specific objective of this work is to develop a systematic methodology to find fuel optimal, near ballistic Halo-to-Halo trajectories between planetary moons, and we achieve this goal by combining dynamical systems theory with a variety of nonlinear programming techniques. The spacecraft is constrained to start at a Halo orbit of a moon and end at another Halo orbit of a second moon. Our approach overcomes the obstacles of the chaotic dynamics by combining multiple “resonant-hopping” gravity assists with manifolds that control the low-energy transport near the Halo orbits of the moons. To help construct good initial guesses, contours of semimajor axes that can be reached by falling off a Halo orbit are presented. An empirical relationship is then derived to find quickly the boundary conditions on the Halo orbits that lead to ballistic capture and escape trajectories, and connect to desired resonances. The overall optimization procedure is broken into four parts of increasing fidelity: creation of the initial guess from unstable resonant orbits and manifolds, decomposition and optimization of the trajectory into two independent ideal three-body portions, end-to-end refinement in a patched three-body model, and transition to an ephemeris model using a continuation method. Each step is based on a robust multiple shooting approach to reduce the sensitivities associated with the close approach trajectories. Numerical results of an intermoon transfer in the Jovian system are presented. In an ephemeris model, using only 55 m/s and 205 days, a spacecraft can transfer between a Halo orbit of Ganymede and a Halo orbit of Europa.

Optimization of low-energy resonant hopping transfers between planetary moons

In response to the scientific interest in Jupiter's Galilean moons, NASA and ESA have plans to send orbiting missions to Europa and Ganymede, respectively. The inter-moon transfers of the Jovian system offer obvious advantages in terms of scientific return, but are also challenging to design and optimize due in part to the large, often chaotic, sensitivities associated with repeated close encounters of the planetary moons. The approach outlined in this paper confronts this shortcoming by exploiting the multi-body dynamics with a patched three-body model to enable multiple “resonant-hopping” gravity assists. Initial conditions of unstable resonant orbits are pre-computed and provide starting points for the elusive initial guess associated with the highly nonlinear optimization problem. The core of the optimization algorithm relies on a fast and robust multiple-shooting technique to provide better controllability and reduce the sensitivities associated with the close approach trajectories. The complexity of the optimization problem is also reduced with the help of the Tisserand–Poincaré (T–P) graph that provides a simple way to target trajectories in the patched three-body problem. Preliminary numerical results of inter-moon transfers in the Jovian system are presented. For example, using only 59m/s and 158 days, a spacecraft can transfer between a close resonant orbit of Ganymede and a close resonant orbit of Europa.

The birth of Saturn's baby moons

Simulations show that Saturn's nearby moons, after forming on the outskirts of the planet's main rings, get pushed clear of them. This model reproduces the moons' orbital locations and remarkably low densities. A population of Saturn's smaller moons contrasts dramatically with the regular moons of the giant planets. Moons like Saturn's Enceladus and Jupiter's Europa orbit in the equatorial plane of their host planet and are thought to have finished their accretion at about the same time as the planets, 4.5 billion years ago. Saturn's icy moonlets are much younger, formed less than 10 million years ago, and as their spectra resemble those of the main rings it has been suggested that they formed by accretion at the rings' edges. A new hybrid numerical simulation of the Saturn system supports this idea, suggesting that the moons formed via viscous spreading of Saturn's main rings beyond the Roche limit (the distance beyond which rings are gravitationally unstable, about 140,000 km from Saturn). After the moons' formation, the edge of the ring migrated inward.

Return to Europa: Overview of the Jupiter Europa orbiter mission

Missions to explore Europa have been imagined ever since the Voyager mission first suggested that Europa was geologically very young. Subsequently, the Galileo spacecraft supplied fascinating new insights into this satellite of Jupiter. Now, an international team is proposing a return to the Jupiter system and Europa with the Europa Jupiter System Mission (EJSM). Currently, NASA and ESA are designing two orbiters that would explore the Jovian system and then each would settle into orbit around one of Jupiter’s icy satellites, Europa and Ganymede. In addition, the Japanese Aerospace eXploration Agency (JAXA) is considering a Jupiter magnetospheric orbiter and the Russian Space Agency is investigating a Europa lander. The Jupiter Europa Orbiter (JEO) would be the NASA-led portion of the EJSM; JEO would address a very important subset of the complete EJSM science objectives and is designed to function alone or in conjunction with ESA’s Jupiter Ganymede Orbiter (JGO). The JEO mission concept uses a single orbiter flight system that would travel to Jupiter by means of a multiple-gravity-assist trajectory and then perform a multi-year study of Europa and the Jupiter system, including 30 months of Jupiter system science and a comprehensive Europa orbit phase of 9 months. The JEO mission would investigate various options for future surface landings. The JEO mission science objectives, as defined by the international EJSM Science Definition Team, include: A. Europa’s ocean: Characterize the extent of the ocean and its relation to the deeper interior. B. Europa’s ice shell: Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface–ice–ocean exchange. C. Europa’s chemistry: Determine global surface compositions and chemistry, especially as related to habitability. D. Europa’s geology: Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration. E. Jupiter system: Understand Europa in the context of the Jupiter system. The JEO orbital mission would provide critical measurements to support the scientific and technical selection of future landed options. The primary challenge of a Europa mission is to perform in Jupiter’s radiation environment, radiation damage being the life-limiting parameter for the flight system. Instilling a system-level radiation-hardened-by-design approach very early in the mission concept would mitigate the pervasive mission and system-level impacts (including trajectory, configuration, fault protection, operational scenarios, and circuit design) that can otherwise result in runaway growth of cost and mass. This paper addresses the JEO mission concept developed by a joint team from JPL and the Applied Physics Laboratory to address the science objectives defined by an international science definition team formed in 2008, while designing for the Jupiter environment.

Cycloid crack sequences on Europa: Relationship to stress history and constraints on growth mechanics based on cusp angles

The original model developed to explain cycloidal cracks on Europa interprets cycloids as tensile fractures that grow in a curved path in response to the constantly rotating diurnal tidal stress field. Cusps form when a new cycloid crack segment propagates at an angle to the first in response to a rotation of the principal tidal stress orientation during a period of no crack growth. A recent revised model states that a cycloid cusp forms through the creation of a secondary fracture called a tailcrack at the tip of an existing cycloid segment during shearing motion induced by the rotating tidal stress field. As the tailcrack propagates away from the cusp, it becomes the next cycloid segment in the chain. The qualitative tailcrack model uniquely accounts for the normal and shear stresses that mechanically must resolve onto the tip of an existing cycloid segment at the instant of cusp formation. In this work, we provide a quantitative framework and test of the hitherto purely conceptual tailcrack model. We first present a relative age sequence inferred from geologic mapping of multiply cross-cutting cycloids in Europa's trailing hemisphere and place this into the context of the global stress history. The age sequence requires a cumulative minimum of 630° of shell reorientation due to nonsynchronous rotation to account for the observed range of orientations of cycloids of different ages. We determined the back-rotated longitudes of formation of two cycloid chain examples and used mathematical modeling of europan tidal stresses to show that the tailcrack model for cusp formation is not only viable, but places constraints on the overall development of a cycloid chain by controlling the timing of cusp development within Europa's orbit. For all cusps analyzed, the exact ratio of resolved shear to normal stress required to form the cusp angles by a process of tailcracking, as governed by the principles of linear elastic fracture mechanics, is produced at the tip of a shearing cycloid segment during Europa's orbit. Cusp formation occurs after the point in the orbit at which the maximum tensile principal tidal stress occurs, implying that tensile tidal stresses are not directly responsible for cusp development. Instead, cusps develop when a tailcrack forms at the tip of a cycloid segment in response to the highly perturbed stress field induced during concomitant opening and shearing at the tip of the cycloid segment.

Stochastic models of hot planetary and satellite coronas: Atomic oxygen in Europa’s corona

This paper analyzes the formation, kinetics, and transport of hot oxygen atoms in the atmosphere of the Jovian satellite Europa. Atmospheric sources of suprathermal oxygen atoms are assumed to be represented by the processes of dissociation of molecular oxygen, which is the main component of the atmosphere, by solar UV radiation and electron fluxes from the inner magnetosphere of Jupiter, as well as by the reaction of dissociative recombination of the main ionospheric ion O2+ which thermal electrons. It is shown that dissociation in Europa’s near-surface atmosphere is balanced by the processes of the loss of atomic oxygen due to the effective escape of suprathermal oxygen atoms into the inner magnetosphere of Jupiter along the orbit of Europa and due to ionization by magnetospheric electrons and catalytic recombination of oxygen atoms on the icy surface of the satellite. It thus follows that atomic oxygen is only a small admixture to the main atmospheric component—molecular oxygen—in the near-surface part of the atmosphere. However, the outer exospheric layers of Europa’s atmosphere are populated mostly by suprathermal oxygen atoms. The near-surface molecular envelope of Europa is therefore surrounded by a tenuous extended corona of hot atomic oxygen.

Europa's atmosphere, gas tori, and magnetospheric implications

A two-dimensional kinetic model calculation for the water group species (H 2O, H 2, O 2, OH, O, H) in Europa's atmosphere is undertaken to determine its basic compositional structure, gas escape rates, and velocity distribution information to initialize neutral cloud model calculations for the most important gas tori. The dominant atmospheric species is O 2 at low altitudes and H 2 at higher altitudes with average day–night column densities of 4.5 × 10 14 and 7.7 × 10 13 cm −2 , respectively. H 2 forms the most important gas torus with an escape rate of ∼ 2 × 10 27 s −1 followed by O with an escape rate of ∼ 5 × 10 26 s −1 , created primarily as exothermic O products from O 2 dissociation by magnetospheric electrons. The circumplanetary distributions of H 2 and O are highly peaked about the satellite location and asymmetrically distributed near Europa's orbit about Jupiter, have substantial forward clouds extending radially inward to Io's orbit, and have spatially integrated cloud populations of 4.2 × 10 33 molecules for H 2 and 4.0 × 10 32 atoms for O that are larger than their corresponding populations in Europa's local atmosphere by a factor of ∼200 and ∼1000, respectively. The cloud population for H 2 is a factor of ∼3 times larger than that for the combined cloud population of Io's O and S neutral clouds and provides the dominant neutral population beyond the so-called ramp region at 7.4–7.8 R J in the plasma torus. The calculated brightness of Europa's O cloud on the sky plane is very dim at the sub-Rayleigh level. The H 2 and O tori provide a new source of europagenic molecular and atomic pickup ions for the thermal plasma and introduce a neutral barrier in which new plasma sinks are created for the cooler iogenic plasma as it is transported radially outward and in which new sinks are created to alter the population and pitch angle distribution of the energetic plasma as it is transported radially inward. The europagenic instantaneous pickup ion rates are peaked at Europa's orbit, dominate the iogenic pickup ion rates beyond the ramp region, and introduce new secondary plasma source peaks in the solution of the plasma transport problem. The H 2 torus is identified as the unknown Europa gas torus that creates both the observed loss of energetic H + ions at Europa's orbit and the corresponding measured ENA production rate for H.