Satellite Io Research Articles

Energetic Charged Particle Measurements During Juno's Two Close Io Flybys

AbstractOn days 2023‐364 and 2024‐034, the Juno spacecraft made close passages of Jupiter's moon Io, at altitudes of about 1,500 km. Data obtained from the first flyby, when the spacecraft was on magnetic field lines connected to both Jupiter and Io, revealed deep flux decreases. In addition, Juno's energetic particle detectors observed tens to hundreds of keV electron and proton beams. Such beams could be generated near Jupiter on field lines associated with Io. The second encounter occurred in the plasma wake and a more modest flux decrease was observed. Furthermore, data from both encounters suggest a spatially extensive decrease in >1 MeV electrons that includes regions inward of Io's orbit. In the immediate vicinity of Io, signatures of absorption likely dominate the data whereas diffusion and wave‐particle interactions are expected to be needed to understand MeV electron data in the wider spatial region around Io.

Super-Earth LHS3844b is Tidally Locked

Short-period exoplanets on circular orbits are thought to be tidally locked into synchronous rotation. If tidally locked, these planets must possess permanent day- and night-sides, with extreme irradiation on the dayside and none on the nightside. However, so far the tidal locking hypothesis for exoplanets is supported by little to no empirical evidence. Previous work showed that the super-Earth LHS 3844b likely has no atmosphere, which makes it ideal for constraining the planet’s rotation. Here we revisit the Spitzer phase curve of LHS 3844b with a thermal model of an atmosphere-less planet and analyze the impact of nonsynchronous rotation, eccentricity, tidal dissipation, and surface composition. Based on the lack of observed strong tidal heating we rule out rapid nonsynchronous rotation (including a Mercury-like 3:2 spin–orbit resonance) and constrain the planet's eccentricity to less than ∼0.001 (more circular than Io's orbit). In addition, LHS 3844b’s phase curve implies that the planet either still experiences weak tidal heating via a small-but-nonzero eccentricity (requiring an undetected orbital companion), or that its surface has been darkened by space weathering; of these two scenarios we consider space weathering more likely. Our results thus support the hypothesis that short-period rocky exoplanets are tidally locked, and further show that space weathering can significantly modify the surfaces of atmosphere-less exoplanets.

Jupiter’s Early Luminosity May Have Driven off Io’s Initial Water Inventory

After its formation, Jupiter was substantially more luminous than it is at the present day (104 times). Right after formation, the inner satellites Io and Europa would have received an order-of-magnitude larger incoming flux from Jupiter, than solar insolation. However, little work has been done to examine the impact this would have had on the Galilean satellites. In this work we explore the possibility that Jupiter’s early luminosity could have caused the loss of any initial water inventory on Io and Europa. We find that, depending on Jupiter’s luminosity after the proto-Jovian disk dissipates, heating from Jupiter could drive rapid water loss via hydrodynamic escape. If early tidal heating dehydrated Io’s accreted silicates, this may provide a mechanism to lose Io’s initial water inventory.

Simulation of Dawn‐To‐Dusk Electric Field in the Jovian Inner Magnetosphere via Region 2‐Like Field‐Aligned Current

AbstractThe presence of the dawn‐to‐dusk electric field of about 4 mV/m in the Jovian inner magnetosphere and its response to the enhancement of the solar wind dynamic pressure are still a mystery of the rotation‐dominated Jovian magnetosphere. Previous studies have suggested that magnetosphere‐ionosphere (M‐I) coupling via Region 2‐like (R2‐like) field‐aligned current (FAC) could be the origin of the Jovian dawn‐to‐dusk electric field. This study investigates whether the dawn‐to‐dusk electric field is formed from this scenario by using a Jovian ionosphere model and a two‐dimensional ionospheric potential solver. Our results show that the dawn‐dusk asymmetry in the ionospheric potential form even at middle latitudes and that the dawn‐to‐dusk electric field is induced in the inner magnetosphere if the electric potential is mapped to the magnetospheric equatorial plane. Around the Io orbit, the calculated electric field strength for the ionosphere without meteoroid influx is too large, 200 mV/m at dawn and 88 mV/m at dusk. One of the solutions is to consider long‐lived meteoric ions in the Jovian ionosphere, which reduce the electric field strength to 15 mV/m at dawn and 12 mV/m at dusk. The model also shows that the electric field strength increases with the intensity of R2‐like FAC, consistent with its response to the solar wind dynamic pressure observed by the Hisaki satellite.

Poynting Fluxes, Field‐Aligned Current Densities, and the Efficiency of the Io‐Jupiter Electrodynamic Interaction

AbstractJuno's highly inclined orbits provide opportunities to sample high‐latitude magnetic field lines connected to the orbit of Io, among the other Galilean satellites. Its payload offers both remote‐sensing and in‐situ measurements of the Io‐Jupiter interaction. These are at discrete points along Io's footprint tail and at least one event (12th perijove) was confirmed to be on a flux tube Alfvénically connected to Io, allowing for an investigation of how the interaction evolves down‐tail. Here we present Alfvén Poynting fluxes and field‐aligned current densities along field lines connected to Io and its orbit. We explore their dependence as a function of down‐tail distance and show the expected decay as seen in UV brightness and electron energy fluxes. We show that the Alfvén Poynting and electron energy fluxes are highly correlated and related by an efficiency that is fully consistent with acceleration from Alfvén wave filamentation via a turbulent cascade process.

Ion-molecule charge exchange in Io's extended atmosphere: Velocity dependence

The Io plasma torus is mostly composed of electrons and ions of O and S, singly and multiply-charged. These ions interact with the neutral species of Io's atmosphere (S, O, SO2 and SO) through symmetrical (e.g., O+ + O ⇒ O + O+) and asymmetrical (e.g., S+ + O ⇒ S + O+) charge exchange reactions. Moreover, molecular ions (SO2+, SO+) produced in Io's atmosphere by ionization or charge exchange are accelerated as they are picked up by the local plasma flow and eventually also charge exchange with the atmospheric neutrals (e.g., SO2+ + SO2 ⇒ SO2 + SO2+).Cross sections for many charge exchange reactions were estimated in Johnson and Strobel (1982) and McGrath and Johnson (1989) for ion speeds of 60 km/s appropriate for torus/neutral cloud interactions where the plasma is approximately at corotation in Io's reference frame. But close to Io, the flow velocity and ion temperature are drastically reduced (Vflow < 10 km/s and Tion < 50 eV) and greatly increased on its flanks (∼80 km/s and 400 eV). Thus, velocity-dependent charge exchange cross sections are critical for numerical simulations of the Io plasma/atmosphere interaction.We provide estimates of charge exchange cross sections of ions on molecular neutrals and of molecular ions on neutrals for the range of velocity [5–120] km/s representative of ion velocities at Io. We combine several methods valid at the low and higher velocities in this range: Two-state impact parameter approximation, quasi-thermal rate constant extrapolation, Langevin orbiting cross sections and scaling from experimental data. We describe these methods in details.Our estimates at 60 km/s are 2–3 times smaller or larger than previously published values, depending on the reaction. Most are generally large (∼60–90 Å2) at small velocities. We discuss the most important reactions relevant to Io's interaction and where improvements and experimental values are needed. We anticipate the effect of these velocity-dependent cross sections in numerical simulations of Io's interaction and show that some of these reactions are of critical importance regarding the plasma composition close to Io, the plasma supply to the torus and the neutral supply to Io's corona and to the giant neutral clouds that extend along Io's orbit.

Rotation models for the Galilean satellites

The primary objective of the present study is to develop and present improved models of the rotational motions of the Galilean satellites Io, Europa, Ganymede, and Callisto. The models include both precessions and nutations of the spin poles, and forced librational oscillations of the rotation rates. Gravitational torques acting on the triaxial figures of these bodies cause their spin poles to precess about their instantaneous orbit poles, whose orientations are quite well known. It is generally expected that the obliquity, or angular separation between spin and orbit poles, for these bodies is small. As a result, there are not yet any good measurements of those angles and thus, the current IAU models for the spin pole motions of these bodies assume zero obliquity. Those models also ignore long period forced librations, driven by deviations of the orbital mean longitudes from linear trends in time. However, near-term future missions to Europa and Ganymede will soon change that situation for the better. We employ a simple model in which the spin pole precessional motion is assumed to be fully damped, and well-known variations in orbital mean longitude drive the librations. Our intent is to both present current estimates of the relevant parameters, and to describe the process used to obtain them, in enough detail to aid in developing better models when the relevant data become available.

Insight Into Io Enabled by Characterization of Its Neutral Oxygen Torus

AbstractThe Jovian system is very intriguing with extremely different particle sources. As the dominant source of particles to Jupiter's magnetosphere, Io is of particular importance. The JAXA Hisaki (SPRINT‐A) mission is providing insightful observations with regard to Io. In particular, its UV neutral oxygen line of sight observations provide the best glimpse so far of Jovian neutral particle populations. This is exciting in that for the first time, the neutral tori can be directly observed on time scales that constrain satellite sources. This data also sheds unprecedented insight into neutral torus distributions, which could subsequently provide essential information about the sources and mechanisms from Io. However, 3‐D modeling is also required to interpret the complex and dynamic observational geometries. Here, we present the first 3‐D distribution of the Io‐generated neutral oxygen torus using JAXA Hisaki neutral oxygen observations as modeling constraints to provide characterization of the Io magnetospheric source and the resulting neutral tori. Major findings of our research include: (a) the effective total original SO2 source rate from Io during a low activity period is ∼800 kg/s; (b) Io has two separate exospheric sources: O at ∼200 kg/s and SO2 at ∼400 kg/s; (c) particles escape from Io preferentially on the Jupiter upstream facing hemisphere; (d) the oxygen neutral torus populates the entire orbit and significantly dominates over the SO2, SO, and S neutral toroidal clouds; and (e) large ionization and dissociation rates limit the significant expansion of Io‐genic neutral particles in Jupiter's magnetosphere beyond ∼2 Rj from Io's orbit.

Loss of Energetic Ions Comprising the Ring Current Populations of Jupiter's Middle and Inner Magnetosphere

AbstractThe low‐altitude, polar orbit of the Juno mission allows the Jupiter Energetic Particle Detector Instrument to view into, and resolve, the loss cone of energetic ions comprising the low‐altitude extension of Jupiter's ring current ions. For regions mapping from just inside Ganymede's orbit to well beyond Ganymede's orbit, energetic ions (&gt;50 keV H+ and &gt;130 keV Oxygen and Sulfur ions) are strongly scattered into the loss cone and lost to the magnetosphere at the “strong diffusion limit” at essentially all times. We conclude, by arguing against magnetic curvature scattering, that the cause is waves, perhaps associated with Alfvénic variations previously documented in this equatorial region. Scattering is generally weak or nonexistent near the orbits of the moons Europa and Io, except for the regions just downstream of the corotating plasmas. For Io, we sometimes observe moderate, but not saturated, scattering within roughly 60° downstream. Significantly, scattering is weak or nonexistent just upstream of Io's position, an asymmetry echoed in some previous wave observations. A preliminary accounting of the total (longitude‐averaged) scattering losses near Io's orbit yields loss rates of order 4%–5% of the strong diffusion limit for H+ and 5%–7% for heavy ions (O + S). We conclude that near Io's orbit, charge exchange losses likely dominate over scattering losses for heavy ions and for the lower energy H+ ions (roughly &lt;200 keV).

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 &lt;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 &lt;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.

A 2020 Observational Perspective of Io

Jupiter's Galilean satellite Io is one of the most remarkable objects in our Solar System. The tidal heating Io undergoes through its orbital resonance with Europa and Ganymede has resulted in a body rich in active silicate volcanism. Over the past decades, Io has been observed from ground-based and Earth-orbiting telescopes and by several spacecraft. In this review we summarize the progress made toward our understanding of the physical and chemical processes related to Io and its environment since the Galileo era. Io science has been revolutionized by the use of adaptive optics techniques on large, 8- to 10-m telescopes. The resultant ever-increasing database, mapping the size, style, and spatial distribution of Io's diverse volcanoes, has improved our understanding of Io's interior structure, its likely composition, and the tidal heating process. Additionally, new observations of Io's atmosphere obtained with these large optical/infrared telescopes and the Atacama Large Millimeter/submillimeter Array reveal the presence of volcanic plumes, the (at times) near-collapse of Io's atmosphere during eclipse, and the interactions of plumes with the sublimation atmosphere. ▪ Extensive new data sets of Io at ultraviolet, mid- to near-infrared, and radio wavelengths have been gathered since the Galileo era. ▪ New data and models inform us about tidal heating, surface properties, and magma composition across Io—although key questions remain. ▪ Atmospheric observations indicate a dominant sublimation-supported component and reinforce the presence of stealth volcanism. ▪ Observations of volcanic plumes show high gas velocities (up to ∼1 km/s) and their effect on Io's atmosphere.

A New Framework to Explain Changes in Io's Footprint Tail Electron Fluxes

AbstractWe analyze precipitating electron fluxes connected to 18 crossings of Io's footprint tail aurora, over altitudes of 0.15 to 1.1 Jovian radii (RJ). The strength of precipitating electron fluxes is dominantly organized by “Io‐Alfvén tail distance,” the angle along Io's orbit between Io and an Alfvén wave trajectory connected to the tail aurora. These fluxes best fit an exponential as a function of down‐tail extent with an e‐folding distance of 21°. The acceleration region altitude likely increases down‐tail, and the majority of parallel electron acceleration sustaining the tail aurora occurs above 1 RJ in altitude. We do not find a correlation between the tail fluxes and the power of the initial Alfvén wave launched from Io. Finally, Juno has likely transited Io's Main Alfvén Wing fluxtube, observing a characteristically distinct signature with precipitating electron fluxes ~600 mW/m2 and an acceleration region extending as low as 0.4 RJ in altitude.

The Librations, Tides, and Interior Structure of Io

AbstractThe melt zone in the volcanically active satellite Io plays a key role in diverse processes such as volcanism, heat transfer, tidal dissipation, and the orbital evolution of the Galilean satellites in the Laplace resonance. Information on the melt distribution, the degree of partial melt, and the possible existence of a global magma ocean has been derived from Galileo magnetic induction measurements, volcanic eruption temperatures, and Hubble Space Telescope auroral spot observations but is currently inconclusive. Here, we calculate the libration amplitude of Io for a diverse set of internal structure models and show that Io's libration can provide insight into the existence of a global magma ocean and into the thickness and rigidity of the crust above it. The diurnal libration amplitude of Io is several times larger if Io has a magma ocean instead of a partial melt asthenosphere and can reach values of above 1 km, making it easily observable for spacecraft flying close by Io on multiple occasions. It also strongly increases with decreasing crustal thickness and increases significantly with increasing rigidity. We demonstrate that the combination of observations of the libration amplitude and the tidal Love number k2 will allow estimating the rigidity and thickness of the crust separately, which is impossible with only one of the quantities observed.

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 &gt;139 RJ. The polar‐orbiting Juno mission carries the Jupiter Energetic particle Detector Instrument that is serendipitously sensitive to ENAs with energies &gt;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.

Combining UV Spectra and Physical Chemistry to Constrain the Hot Electron Fraction in the Io Plasma Torus

AbstractWe have developed a spectral emission model that is a function of the plasma composition, electron temperature, and density in the Io plasma torus. The lines are excited by electron collisions and spontaneously decay resulting in UV emission that is diagnostic of the plasma conditions. In a previous study we used a single Maxwellian distribution to model the UV spectra obtained by the Cassini UVIS instrument in January 2001. We now try to determine the fraction of hot electrons using a double Maxwellian distribution where the core, thermal electron distribution is combined with a hot electron distribution, also assumed to be a Maxwellian. This spectral emission model does not well constrain the fraction of hot electrons, which can be seen by the χ2 output. By using physical chemistry modeling of equilibrium conditions, we determine the fraction of hot electrons. Our physical chemistry model shows that in order to match the plasma composition and temperatures near the orbit of Io during the Cassini flyby of Jupiter we need the hot electrons to comprise &lt;0.3% of the total electron density.

Equilibrium Tidal Response of Jupiter: Detectability by the Juno Spacecraft

Abstract An observation of Jupiter's tidal response is anticipated for the ongoing Juno spacecraft mission. We combine self-consistent, numerical models of Jupiter's equilibrium tidal response with observed Doppler shifts from the Juno gravity science experiment to test the sensitivity of the spacecraft to tides raised by the Galilean satellites and the Sun. The concentric Maclaurin spheroid (CMS) method finds the equilibrium shape and gravity field of a rotating, liquid planet with the tide raised by a satellite, expanded in Love numbers (k nm ). We present improvements to the CMS theory that eliminate an unphysical center-of-mass offset and study in detail the convergence behavior of the CMS approach. We demonstrate that the dependence of k nm with orbital distance is important when considering the combined tidal response for Jupiter. Conversely, the details of the interior structure have a negligible influence on k nm for models that match the zonal harmonics J 2, J 4, and J 6, already measured to high precision by Juno. As the mission continues, improved coverage of Jupiter’s gravity field at different phases of Io’s orbit is expected to yield an observed value for the degree-two Love number (k 22) and potentially select higher-degree k nm . We present a test of the sensitivity of the Juno Doppler signal to the calculated k nm , which suggests the detectability of k 33, k 42, and k 31, in addition to k 22. A mismatch of a robust Juno observation with the remarkably small range in calculated Io equilibrium, k 22 = 0.58976 ± 0.0001, would indicate a heretofore uncharacterized dynamic contribution to the tides.

Spatially Asymmetric Increase in Hot Electron Fraction in the Io Plasma Torus During Volcanically Active Period Revealed by Observations by Hisaki/EXCEED From November 2014 to May 2015

AbstractThe satellite Io, which has volcanoes and is located at 5.9 RJ from the center of Jupiter, is a powerful plasma source in the magnetosphere. The heavy ions originating from Io form a torus‐like structure and emit radiation. The pickup energy and hot electrons are believed to power the Io plasma torus. Voyager data showed that a trace amount of hot electrons (at several hundreds of eV) exist in the torus. The origin of hot electrons, that is, plasma heating and/or transport mechanisms, have been mentioned in previous research. However, the contribution of each mechanism toward supplying hot electrons remains poorly understood. To address this issue, we explored the time variation and spatial structure of hot electrons by spectroscopic observations using the Hisaki satellite. In this study, the radial distributions of plasma densities and temperatures were derived from the emission line intensities in the extreme ultraviolet range of day of year (DOY) 331 in 2014 to DOY 134 in 2015, which includes the Io's volcanically active period. We found that hot electrons inside the torus began to increase particularly on the duskside ~40 days after the onset of volcanic activity. This result suggests that the mass increase in the torus with volcanic activity enhanced the plasma transport from the outside within a specific region or via a local heating process.

Proton Acceleration by Io's Alfvénic Interaction

AbstractThe Jovian Auroral Distributions Experiment aboard Juno observed accelerated proton populations connected to Io's footprint tail aurora. While accelerated electron populations have been previously linked with Io's auroral footprint tail aurora, we present new evidence for proton acceleration due to Io's Alfvénic interaction with Jupiter's magnetosphere. Separate populations were accelerated above the Io torus and at high latitudes near Jupiter. The timing suggests the acceleration is due to Alfvén waves associated with Io's Main Alfvén Wing. The inferred high‐latitude proton acceleration region spans 0.9–2.5 Jovian radii in altitude, comparable to the expected location for electron acceleration, and suggests the associated Alfvén waves are able to accelerate electrons and protons in similar locations. The proton populations magnetically connected to Io's orbit are recently perturbed, equilibrating with the nominal torus plasma population on a timescale smaller than Io's System III orbital period of ~13 h, likely due to wave‐particle interactions. The tail populations are split into a wake‐like structure with distinct inner and outer regions, where the inner region maps to an equatorial width nearly identical to the diameter of Io. The approximately symmetric surrounding outer regions are each slightly smaller than the central region and may be related to Io's atmospheric extent. The nominal, corotational torus proton population exhibits energization throughout all regions, peaking at the anti‐Jovian flank of the inner core region mapping to Io's diameter. These proton observations suggest Alfvén waves are capable of accelerating protons in multiple locations and provide further evidence that Io's Alfvénic interaction is bifurcated.

Transient Change of Io's Neutral Oxygen Cloud and Plasma Torus Observed by Hisaki

AbstractIo's atmospheric oxygen atoms are heated by atmospheric sputtering and escape from Io's gravity, forming a neutral oxygen cloud around Io's orbit. This neutral cloud is important as a source of the Io oxygen plasma torus. Previous studies derived the distribution and density of the equilibrium neutral oxygen cloud. However, little is known about the evolution of the neutral cloud. In this study, we analyzed Hisaki satellite observations of the spatial distribution of OI 130.4 nm emissions around Io's orbit during transient strong density enhancement in the torus in 2015 (called high density period). Comparing time variations of OI and OII 83.4 nm emissions, we estimated that the lifetimes of O+ in this period were about 21 days in the high density period and 41 days in the normal density period. Hisaki observations are consistent with a decrease in the lifetime of O+ when the density in the torus increases. The radial distribution showed the neutral oxygen cloud spread outward up to 8.6 Jupiter radii during the high density period. We also show that during the high density period, the neutral oxygen number density at Io's orbit (where north‐south thickness is assumed to be 1.2 Jupiter radii) increased to cm−3, more than three times the value during the normal density period ( cm−3). The azimuthal distribution showed a dense region around Io and a longitudinally uniform, diffuse region distributed along Io's orbit that enlarges during the high density period.

The Galilean Satellites Formed Slowly from Pebbles

Abstract It is generally accepted that the four major (Galilean) satellites formed out of the gas disk that accompanied Jupiter’s formation. However, understanding the specifics of the formation process is challenging, as both small particles (pebbles) and the satellites are subject to fast migration processes. Here we hypothesize a new scenario for the origin of the Galilean system, based on the capture of several planetesimal seeds and subsequent slow accretion of pebbles. To halt migration, we invoke an inner disk truncation radius, and other parameters are tuned for the model to match physical, dynamical, compositional, and structural constraints. In our scenario it is natural that Ganymede’s mass is determined by pebble isolation. Our slow pebble accretion scenario then reproduces the following characteristics: (1) the mass of all the Galilean satellites; (2) the orbits of Io, Europa, and Ganymede captured in mutual 2:1 mean motion resonances; (3) the ice mass fractions of all the Galilean satellites; and (4) the unique ice-rock partially differentiated Callisto and the complete differentiation of the other satellites. Our scenario is unique to simultaneously reproduce these disparate properties.