Galilean Satellites Research Articles

On the Relation Between Auroral Morphologies and Compression Conditions of Jupiter's Magnetopause: Observations From Juno and the Hubble Space Telescope

AbstractJupiter displays the most powerful auroral emissions in our solar system, which result from strong energy dissipation in Jupiter's surrounding space environment. Although mass and energy in Jupiter's magnetosphere mostly come from the innermost Galilean moon Io's volcanic activity and Jupiter's rotation, solar wind perturbations can play crucial roles in releasing magnetospheric energy. The systematic response of the aurora to a solar wind compression remains poorly understood because of timing uncertainties. Here we report the analysis of a set of auroral images from the Hubble Space Telescope with contemporaneous in situ magnetopause detections from Juno, allowing for a more direct comparison. By analyzing the dawn side main auroral emission, we distinguish two non‐mutually exclusive types of auroral enhancements: auroral dawn storm (ADS) featured with latitudinal extension in limited longitudes, and a long‐lasting main auroral brightening with limited extension in latitudes while extending over a large longitude range. Only the latter systematically appears under a compressed magnetopause, while the dawn storms could occur whatever the state of the magnetopause. The results could provide important constraints to improve theoretical models and numerical simulations. During expanded magnetopause conditions, Jupiter's aurora displayed either quiet or dawn storm morphology. The result is consistent with recent discovery of the initiation of ADSs in midnight and post‐midnight, possibly driven by magnetic reconnection plasma instabilities in night magnetotail. Our results show that some typical auroral morphologies could be used as a diagnostic of solar wind conditions at Jupiter.

The PHEMU21 catalogue and astrometric results of the observations of the mutual occultations and eclipses of the Galilean satellites of Jupiter made in 2021

ABSTRACT 2021 was the year of Jupiter’s equinox, that is the Sun and the Earth passed through the equatorial plane of the planet and therefore the orbital planes of its main satellites. This occurrence made it possible to observe mutual occultations and eclipses between the satellites. Our former experience shows that observations of such events provide accurate astrometric data that can be used to obtain new information on the dynamics of the Galilean satellites. The observations are a series of photometric measurements of a satellite which are carried out through the organization of a world wide campaign of observations thus maximizing the number and the quality of the data obtained. This work focuses on processing the photometric observations of the mutual occultations and eclipses of the Galilean satellites of Jupiter made during the international campaign in 2021. The final goal is to derive new accurate astrometric data. We used an accurate photometric model of mutual events in conjunction with the accuracy of observation. We obtained and processed the 84 light curves obtained during the campaign. As compared with the current best ephemerides, the rms of ‘O–C’ residuals are equal to 49 and 48 mas in right ascension and declination, respectively.

Decoupled and coupled moons’ ephemerides estimation strategies application to the JUICE mission

When reconstructing natural satellites' ephemerides from space missions' tracking data, the dynamics of the spacecraft and natural bodies are often solved for separately, in a decoupled manner. Alternatively, the ephemeris generation and spacecraft orbit determination can be performed concurrently. This method directly maps the available data set to the estimated parameters' covariances while fully accounting for all dynamical couplings. It thus provides a statistically consistent solution to the estimation problem, whereas this is not directly ensured with the decoupled strategy. For the Galilean moons in particular, the JUICE mission provides a unique, although challenging, opportunity for ephemerides improvement. For such a dynamically coupled problem, choosing between the two state estimation strategies will be influential. This paper quantifies the Galilean moons' state uncertainties attainable when applying a coupled estimation strategy to simulated JUICE data, and discusses the challenges that remain to be addressed to achieve such a coupled solution from real observations. We first provide a detailed, explicit formulation for the coupled approach, which was still missing in the literature although already used in past studies. We then assessed the relative performances of the two ephemerides generation techniques for the JUICE test case. To this end, we used both decoupled and coupled models on simulated JUICE radiometric data. We compared the resulting covariances for the Galilean moons' states, and showed that the decoupled approach yields slightly lower formal errors for the moons' tangential positions. However, the coupled model can reduce the state uncertainties by more than one order of magnitude in the radial direction ( i.e. towards the central body). It also proved more sensitive to the dynamical coupling between Io, Europa and Ganymede, allowing the state solutions for the first two moons to fully benefit from JUICE orbital phase around Ganymede. On the other hand, we showed that the choice of state estimation methods does not strongly affect the moons' gravity field determination. Many issues still remain to be solved before a concurrent estimation strategy can be successfully applied, especially to reconstruct the moons’ dynamics over long timescales. Nonetheless, our analysis highlights promising ephemerides improvements and thus motivates future efforts to reach a coupled state solution for the Galilean moons.

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.

Search for Jovian decametric emission induced by Europa on the extensive Nançay Decameter Array catalog

Context. The electrodynamic interaction between the Galilean satellites and the Jovian magnetosphere generates Alfvén wings that connect the satellites to the polar atmosphere of Jupiter and induce auroral radiation through the cyclotron-maser instability. The satellite control of the Jovian decametric emission is widely known and has been studied since the 1960s, being first discovered with regard to Io and, more recently, Ganymede. The partial control of these emission by Europa and Callisto, however, has not yet been confirmed, however, hints of this control have already been found. Aims. The goal of this work is to search for evidence of control of the Jovian decametric emission by the satellite Europa. Methods. For this purpose, we analyzed the extensive digital catalog of Jovian decametric emission detected by the Nançay Decameter Array from 1990 to 2020. We analyzed distributions of the occurrence probability of the emission not induced by Io nor by Ganymede as a function of Europa phase and of the Array’s longitude with regard to the Jovian central meridian of longitude. Results. As a result, we selected 267 possible Europa-induced emission, from which 186 are from source A (Eu-A), 56 are from source C (Eu-C), and 25 are from source D (Eu-D). The general maximum frequency and duration of these emission are presented and compared to those of the other emission in the catalog and their average power is estimated as a function of the average power of the Io-induced emission. Conclusions. We conclude that Europa, just as in the case of Io and Ganymede, induces a portion of the Jovian decametric emission.

Orbital Stability and Invariant Manifolds on Distant Retrograde Orbits around Ganymede and Nearby Higher-Period Orbits

In the past few years, distant retrograde orbits (DROs) have become increasingly popular due to their conspicuous stability. Nevertheless, it is this characteristic that results in the challenge to the design of transfer orbits into/out of DROs. This paper investigates the DROs around Ganymede in order to utilize their dynamical characteristics for Jupiter system exploration. In particular, the DRO family is calculated by numerical integration and numerical continuation, higher-period orbits near the DROs are detected using bifurcation theory, and characteristics including orbital stability and invariant manifolds of these orbits are investigated through stability indices and manifold theory. The stability of DROs and the higher-period orbits are first investigated in the circular restricted three-body problem and are then verified in a third-body gravitation perturbation model. The results show that the strong stability of DROs makes it possible to observe the Galilean moons for long periods and that the higher-period orbits that bifurcate from the DROs offer additional insight into the motion of probes approaching/departing from the vicinities of the DROs. Further investigation of the invariant manifolds around higher-period orbits reveals the feasibility of utilizing the DRO family and the nearby unstable structures for multi-target exploration and low-energy transfer between the Galilean moons.

Jupiter’s Satellites, from Cassini to Today’s Icy Worlds: 350 Years of Observation and Study of the Galilean Satellites of Jupiter at Paris Observatory

The satellites of Jupiter discovered by Galileo in 1610 are among the most interesting and difficult objects to study. These are complex worlds whose size is close to that of Mars. The four satellites are similar to a small solar system, their motions bringing together all the difficulties of celestial mechanics. These satellites, known as the Galilean satellites also have the peculiarity of being regularly eclipsed by the planet Jupiter, a phenomenon easily observed from Earth. As soon as the Paris Observatory was created, it soon became clear that these satellites were not only objects of study for astronomers, but also valuable auxiliaries for geographers and explorers, helping to define a universal time. Thus, they were used to perfect the cartography of France as desired by Louis XIV during the creation of the observatory. They also helped demonstrate the finite speed of light. The greatest astronomers were interested by these satellites, especially Laplace, who first, understood and described the dynamics of the Jovian system. Delambre undertook to observe and collect many observations of the eclipses of these satellites which are still useful today. More recently, space exploration revived these studies; the Voyager probe sent us unprecedented images revealing their icy surface. The continuous gain in accuracy of astrometric observations allowed to refine their dynamics, modeling and constraining their internal structure: the tidal effects of Jupiter on its satellites are now observable from Earth. These satellites remain a chosen field of study at Paris Observatory; where 350 years later their story continues to be told.

The Dynamical Viability of an Extended Jupiter Ring System

Planetary rings are often speculated as being a relatively common attribute of giant planets, partly based on their prevalence within the solar system. However, their formation and sustainability remain a topic of open discussion, and the most massive planet within our planetary system harbors a very modest ring system. Here, we present the results of an N-body simulation that explores dynamical constraints on the presence of substantial ring material for Jupiter. Our simulations extend from within the rigid satellite Roche limit to 10% of the Jupiter Hill radius, and include outcomes from 106 and 107 yr integrations. The results show possible regions of a sustained dense ring material presence around Jupiter that may comprise the foundation for moon formation. The results largely demonstrate the truncation of stable orbits imposed by the Galilean satellites, and dynamical desiccation of dense ring material within the range ∼3–29 Jupiter radii. We discuss the implications of these results for exoplanets, and the complex relationship between the simultaneous presence of rings and massive moon systems.

Frequency-dependent Ganymede’s tidal Love number [formula omitted] detection by JUICE’s 3GM experiment and implications for the subsurface ocean thickness

Ganymede will be the first icy satellite in the Solar System orbited by a spacecraft, ESA’s JUpiter ICy moons Explorer (JUICE). JUICE launch is scheduled for April 2023 and the arrival at Ganymede is foreseen in 2035.Thanks to the advanced Ka-band radio tracking system, the Geodesy and Geophysics of Jupiter and Galilean Moons (3GM) experiment aboard the mission will provide range and range-rate measurements that will be used to infer the static (up to degree 35–45) and time-varying gravity field and the internal structure of the moon. Ganymede is subject to tidal deformation, which periodically modifies its gravity field. The larger contribution to the time-varying gravity is due to the tidal interaction with Jupiter, modulated by the eccentricity and the inclination of the moon’s orbit. In addition, Ganymede also experiences a lower amplitude time-varying tidal interaction with Io, Europa and Callisto. To a good approximation, the corresponding gravitational signals are periodic and they contain several harmonics of the fundamental synodic frequencies. The elastic response of Ganymede is expected to be frequency-dependent as well. In this work we describe a procedure to estimate the Ganymede’s tidal Love number k2 at different frequencies in the orbit determination process of the 3GM experiment, supported by numerical simulations. Finally, we show that 3GM measurements can be used to characterize the interior structure of Ganymede, providing a tight constraint on the subsurface ocean thickness. The ocean density can be also constrained, but with a lower precision.

Exosphere-mediated migration of volatile species on airless bodies across the solar system

Surface-bound exospheres facilitate volatile migration across the surfaces of nearly airless bodies. However, such transport requires that the body can both form and retain an exosphere. To form a sublimation exosphere requires the surface of a body to be sufficiently warm for surface volatiles to sublime; to retain an exosphere, the ballistic escape and photodestruction rates and other loss mechanisms must be sufficiently low. Here we construct a simple free molecular model of exospheres formed by volatile desorption or sublimation. We consider the conditions for forming and retaining exospheres for common volatile species across the Solar System, and explore how three processes (desorption/sublimation, ballistic loss, and photodestruction) shape exospheric dynamics on airless bodies. Our model finds that the CO2 exosphere of Callisto is much too dense to be sustained by impact-delivered volatiles, but could be maintained by only ~7 ha (~0.07 km2) of exposed CO2 ice distributed across Callisto (and refreshed through mass wasting). We use our model to predict the peak surface locations of Callisto's CO2 exosphere along with other Galilean moons, which could be tested by JUICE observations. Our model finds that to maintain Iapetus' two-tone appearance, its dark Cassini Regio likely has unresolved exposures of water ice, perhaps in sub-resolution impact craters, that amount to up to approximately ~0.06% of its surface. In the Uranian system, we find that the CO2 deposits on Ariel, Umbriel, Titania, and Oberon are unlikely to have been delivered via impacts, but are consistent with both a magnetospheric origin, (as has been previously suggested) or sourced endogenously. We suggest that the leading/trailing CO2 asymmetries on these moons could result from exosphere-mediated volatile transport, and may be a seasonal equinox feature that could be largely erased by pole-to-pole volatile migration during the Uranian solstices. We calculate that ~2.4–6.4 mm thick layer of CO2 (depending the moon) could migrate about the surface of Uranus' large moons during a seasonal cycle. Our model also confirms that water migration to Mercury's polar cold traps is inefficient without self-shield against photodestroying UV light, and that Callisto's bright spires could be formed/maintained by exospherically deposited H2O.

A test of general relativity with ESA’s JUICE mission

The ESA’s JUICE mission is planned for launch in April 2023. After a 8.3-year interplanetary cruise it will reach the Jovian system in mid 2031. The mission is devoted to the study of Jupiter and three of the Galilean moons (Europa, Callisto and Ganymede). The 3GM (Geodesy and Geophysics of Jupiter and the Galilean Moons) radio-science package onboard the spacecraft comprises a Ka-band Transponder (KaT) that will enable a highly stable two-way coherent Ka/Ka link to study the gravity fields of the three Jovian satellites and their internal structure, and an Ultra Stable Oscillator (USO) to probe the atmosphere of Jupiter and the ionospheres of the three moons. The KaT can be used together with the onboard Deep Space Transponder (DST), enabling a full multi-frequency radio link system capable to suppress the dispersive noise, due to solar plasma and ionospheric effects, at nearly all solar elongation angles. The cruise phase of the mission provides three opportunities to perform classical tests of general relativity by measuring the relativistic time-delay and the frequency shift experienced by radio signals during superior solar conjunctions. A similar experiment, performed by Cassini in 2002, provided the most accurate measurement of the post-Newtonian parameter γ so far, with an accuracy of 2.3×10-5. The MORE experiment onboard the ESA/JAXA BepiColombo mission to Mercury is expected to improve the result obtained by Cassini up to a factor of 10 while orbiting the planet. Less significant improvements are expected in the cruise phase. The advanced radio-tracking system of JUICE and the favorable dynamical environment experienced in the outer solar system make a repetition of the Cassini and BepiColombo cruise experiments quite significant. In this work, we show the results of numerical simulations of JUICE superior conjunction experiments. We have found that 3GM can estimate γ with an accuracy at the level of 4.8×10-7, which represents a constraint on the value of γ 50 times tighter than the one obtained by Cassini and about four times better than the expected result from BepiColombo at the end of the mission.

On the origin of molecular oxygen on the surface of Ganymede

Since its first identification on the surface of Ganymede in 1995, molecular oxygen (O2) ice has been at the center of a scientific debate as the surface temperature of the Jovian moon is on average well above the freezing point of O2. Laboratory evidence suggested that solid O2 may either exist in a cold (<50 K) subsurface layer of the icy surface of Ganymede, or it is in an atmospheric haze of the moon. Alternatively, O2 is constantly replenished at the surface through ion irradiation of water-containing ices. A conclusive answer on the existence of solid O2 on the surface of Ganymede is hampered by the lack of detailed, extensive observational datasets. We present new ground-based, high-resolution spectroscopic observations of Ganymede's surface obtained at the Telescopio Nazionale Galileo. These are combined with dedicated laboratory measurements of ultraviolet-visible (UV–vis) photoabsorption spectra of O2 ice, both pure and mixed with other species of potential interest for the Galilean satellites. Our study confirms that the two bands identified in the visible spectra of Ganymede's surface are due to the (1,0) and (0,0) transition bands of O2 ice. Oxygen-rich ice mixtures including water (H2O) and carbon dioxide (CO2) can reproduce observational reflectance data of the Ganymede's surface better than pure O2 ice in the temperature range 20–35 K. Solid H2O and CO2 also provide an environment where O2 ice can be trapped at higher temperatures than its pure ice desorption under vacuum space conditions. Our experiments at different temperatures show also that the (1,0)/(0,0) ratio in case of the CO2:O2 = 1:2 ice mixture at 35 K has the closest value to observations, while at 30 K the (1,0)/(0,0) ratio seems to be mixture independent with the exception of the N2:O2 = 1:2 ice mixture. The present work will be part of a laboratory spectroscopic database in support of the ESA/JUICE mission to the Jovian system.

Milliarcsecond Astrometry for the Galilean Moons Using Stellar Occultations

A stellar occultation occurs when a Solar System object passes in front of a star for an observer. This technique allows the sizes and shapes of the occulting body to be determined with kilometer precision. In addition, this technique constrains the occulting body’s positions, albedos, densities, and so on. In the context of the Galilean moons, these events can provide their best ground-based astrometry, with uncertainties in the order of 1 mas (∼3 km at Jupiter’s distance during opposition). We organized campaigns and successfully observed a stellar occultation by Io (JI) in 2021, one by Ganymede (JIII) in 2020, and one by Europa (JII) in 2019, with stations in North and South America. We also re-analyzed two previously published events: one by Europa in 2016 and another by Ganymede in 2017. We then fit the known 3D shape of the occulting satellite and determine its center of figure. This resulted in astrometric positions with uncertainties in the milliarcsecond level. The positions obtained from these stellar occultations can be used together with dynamical models to ensure highly accurate orbits of the Galilean moons. These orbits can help when planning future space probes aiming at the Jovian system, such as JUICE by ESA and Europa Clipper by NASA. They also allow more efficient planning of flyby maneuvers.

A Comprehensive Set of Juno In Situ and Remote Sensing Observations of the Ganymede Auroral Footprint

AbstractJupiter's satellite auroral footprints are a manifestation of the satellite‐magnetosphere interaction of the Galilean moons. Juno's polar elliptical orbit enables crossing the magnetic flux tubes connecting each Galilean moon with their associated auroral emission. Its payload allows measuring the fields and particle population in the flux tubes while remotely sensing their associated auroral emissions. During its thirtieth perijove, Juno crossed the flux tube directly connected to Ganymede's leading footprint spot, a unique event in the entire Juno prime mission. Juno revealed a highly‐structured precipitating electron flux, up to 316 mW/m2, while measuring both a small perturbation in the magnetic field azimuthal component and small Poynting flux with an estimated total downward current of 4.2 ± 1.2 kA. Based on the evolution of the footprint morphology and the field and particle measurements, Juno transited for the first time through a region connected to the transhemispheric electron beam of the Ganymede footprint.

Occultation Mapping of Io’s Surface in the Near-infrared. I. Inferring Static Maps

Abstract With hundreds of active volcanoes varying in intensity on different timescales, Jupiter’s moon Io is the most volcanically active body in the solar system. Io has been observed from Earth using high-cadence near-infrared photometry during occultations by Jupiter and other Galilean moons since the 1980s. These observations encode a wealth of information about the volcanic features on its surface. We built a generative model for the observed occultation light curves using the code starry, which enables fast, analytic, and differentiable computation of occultation light curves in emitted and reflected light. Using this model, we are able to recover surface thermal emission maps of Io containing known volcanic hot spots without having to make assumptions about the locations, shapes, or number of hot spots. Our model is also directly applicable to the problem of mapping the surfaces of stars and exoplanets.

Geodetic Precession of the Sun, Solar System Planets, and their Satellites

Abstract The effect of the geodetic precession is the most significant relativistic effect in the rotation of celestial bodies. In this article, the new geodetic precession values for the Sun, the Moon, and the Solar System planets have been improved over the previous version by using more accurate rotational element values. For the first time, the relativistic effect of the geodetic precession for some planetary satellites (J1–J4, S1–S6, S8–S18, U1–U15, N1, and N3–N8) with known quantities of the rotational elements was studied in this research. The calculations of the values of this relativistic effect were carried out by the method for studying any bodies of the Solar System with long-time ephemeris. As a result, the values of the geodetic precession were first determined for the Sun, planets in their rotational elements, and for the planetary satellites in the Euler angles relative to their proper coordinate systems and in their rotational elements. In this study, with respect to the previous version, additional and corrected values of the relativistic influence of Martian satellites (M1 and M2) on Mars were calculated. The largest values of the geodetic rotation of bodies in the Solar System were found in Jovian satellite system. Further, in decreasing order, these values were found in the satellite systems of Saturn, Neptune, Uranus, and Mars, for Mercury, for Venus, for the Moon, for the Earth, for Mars, for Jupiter, for Saturn, for Uranus, for Neptune, and for the Sun. First of all, these are the inner satellites of Jupiter: Metis (J16), Adrastea (J15), Amalthea (J5), and Thebe (J14) and the satellites of Saturn: Pan (S18), Atlas (S15), Prometheus (S16), Pandora (S17), Epimetheus (S11), Janus (S10), and Mimas (S1), whose values of geodetic precession are comparable to the values of their precession. The obtained numerical values for the geodetic precession for the Sun, all the Solar System planets, and their satellites (E1, M1, M2, J1–J5, J14–J16, S1–S6, S8–S18, U1–U15, N1, and N3–N8) can be used to numerically study their rotation in the relativistic approximation and can also be used to estimate the influence of relativistic effects on the orbital–rotational dynamics of bodies of exoplanetary systems.

Geodetic investigations of the mission concept MAGIC to reveal Callisto's internal structure

Geodetic and geophysical investigations of the Galilean moon Callisto address fundamental questions regarding the formation and evolution of the Jovian system. Callisto's evolution and internal structure appear to significantly differ from the other Jovian satellites. Similarly-sized Ganymede is a highly evolved ice-rock moon with a differentiated interior, intrinsic magnetic field, and abundant surface evidence of internal activity. In contrast, Callisto's surface is ancient, and Galileo spacecraft data suggest its interior is only incompletely differentiated, despite the presumed presence of a sub-surface ocean. These properties make Callisto uniquely able to constrain the timing and nature of the Jovian system formation. The Magnetics, Altimetry, Gravity, and Imaging of Callisto (MAGIC) mission concept is conceived to fully characterize the properties of this enigmatic moon from its deep interior to the icy shell. Three main instruments are included as a scientific payload. Highly accurate measurements of Callisto's topography, magnetic field, and morphology are obtained by the onboard laser altimeter, magnetometer, and camera, respectively. The telecommunication system supports an additional gravity and radio science investigation. Long- and short-wavelength gravity anomalies afford powerful constraints on internal differentiation and the properties of the hydrosphere (water and ice). Comprehensive numerical simulations and covariance analyses of MAGIC mission scenarios presented in this paper show that the gravitational degree-2 normalized coefficients and the pole obliquity enable the determination of the moment of inertia with an accuracy better than 0.015%. The combination of gravity and altimetry measurements acquired by MAGIC are essential to the characterization of Callisto's interior if – as is likely – the degree-2 gravity includes non-hydrostatic terms. MAGIC's radio science data yield the estimation of Callisto's gravity field with spatial resolutions of <100 km. The combination of gravitational and deformation tides that are retrieved by the radio science and altimetry investigations, respectively, leads to the recovery of the rigid ice shell thickness to within ∼3 km. Together these datasets would resolve ambiguities inherent in Galileo flyby data, revealing Callisto's interior structure as well as the existence and properties of its postulated internal ocean.

Crater Distributions of Uranus's Mid-sized Satellites and Implications for Outer Solar System Bombardment

Abstract Outer solar system impact bombardment is largely unconstrained. Although recent data from the Jupiter, Saturn, and Pluto systems have produced new constraints, analysis is incomplete without inclusion of the Uranus system. We reanalyze Uranus system crater populations with recent improvements in processing of Voyager 2 imaging. No consensus in crater populations on mid-sized Uranian satellites, Miranda, Ariel, Umbriel, Titania, and Oberon, was resolved during the Voyager era. For satellites with available data, we find variability in crater size–frequency distributions (SFDs) for diameters (D) &lt; 10 km. Most terrains on Miranda show a shallower slope (ratio of smaller to larger craters is smaller), while Inverness Corona on Miranda and Ariel's terrains show a steeper slope (ratio increases). For D &gt; 10 km, satellites with available data show a steeper slope. Shallower-sloped SFDs for D &lt; 10 km and steeper slopes for D &gt; 10 km agree with Pluto system data—a proxy for the heliocentric impactor population originating from the Kuiper Belt—implying these SFDs represent heliocentric bombardment in the Uranus system. The shallow-sloped population for smaller diameters is also observed on Jovian satellites, but not on mid-sized, heavily cratered Saturnian satellites or Triton (Neptune), which have steeper slopes. This implies the heliocentric impactor population originating from the Kuiper Belt reaches throughout the outer solar system, but that the Saturnian, Neptunian, and maybe Uranian systems also might have their own planet-specific impactors. Finally, we find Ariel appears overall younger than the other Uranian satellites, supporting relatively recent geologic activity.

Variability of Jupiter's Main Auroral Emission and Satellite Footprints Observed With HST During the Galileo Era

AbstractHubble Space Telescope images of Jupiter's UV aurora show that the main emission occasionally contracts or expands, shifting toward or away from the magnetic pole by several degrees in response to changes in the solar wind dynamic pressure and Io's volcanic activity. When the auroral footprints of the Galilean satellites move with the main emission this indicates a change in the stretched field line configuration that shifts the ionospheric mapping of a given radial distance at the equator. However, in some cases, the main emission shifts independently from the satellite footprints, indicating that the variability stems from some other part of the corotation enforcement current system that produces Jupiter's main auroral emissions. Here, we analyze HST images from the Galileo era (1996–2003) and compare latitudinal shifts of the Ganymede footprint and the main auroral emission. We focus on images with overlapping Galileo measurements because concurrent measurements are available of the current sheet strength, which indicates the amount of field line stretching and can influence both the main emission and satellite footprints. We show that the Ganymede footprint and main auroral emission typically, but do not always, move together. Additionally, we find that the auroral shifts are only weakly linked to changes in the current sheet strength measured by Galileo. We discuss implications of the observed auroral shifts in terms of the magnetospheric mapping. Finally, we establish how the statistical reference main emission contours vary with central meridian longitude and show that the dependence results from magnetospheric local time asymmetries.

ANN-based method for fast optimization of Jovian-moon gravity-assisted trajectories in CR3BP

Gravity assist is crucial for tour exploration of Jupiter’s Galilean moons. To achieve high accuracy and low cost gravity-assisted transfer trajectories, the circular, restricted, three-body problem (CR3BP) model is adopted. Since there is no analytical solution for CR3BP, the orbit propagation must be calculated by numerical integration, which requires considerable time for the design of complex Jupiter exploration missions. Artificial neural networks (ANNs) that can fully approximate complex nonlinear relationships are used to model accurate and computationally efficient maps between the orbit states before and after gravity assist. Thereafter, a new framework based on the ANN model is developed for the fast design of Jovian-moon gravity-assisted transfers, by which optimal trajectories are obtained using a genetic algorithm (GA). Finally, the accuracy and performance of the proposed method are verified by two representative test cases of the Jupiter-Europa and Jupiter-Ganymede systems. The simulation results show that using the trained ANN model can significantly improve the convergence speed of the genetic algorithm, and high-precision, optimal solutions can be obtained rapidly.