Icy Satellites Research Articles

On detecting and characterizing planetary oceans in the solar system using a distance-based ensemble modelling approach: application to the Uranus system.

The discovery of Europa's subsurface ocean has spawned a strong desire by the planetary community to return and assess the ocean's habitability using the magnetic induction signal that Europa generates. NASA has since formulated and developed the Europa Clipper mission with that same goal, anticipating its arrival in the Jovian system in the early 2030s. In parallel, ESA has developed the JUpiter Icy moons Explorer mission to further investigate the interior of Ganymede and other Jovian moons, scheduled to arrive approximately one year later. As a result, extensive work has now been devoted to developing and refining methods to analyse magnetic induction measurements with the goal of characterizing oceans within icy moons, including those in the Neptune and Uranus systems, which are ideal laboratories for such investigations. We present one such method, involving a distance-based inverse and forward modelling approach that leverages self-consistent interior models used to infer ocean and ice-shell properties of various moons that respond inductively to the dynamic magnetic environments in which they reside. We demonstrate the method on a hypothetical ocean within Umbriel, showing the ocean thickness and conductivity constraints that can be inferred from a Monte Carlo error analysis using a three-flyby mission concept.This article is part of the theme issue 'Magnetometric remote sensing of Earth and planetary oceans'.

Monte Carlo Calculation of Electron Energy Degradation in a Pure O2 Atmosphere

Electron degradation serves as a significant energy source in planetary upper atmospheres. In this study, a Monte Carlo model is constructed to investigate the degradation of electrons in a pure O2 atmosphere under the local approximation. Both elastic and inelastic collision processes between electrons and O2 molecules are considered in the model. The yield spectra, characterizing the occurrence of various inelastic collisions for a specified pair of incident and post-collision energies, are obtained from the model. When combined with the information of e-O2 collision cross sections, the yields of each inelastic process are then determined. Furthermore, we derive the mean energy per ion pair and the efficiencies of various inelastic processes, along with the yields of secondary (and higher-order) electrons. The calculations presented here are beneficial for understanding the ionization and heating budgets in planetary atmospheres containing a significant amount of O2 such as Earth and Jovian icy satellites (Europa, Ganymede, and Callisto).

Ocean weather systems on icy moons, with application to Enceladus.

We explore ocean circulation on a rotating icy moon driven by temperature gradients imposed at its upper surface due to the suppression of the freezing point of water with pressure, as might be induced by ice thickness variations on Enceladus. Using high-resolution simulations, we find that eddies dominate the circulation and arise from baroclinic instability, analogous to Earth's weather systems. Multiple alternating jets, resembling those of Jupiter's atmosphere, are sustained by these baroclinic eddies. We establish a theoretical model of the stratification and circulation and present scaling laws for the magnitude of the meridional heat transport. These are tested against numerical simulations. Through identification of key nondimensional numbers, our simplified model is applied to other icy moons. We conclude that baroclinic instability is central to the general circulation of icy moons.

The Neutral Water Torus of Europa

AbstractThe neutral particles that escape Europa form a neutral torus which asymmetrically encircles the giant planet Jupiter. The study of this structure can inform on neutral escape at the icy moon and on the loading of the Jovian magnetosphere with Europan material. In this letter, we present the first Monte Carlo model of the neutral water torus of Europa. Water molecules are released to high altitude by ion sputtering of the surface and get dissociated and ionized mostly by electron impact. We find that the density of H2O molecules can be as large as atomic O in a large region of the Jovian magnetosphere. The neutral torus of Europa may therefore have a more diverse composition than previously thought. Plasma instrumentation able to separate H2O+ from O+ pick‐up ions could help to advance our understanding of the neutral torus of Europa.

Utility-Driven Approach to Onboard Scheduling and Execution for an Autonomous Europa Lander Mission

Jupiter’s icy moon Europa is among the most promising locations to explore for signs of extraterrestrial life in our solar system. However, searching for biosignatures on the surface of Europa presents an unprecedented set of challenges: immense uncertainty, limited energy, and few opportunities for operator feedback. Effectively carrying out a robotic science campaign under these conditions will require a system with a greater degree of autonomy than any planetary exploration mission to date. In particular, onboard scheduling and execution are required for robustness to uncertainty in surface conditions, variation in lander performance, and science discoveries to maximize the quantity and quality of science data returned to Earth during a fixed, limited lander lifetime. Here we represent a proposed Europa Lander surface mission as a utility-driven hierarchical task network and establish through analysis that onboard autonomy using automated mission planning with execution feedback and predictive task models results in mission execution that is more consistently productive compared to traditional static approaches. We design a simulated onboard autonomy framework built by integrating two software components—Multi-mission EXECutive and Europa Lander Autonomy Prototype—to properly simulate the Europa Lander domain and demonstrate empirically that the proposed planning and execution system is capable of commanding a set of realistic surface scenarios as part of a larger Europa Lander surface autonomy software prototype. We expect that an approach to scheduling and execution grounded in decision theory will be an enabling technology for future tightly constrained planetary surface missions.

Calibration of MAJIS (Moons and Jupiter Imaging Spectrometer). IV. Radiometric calibration (invited)

The MAJIS (Moons and Jupiter Imaging Spectrometer) instrument is an imaging spectrometer on-board the JUICE (JUpiter ICy moons Explorer) spacecraft. MAJIS covers the spectral range from 0.5 to 5.54 μm with two channels [visible–near infrared (VISNIR) and IR]. A comprehensive campaign of on-ground MAJIS calibration was conducted in August and September 2021 in the IAS (Institut d’Astrophysique Spatiale, CNRS/Université Paris-Saclay) facilities. In this article, we present the results relevant for the radiometric calibration of MAJIS. Due to the specific characteristics of the MAJIS detectors (H1RG from Teledyne), an extensive detector characterization campaign was implemented for both the VISNIR and IR detectors before integration so as to validate readout procedures providing precision and accuracy. The characterization also provided critical information on linearity and operability as a function of the integration time and operating temperature. The radiometric calibration of the integrated MAJIS instrument focused on the determination of the instrument transfer function in terms of DN output per unit of radiance for each MAJIS data element as a function of its position in the field of view of MAJIS and its central wavelength. The radiometric calibration of the VISNIR channel required a specific procedure due to stray light at short wavelengths. Observations of an internal calibration source during calibration and after launch (April 14, 2023) showed that there were minor changes in both the VISNIR and IR channels. The instrument transfer functions to be used in flight have been updated on this basis.

The Influence of Non‐Thermal Collisions in Europa's Atmosphere

AbstractIn this study, we show that non‐thermal collisions can play a significant role in shaping Europa's exospheric structure. Collisions between radiolytically produced and the O produced via electron impact dissociation of affect the exospheric structure and escape rates. Specifically, O + collisions lead to the production of a non‐thermal population, and increase the escape while decreasing the O escape. These collisions are dependent on three specific physical parameters: (a) the density of , (b) the electron impact dissociation rate of , and (c) the O + collision cross section. We demonstrate here that O + collisions affect Europa's atmosphere even in the lowest limits considered. Thus, to more accurately determine the influence O + collisions have on Europa's atmosphere in preparation for the forthcoming spacecraft missions, Europa Clipper and the JUpiter ICy moons Explorer (JUICE), these physical parameters need to be better constrained.

How to infer ocean freezing rates on icy satellites from measurements of ice thickness

ABSTRACT Liquid-water oceans likely underlie the ice shells of Europa and Enceladus, but ocean properties are challenging to measure due to the overlying ice. Here, we consider gravity-driven flow of the ice shells of icy satellites and relate this to ocean freeze and melt rates. We employ a first-principles approach applicable to conductive ice shells in a Cartesian geometry. We derive a scaling law under which ocean freeze/melt rates can be estimated from shell-thickness measurements. Under a steady-state assumption, ocean freeze/melt rates can be inferred from measurements of ice thickness, given a basal viscosity. Depending on a characteristic thickness scale and basal viscosity, characteristic freeze/melt rates range from around O(10$^{-1}$) to O(10$^{-5}$) mm/yr. Our scaling is validated with ice-penetrating radar measurements of ice thickness and modelled snow accumulation for Roosevelt Island, Antarctica. Our model, coupled with observations of shell thickness, could help estimate the magnitudes of ocean freeze/melt rates on icy satellites.

Ice Formation, Exsolution, and Multiphase Equilibria in the Methane–Ethane–Nitrogen System at Titan Surface Conditions

Titan is unique among the icy satellites in that it has a thick atmosphere, stable surficial bodies of liquid, and a precipitation system that promotes interactions between the two. Although Titan’s surface conditions are typically assumed to be above the freezing point temperatures of the major constituent species of the climate system (methane, ethane, and nitrogen), conditions may be sufficiently cool across parts of Titan to allow for ice formation alongside known liquid-vapor phases. In this study, we used Raman spectroscopy, visual inspection, and the CRYOCHEM 2.0 equation of state to map the appearance of first ice and to quantify the amount of nitrogen dissolution into liquid in the methane–ethane–nitrogen system along a 1.5 bar isobaric cooling path in the temperature range 80–95 K. This was with the intent of (1) determining the effects nitrogen has on the phase behaviors of the methane–ethane binary system, and (2) establishing the temperatures and ternary mixing ratios needed for ice formation on Titan’s surface. We found that ethane-rich mixtures enter a three-phase solid–liquid–vapor equilibrium and are characterized by nitrogen-rich exsolution upon freezing and ice that form at the bottom of the sample. With sufficient methane content, the mixtures cross a univariant four-phase solid–liquid–liquid–vapor boundary, which contributes to a distinct isothermal freezing point profile and ice that forms starting at the liquid–liquid interface. Our results generally agree with findings from previous studies of the methane–ethane–nitrogen system and are intended to add to our current understanding of Titan’s geochemical processes.

Calibration of MAJIS (Moons and Jupiter Imaging Spectrometer): V. Validation with mineral samples and reference materials.

MAJIS (Moons and Jupiter Imaging Spectrometer) is the imaging spectrometer onboard ESA's JUICE (JUpiter and ICy Moons Explorer) spacecraft that operates in the visible and near/mid-infrared between 0.5 and 5.54μm. Before the launch of JUICE in April 2023, MAJIS underwent a comprehensive on-ground calibration campaign in between August and September 2021 in the IAS (Institut d'Astrophysique Spatiale, Université Paris-Saclay) calibration facilities. Among all the operations, calibration sequences using a set of natural mineral samples and synthetic reference materials were acquired in order to characterize MAJIS performances under conditions assumed to be close to certain future observation configurations. Here, we analyze these calibration measurements using comparison with laboratory reference spectra to quantify MAJIS spectral and spatial performances while observing these solid surfaces. We first assess the MAJIS absolute spectral calibration of the visible and near-infrared channel covering half of the wavelength range. We then quantify spectral performances in terms of global spectral slopes, band detection, band shape, and depth retrievals, over most of the spectral range using six mineral samples. We conclude that for most configurations, the MAJIS instrument demonstrates excellent spectral performances compliant with the requirements. MAJIS can, however, be affected by stray light contributions, notably for wavelengths lower than about 1.2μm, and some performances of the instrument may then be significantly impacted depending on viewing conditions. In particular, we have identified cases of spectral contrast reduction up to 40%, absolute spectral shifts up to 2-3nm, and spectral smile variability by +/1nm. Finally, we used the MAJIS internal scanning mirror to test its ability to construct hyperspectral images of a few samples: we present the first band depth maps derived with MAJIS while observing a serpentine/carbonate sample, as well as an evaluation of MAJIS spatial point spread function. Overall, the analysis of MAJIS behavior while observing samples confirms most MAJIS expected performance requirements, while revealing subtle spectral perturbations that may be related to stray light and viewing conditions. These differences will be further investigated in-flight during the cruise, with a solar reflected target such as the Moon, as well as Jupiter before the JUICE orbital insertion.

LunaIcy: Exploring Europa’s Icy Surface Microstructure through Multiphysics Simulations

Abstract A multiphysics simulation model incorporating a sintering model coupled with the MultIHeaTS thermal solver was developed to study the evolution of icy moons’ microstructure. The sintering process is highly dependent on temperature, and this study represents the first attempt in planetary science to examine the coupled interaction between heat transfer and sintering. Our approach to ice sintering is based upon the literature while offering a refined description of the matter exchange between grains, bonds, and the pore space. By running the numerical framework, we simulate the evolution of ice microstructure on Galilean satellites, specifically tracking the changes in the ice grain and bond radii over time. LunaIcy, our multiphysics model, was applied to study the evolution of Europa’s ice microstructure over 1 million yr along its orbit, with a parameter exploration to investigate the diverse configurations of the icy surface. Our results indicate that effective sintering can take place in regions where daily temperatures briefly surpass 115 K, even during short intervals of the day. Such sintering could not have been detected without the diurnal thermal coupling of LunaIcy due to the cold daily mean temperature. In these regions, sintering occurs within timescales shorter than Europa’s ice crust age, suggesting that, in present times, their surface is made of an interconnected ice structure.

Monitoring JUICE deployment operations with high-accuracy accelerometer data

The Jupiter Icy Moons Explorer (JUICE) mission is a cornerstone of ESA's Cosmic Vision 2015–2025 program devoted to the scientific exploration of Jupiter and its icy moons. Launched on April 14, 2023, from French Guiana, JUICE will perform multiple gravity assists, including an unprecedented lunar-Earth double gravity assist, in order to shape its trajectory to Jupiter, where it will arrive in July 2031. Thanks to a suite of 10 scientific instruments, JUICE will carry out a comprehensive investigation of the jovian system, with special focus on Ganymede. Among these experiments is the Gravity and Geophysics of Jupiter and the Galilean Moons (3GM) radio science and planetary geodesy experiment, supported by the onboard High Accuracy Accelerometer (HAA). The HAA, a three-axis spring mass accelerometer, measures the non-gravitational perturbations acting on the spacecraft, that need to be measured in order to fully exploit the highly accurate range and range rate measurements of 3GM. This paper analyzes the HAA data collected in the very early phase of the mission, to monitor the initial deployment of the spacecraft's moving appendages. The vibration modes of the magnetometer boom and solar arrays were clearly detected during the appendages' deployment, as well as the latching of the Langmuir probe hinges. The detected resonance frequencies of the first and second magnetometer boom bending modes are equal to 0.44 and 0.46 Hz, respectively. The HAA data contributed to identifying the root-cause of the anomalous Radar for Icy Moon Exploration (RIME) antenna deployment. The strong external perturbations due to the actuators' activation have been used to characterize the spacecraft tranquillization time. Despite the significant increase in the spacecraft noise floor during deployment events, the full sensitivity of the instrument was regained within 10 min. This information can be used to plan the spacecraft operations during the scientific phase of the mission.

Looking for Subsurface Oceans Within the Moons of Uranus Using Librations and Gravity

AbstractSeveral of the icy moons in the Jupiter and Saturn systems appear to possess internal liquid water oceans. Our knowledge of the Uranian moons is more limited but a future tour of the system has the potential to detect subsurface oceans. Planning for this requires an understanding of how the moons' internal structures—with and without oceans—relate to observable quantities. Here, we show that the amplitude of forced physical librations could be diagnostic of the presence or absence of subsurface oceans within the Uranian moons. In the presence of a decoupling global ocean, ice shell libration amplitudes at Miranda, Ariel, and Umbriel will exceed 100 m if the shells are thick. The presence of oceans could also imply significant tidal heating within the last few hundred million years. Combining librations with the quadrupole gravity field could provide comprehensive constraints on the internal structures and histories of the Uranian moons.

Chemolithoautotrophic bacteria flourish at dark water-ice interfaces of an emerged Arctic cold seep.

Below their ice shells, icy moons may offer a source of chemical energy that could support microbial life in the absence of light. In the Arctic, past and present glacial retreat leads to isostatic uplift of sediments through which cold and methane-saturated groundwater travels. This fluid reaches the surface and freezes as hill-shaped icings during winter, producing dark ice-water interfaces above water ponds containing chemical energy sources. In one such system characterized by elevated methane concentrations - the Lagoon Pingo in Adventdalen, Svalbard (~10mg/L CH4, <0.3mg/L O2, -0.25°C, pH7.9), we studied amplicons of the bacterial and archaeal (microbial) 16S rRNA gene and transcripts in the water pond and overlaying ice. We show that active chemolithoautotrophic sulfur-oxidizing microorganisms (Sulfurimonas, Thiomicrorhabdus) dominated a niche at the bottom of the ice in contact with the anoxic water reservoir. There, the growing ice offers surfaces interfacing with water, and hosts favorable physico-chemical conditions for sulfide oxidation. Detection of anaerobic methanotrophs further suggests that the ice led to a steady-state dark and cold methane sink under the ice throughout winter, in two steps: first methane is oxidized to carbon dioxide and sulfates concomitantly reduced to sulfides by the activity of ANME-1a and SEEP-SRB1 consortia, in a second time energy from sulfides is used by sulfur- oxidizing microorganisms to fix carbon dioxide into organic carbon. Our results underline ice- covered and dark ecosystems as a hitherto overlooked oasis of microbial life and emphasize the need to study microbial communities in icy habitats.

Planetary radio interferometry and Doppler experiment as an operational component of the Jupiter Icy Moons Explorer mission

AbstractWe present an overview of the operations and engineering interface for Planetary Radio Interferometry and Doppler Experiment (PRIDE) radio astronomy observations as a scientific component of the ESA’s Jupiter Icy Moons Explorer (JUICE) mission, as well as other prospective planetary and space science missions. The article discusses advanced scheduling and planning methods that make it possible to create observing schedules for observations of specific spacecraft in concurrence with observations of natural radio sources. In order to put this into practice and find suitable natural background calibrator sources for PRIDE of JUICE mission, we developed planning and scheduling software. The conventional scheduling software for natural celestial radio sources is not set up to include spacecraft as observation targets in the necessary control files. Therefore, difficulties already arise during observation planning. We report on the development of new and the adaptation of existing routines used in astrophysical and geodetic VLBI for satellite scheduling and planning. The analysis of the PRIDE science observations led to improved observational planning, and the mission’s scheduling methodologies were studied using a systems engineering approach. In addition, we highlighted the new procedures, like finding charts for selecting calibrator radio sources over a range of frequency bands and the outcomes of those strategies for science operation planning. A simulation of the flyby of Venus during the cruise phase of the JUICE spacecraft, based on the Tudat software, is also presented, resulting in a promising opportunity to test PRIDE techniques and evaluate the improvements that PRIDE observables can make to natural bodies’ ephemerides. The first $$\hbox {K}_{a}$$ K a -band (32 GHz) observations of the ESA’s BepiColombo by a radio telescope in the VLBI network, which employs a similar radio communications system as JUICE, were also demonstrated as a test case. The primary objective of these activities is to serve as a practice run for the upcoming operational PRIDE JUICE operations. We showcase the capabilities of the planning and scheduling software for other space missions.

Triton’s Captured Youth: Tidal Heating Kept Triton Warm and Active for Billions of Years

Neptune’s moon Triton has two remarkable attributes: its retrograde orbit suggests that it was captured from the Kuiper Belt, and Triton has one of the youngest surfaces of all the icy satellites in the solar system. Soon after capture, Triton experienced strong diurnal tides raised by Neptune, which caused intense deformation, heating, and melting of its ice shell as its highly eccentric initial orbit was circularized. While previous studies have suggested that Triton’s orbit would have circularized early in solar system history, we show that internal feedbacks between tidal heating and ice shell melting significantly reduce the orbital evolution rate, causing strong tidal heating to persist for billions of years. We simulate Triton’s post-capture evolution over a range of initial semimajor axes and ice shell properties. We find that Triton’s ice shell would have been extremely thin (1–10 km) for a period of 1–4 billion years, with tidal stresses strong enough to fracture the entire ice shell down to the subsurface ocean. A final phase of intense geologic activity may have occurred after tidal dissipation waned, in which late-stage ice shell thickening caused ocean pressurization potentially sufficient to refracture the ice shell and push water to the surface. Such overpressurization could have caused recent massive cryovolcanic resurfacing, perhaps explaining Triton’s geologically young surface. It is therefore possible that Triton’s youthful surface and its origin as a captured satellite may in fact be related. A long-lived subsurface ocean and extended thin ice shell period also greatly increase Triton’s astrobiological potential.

Surface Charging of the Jupiter Icy Moons Explorer (JUICE) Spacecraft in the Solar Wind at 1 AU

AbstractThis article presents the first study of the interaction between the Jupiter Icy Moons Explorer (JUICE) spacecraft and the solar wind environment at 1 AU. The state‐of‐the‐art software Spacecraft Plasma Interaction Software was used to simulate the surface charging of the spacecraft and the altered particle environment around the spacecraft. The simulations show that for a typical solar wind environment the spacecraft will charge to around 6 V, with the different dielectric parts of the spacecraft charging to potentials from around −36 to 8 V. For the studied extreme solar wind environment, similar to the environment found in the sheath region inside the shock front of an Interplanetary Coronal Mass Ejection, the surface potential of the spacecraft is lower due to the increased accumulation of electrons. The spacecraft will charge to around 3 V, with the different dielectric surfaces charging from around −45 to 9 V. We also show how the interaction between the spacecraft and its environment alters the ion and electron particle environment around the spacecraft. This study is the first step toward developing correction techniques for the impact that the interaction between the JUICE spacecraft and its environment has on the JUICE charged particle and field measurements.

Icy ocean worlds - astrobiology research in Germany

Icy bodies with subsurface oceans are a prime target for astrobiology investigations, with an increasing number of scientists participating in the planning, development, and realization of space missions to these worlds. Within Germany, the Ocean Worlds and Icy Moons working group of the German Astrobiology Society provides an invaluable platform for scientists and engineers from universities and other organizations with a passion for icy ocean worlds to share knowledge and start collaborations. We here present an overview about astrobiology research activities related to icy ocean worlds conducted either in Germany or in strong collaboration with scientists in Germany. With recent developments, Germany offers itself as a partner to contribute to icy ocean world missions.

Lower magnetic field measurement limit of the coupled dark state magnetometer

Abstract The Coupled Dark State Magnetometer (CDSM) is an optically pumped magnetometer. For the Jupiter Icy Moons Explorer (JUICE) mission, the CDSM and two fluxgate magnetometers are combined in the J-MAG instrument to measure the static and low frequency magnetic field in the Jupiter system. During certain calibration manoeuvres, the CDSM has to be able to measure magnetic field strengths down to 100 nT with an accuracy of 0.2 nT (1 σ). At such low magnetic fields, the CDSM’s operational parameters must be carefully selected to obtain narrow resonance structures. Otherwise, the coupled dark state resonances, used for the magnetic field detection in different instrument modes, overlap and result in a systematic error. In this paper we demonstrate that with the found instrument settings the CDSM is able to measure magnetic field strengths below 100 nT with a systematic error less than 0.2 nT resulting from the overlap of the resonances.

Symmetry breaking of rotating convection due to non-Oberbeck–Boussinesq effects

The non-Oberbeck–Boussinesq (NOB) effects arising from variations in thermal expansivity are theoretically and numerically studied in the context of rotating Rayleigh–Bénard convection in forms of two-dimensional rolls. The thermal expansivity increases with pressure (depth), and its variation is measured by a dimensionless factor ε. Utilizing an asymptotic expansion with weak nonlinearity, we derive an amplitude equation, revealing that NOB effects amplify the magnitude of convection. An ε2-order NOB correction leads to a symmetry breaking about the horizontal mid-plane, manifested in the strengthening of convection near the bottom and its weakening near the top, forming bottom-heavy profiles. At ε3-order, the conjunction of NOB effects and nonlinear advection leads to a horizontal symmetry breaking. The values of Taylor number and Prandtl number determine whether upward or downward plumes are stronger. Numerical calculations validate the theoretical analyses in weakly nonlinear regime. This work advances our understanding of hydrothermal plumes in some winter lakes on Earth and in the subglacial oceans on icy moons as well as tracer transport from the seafloor to the ice shell.