Jovian Moon Europa Research Articles

Ice Transit and Performance Analysis for Cryorobotic Subglacial Access Missions on Earth and Europa.

Ice-covered ocean worlds, such as the Jovian moon Europa, are some of the prime targets for planetary exploration due to their high astrobiological potential. While upcoming space exploration missions, such as the Europa Clipper and JUICE missions, will give us further insight into the local cryoenvironment, any conclusive life detection investigation requires the capability to penetrate and transit the icy shell and access the subglacial ocean directly. Developing robust, autonomous cryorobotic technology for such a mission constitutes an extremely demanding multistakeholder challenge and requires a concentrated interdisciplinary effort between engineers, geoscientists, and astrobiologists. An important tool with which to foster cross-disciplinary work at an early stage of mission preparation is the virtual testbed. In this article, we report on recent progress in the development of an ice transit and performance model for later integration in such a virtual testbed. We introduce a trajectory model that, for the first time, allows for the evaluation of mission-critical parameters, such as transit time and average/overall power supply. Our workflow is applied to selected, existing cryobot designs while taking into consideration different terrestrial, as well as extraterrestrial, deployment scenarios. Specific analyses presented in this study show the tradeoff minimum transit time and maximum efficiency of a cryobot and allow for quantification of different sources of uncertainty to cryobot's trajectory models.

Abiotic molecular oxygen production-Ionic pathway from sulfur dioxide.

Molecular oxygen, O2, is vital to life on Earth and possibly also on exoplanets. Although the biogenic processes leading to its accumulation in Earth’s atmosphere are well understood, its abiotic origin is still not fully established. Here, we report combined experimental and theoretical evidence for electronic state–selective production of O2 from SO2, a chemical constituent of many planetary atmospheres and one that played an important part on Earth in the Great Oxidation Event. The O2 production involves dissociative double ionization of SO2 leading to efficient formation of the ion, which can be converted to abiotic O2 by electron neutralization or by charge exchange. This formation process may contribute substantially to the abundance of O2 and related ions in planetary atmospheres, such as the Jovian moons Io, Europa, and Ganymede. We suggest that this sort of ionic pathway for the formation of abiotic O2 involving multiply charged molecular ion decomposition may also exist for other atmospheric and planetary molecules.

Single‐ and Multi‐Pass Magnetometric Subsurface Ocean Detection and Characterization in Icy Worlds Using Principal Component Analysis (PCA): Application to Triton

AbstractMany moons in the solar system are thought to potentially harbor hidden oceans based on the features observed at their surfaces. However, the magnetic induction signatures measured in the vicinity of these moons provide the most compelling evidence for the presence of a subsurface ocean, specifically for the Jovian moons Europa and Callisto. Interpretation of these magnetic signatures can be challenging due to the various systematic and random sources of noise that are present in the magnetic field measurement. In this work, a novel magnetometric ocean detection methodology based on Principal Component Analysis is presented and shown to provide enhanced discrimination and geophysical characterization of ocean properties in the presence of noise and error sources. The proposed methodology is robust for a single‐encounter mission or an orbiting mission with multiple flybys. Here, it is applied to the Neptunian moon Triton as a prime example of an active, potential ocean world residing in the requisite time‐varying magnetic field environment that enables magnetic induction investigation of its interior. In addition to the usual noise sources, other confounding factors are addressed, including the presence of an intense conductive ionosphere, the small amplitude of Neptune's driving magnetic field, and the uncertainty of Neptune's magnetic phase at the time‐of‐arrival which can potentially hinder accurate ocean detection and characterization. The proposed methodology is applicable to any moon in the solar system residing in a time‐varying magnetic field environment.

Stress-activated electric currents in icy planetary bodies: H2O2-doped H2O ices

Planetary satellites such as the Jovian moon Europa and several Kuiper Belt objects (including Pluto) have surfaces consisting largely of H2O ice, which is brittle and behaves as a rock at the low temperatures that prevail in the outer Solar System. Several of those ice-crusted bodies show evidence of tectonic activity indicating high levels of stress. This paper reports on laboratory experiments with pure H2O ice and H2O2–doped H2O ices with H2O2 concentration levels comparable to those in the ices of Europa. Elongated rectangular ice blocks at T = −81 °C [192 K] were stressed at one end to test whether electric currents are generated and capable of flowing down the stress gradient. Pure H2O ice was found to not produce currents above the 10−10 A background level except for occasional transients in the 10−9–10−8 A range during fracture or rapid plastic deformation due to electrons, e’. By contrast, stressing H2O2-doped H2O ices consistently produced electric currents in the 10−7–10−5 A range, due to holes, h•, propagating from the stressed end to the unstressed end. The h• charge carriers are generated by the break-up of peroxy bonds of H2O2 molecules, leading to O−, equivalent to defect electrons or holes h• in the O2− matrix.

Lunar Secondary Craters and Estimated Ejecta Block Sizes Reveal a Scale‐Dependent Fragmentation Trend

AbstractPlanetary impact events eject large volumes of surface material. Crater excavation processes are difficult to study, and in particular the details of individual ejecta fragments are not well understood. A related, enduring issue in planetary mapping is whether a given crater resulted from a primary impact (asteroid or comet) or instead is a secondary crater created by an ejecta fragment. With mapping and statistical analyses of six lunar secondary crater fields (including Orientale, Copernicus, and Kepler) we provide three new constraints on these issues: (1) estimation of the maximum secondary crater size as a function of distance from a primary crater on the Moon, (2) estimation of the size and velocity of ejecta fragments that formed these secondaries, and (3) estimation of the fragment size ejected at escape velocity. Through this analysis, we confirmed and extended a suspected scale‐dependent trend in ejecta size‐velocity distributions. Maximum ejecta fragment sizes fall off much more steeply with increasing ejection velocity for larger primary impacts (compared to smaller primary impacts). Specifically, we characterize the maximum ejecta sizes for a given ejection velocity with a power law and find that the velocity exponent varies between approximately −0.3 and −3 for the range of primary craters investigated here (0.83–660 km in diameter). Data for the Jovian moons Europa and Ganymede confirm similar trends for icy surfaces. This result is not predicted by analytical theories of formation of Grady‐Kipp fragments or spalls during impacts and suggests that further modeling investigations are warranted to explain this scale‐dependent effect.

Crevasse Propagation on Brittle Ice: Application to Cycloids on Europa

AbstractExisting lineaments on the surface of the Jovian moon Europa are thought to be the result of ongoing brittle crack formation in the elastic regime. Arcuate features are called cycloids and can be modeled using linear elastic fracture mechanics. Here, we build on existing terrestrial models of rift propagation and extend them to cycloids on the moon. We propose that these cracks tend to grow as a series of nearly instantaneous events, spaced by periods of inactivity. The behavior is similar to what is observed on Antarctic ice shelves, where rifts can remain dormant for years. We argue that dormant periods between growth events could explain the presence of cycloids on Europa even without invoking secular motion of the crust. Furthermore, being able to model propagation events and their timing should help future missions exploring the moon.

A small spacecraft to probe the interior of the Jovian moon Europa: Europa Tomography Probe (ETP) system design

We propose the system design of a small orbiter of the Jovian moon Europa (Europa Tomography Probe, ETP in short) aimed at unveiling its interior structure. ETP is conceived as a piggyback probe of a larger spacecraft to the Jovian system (such as Europa Clipper). Its payload comprises only a magnetometer and transponder. The former will be used to measure the time-varying, induction magnetic field of the moon at different orbital and rotational frequencies, a measurement inaccessible to a flyby spacecraft. An Inter-Satellite Link (ISL) between ETP and the main spacecraft, enabled by the on-board transponder, will be used to accurately determine Europa's gravity field, rotational state and tidal deformation. By combining magnetic and gravity field measurements, ETP could characterize the interior structure of Europa with an accuracy only attainable by a low-altitude orbiter, thus constraining the thickness and conductivity of the subsurface ocean. Following the announcement that a 250-300-kg mass allowance was available on the upcoming NASA Europa Clipper spacecraft, we propose ETP's mission and spacecraft design assuming Clipper's nominal trajectory as a baseline. The concept can be adapted to a class of missions to the Jovian and Saturnian systems based upon a mother-daughter spacecraft system. The spacecraft design has been pursued under the philosophy of determining the minimum required total mass and volume that allows to meet the scientific requirements, rather than finding out what science return could be obtained with pre-assigned system constraints. Since ETP shall autonomously reach its final orbit, the propulsion system has been one of the primary focuses. The radiation analysis has also been essential because the shielding structure affects both the mission duration and the mass budget. We show that the ETP concept could be, indeed, technically feasible with an overall system mass budget just above 250 kg, thus providing a valuable yet affordable augmentation to a larger flyby mission to Europa.

Experimental Petrology to Understand Europa's Crust

AbstractThe Jovian moon Europa is a prime target for astrobiology. A global subsurface water ocean and a geologically young surface provide evidence of an active planetary body with a potential deep habitable environment. Tectonism and cryomagmatism are both agents of resurfacing, with structures on the surface spatially related to reddish non‐icy materials that could represent crystallized volatile and salt‐rich fluids from the interior, possibly from the ocean or shallower aqueous bodies. Cryomagmatism could therefore be a mechanism for exposing the underlying liquid layers to the surface and could hold paramount importance for understanding the physical and chemical evolution of fluids during their ascent and emplacement and their connection with geological features at the surface. With these premises, we perform a set of laboratory experiments simulating the evolution of different fluids under the conditions in Europa's crust. These experiments allow us to constrain the physico‐chemical and textural changes experienced by the different fluids and solids that are potentially emplaced within the icy crust and determine how they are affected by such secondary processes as reheating, melting, and ultimate recrystallization (e.g., in response to the emplacement of a second diapir close to the first one or tidal reheating). Based on these experimental results, we explore the connection of cryomagmas and their evolution near the surface to geologic features present on Europa's surface, such as pits, uplifts/domes, and microchaos regions, as well as the link with explosive cryovolcanism responsible for putative plumes at Europa.

Updating the Jovian Electron Plasma Environment

Reanalysis of the plasma science (PLS) experiments on the Voyagers (~10 eV to ~5 keV) and Galileo (~1 to ~50 keV) has refined our understanding of the electron environment responsible for spacecraft charging at and near the Jovian moon Europa. The low-energy 1–50-keV environment measured by the Galileo PLS has previously been at best poorly defined as the PLS electron data were found to have been contaminated by high-energy electrons. To address this issue, the raw PLS data have been reanalyzed orbit by orbit and detector by detector. The results indicate that the Voyager and the Galileo spectra, previously fit by the Maxwell–Boltzmann distributions, may be better fit by a more complex combination of Maxwell–Boltzmann plus kappa or a polynomial. The new spectra derived from the Voyager and Galileo PLS instruments are presented for a range of applicability from ~8 to ~40 Rj.

Element history of the Laplace resonance: a dynamical approach

Context. We consider the three-body mean motion resonance defined by the Jovian moons Io, Europa, and Ganymede, which is commonly known as the Laplace resonance. In terms of the moons’ mean longitudes λ1 (Io), λ2 (Europa), and λ3 (Ganymede), this resonance is described by the librating argument φL ≡ λ1 − 3λ2 + 2λ3 ≈ 180°, which is the sum of φ12 ≡ λ1 − 2λ2 + ϖ2 ≈ 180° and φ23 ≡ λ2 − 2λ3 + ϖ2 ≈ 0°, where ϖ2 denotes Europa’s longitude of perijove. Aims. In particular, we construct approximate models for the evolution of the librating argument φL over the period of 100 yr, focusing on its principal amplitude and frequency, and on the observed mean motion combinations n1 − 2n2 and n2 − 2n3 associated with the quasi-resonant interactions above. Methods. First, we numerically propagated the Cartesian equations of motion of the Jovian system for the period under examination, and by comparing the results with a suitable set of ephemerides, we derived the main dynamical effects on the target quantities. Using these effects, we built an alternative Hamiltonian formulation and used the normal forms theory to precisely locate the resonance and to semi-analytically compute its main amplitude and frequency. Results. From the Cartesian model we observe that on the timescale considered and with ephemerides as initial conditions, both φL and the diagnostics n1 − 2n2 and n2 − 2n3 are well approximated by considering the mutual gravitational interactions of Jupiter and the Galilean moons (including Callisto), and the effect of Jupiter’s J2 harmonic. Under the same initial conditions, the Hamiltonian formulation in which Callisto and J2 are reduced to their secular contributions achieves larger errors for the quantities above, particularly for φL. By introducing appropriate resonant variables, we show that these errors can be reduced by moving in a certain action-angle phase plane, which in turn implies the necessity of a tradeoff in the selection of the initial conditions. Conclusions. In addition to being a good starting point for a deeper understanding of the Laplace resonance, the models and methods described are easily generalizable to different types of multi-body mean motion resonances. Thus, they are also prime tools for studying the dynamics of extrasolar systems.

Distant retrograde orbits and the asteroid hazard

Distant Retrograde Orbits (DROs) gained a novel wave of fame in space mission design because of their numerous advantages within the framework of the US plans for bringing a large asteroid sample in the vicinity of the Earth as the next target for human exploration. DROs are stable solutions of the three-body problem that can be used whenever an object, whether of natural or artificial nature, is required to remain in the neighborhood of a celestial body without being gravitationally captured by it. As such, they represent an alternative option to Halo orbits around the collinear Lagrangian points L1 and L2. Also known under other names (e.g., quasi-satellite orbits, cis-lunar orbits, family-f orbits) these orbital configurations found interesting applications in several mission profiles, like that of a spacecraft orbiting around the small irregularly shaped satellite of Mars Phobos or the large Jovian moon Europa. In this paper a basic explanation of the DRO dynamics is presented in order to clarify some geometrical properties that characterize them. Their accessibility is then discussed from the point of view of mission analysis under different assumptions. Finally, their relevance within the framework of the present asteroid hazard protection programs is shown, stressing the significant increase in warning time they would provide in the prediction of impactors coming from the direction of the Sun.

Power Subsystem Approach for the Europa Mission

NASA is planning to launch a spacecraft on a mission to the Jovian moon Europa, in order to conduct a detailed reconnaissance and investigation of its habitability. The spacecraft would orbit Jupiter and perform a detailed science investigation of Europa, utilizing a number of science instruments including an ice-penetrating radar to determine the icy shell thickness and presence of subsurface oceans. The spacecraft would be exposed to harsh radiation and extreme temperature environments. To meet mission objectives, the spacecraft power subsystem is being architected and designed to operate efficiently, and with a high degree of reliability.

An analytical model of sub‐Alfvénic moon‐plasma interactions with application to the hemisphere coupling effect

AbstractWe develop a new analytical model of the Alfvén wing that is generated by the interaction between a planetary moon's ionosphere and its magnetospheric environment. While preceding analytical approaches assumed the obstacle's height‐integrated ionospheric conductivities to be spatially constant, the model presented here can take into account a continuous conductance profile that follows a power law. The electric potential in the interaction region, determining the electromagnetic fields of the Alfvén wing, can then be calculated from an Euler‐type differential equation. In this way, the model allows to include a realistic representation of the “suspension bridge”‐like conductance profile expected for the moon's ionosphere. The major drawback of this approach is its restriction to interaction scenarios where the ionospheric Pedersen conductance is large compared to the Hall conductance, and thus, the Alfvénic perturbations are approximately symmetric between the planet‐facing and the planet‐averted hemispheres of the moon. The model is applied to the hemisphere coupling effect observed at Enceladus, i.e., to the surface currents and the associated magnetic discontinuities that arise from a north‐south asymmetry of the obstacle to the plasma flow. We show that the occurrence of this effect is very robust against changes in the conductance profile of Enceladus' plume, and we derive upper limits for the strength of the magnetic field jumps generated by the hemisphere coupling effect. During all 11 reported detections of the hemisphere coupling currents at Enceladus, the observed magnetic field jumps were clearly weaker than the proposed limits. Our findings are also relevant for future in situ studies of putative plumes at the Jovian moon Europa.

Polymerization of Building Blocks of Life on Europa and Other Icy Moons.

The outer Solar System may provide a potential habitat for extraterrestrial life. Remote sensing data from the Galileo spacecraft suggest that the jovian icy moons--Europa, Ganymede, and possibly Callisto--may harbor liquid water oceans underneath their icy crusts. Although compositional information required for the discussion of habitability is limited because of significantly restricted observation data, organic molecules are ubiquitous in the Universe. Recently, in situ spacecraft measurements and experiments suggest that amino acids can be formed abiotically on interstellar ices and comets. These amino acids could be continuously delivered by meteorite or comet impacts to icy moons. Here, we show that polymerization of organic monomers, in particular amino acids and nucleotides, could proceed spontaneously in the cold environment of icy moons, in particular the jovian icy moon Europa as a typical example, based on thermodynamic calculations, though kinetics of formation are not addressed. Observed surface temperature on Europa is 120 and 80 K in the equatorial region and polar region, respectively. At such low temperatures, Gibbs energies of polymerization become negative, and the estimated thermal structure of the icy crust should contain a shallow region (i.e., at a depth of only a few kilometers) favorable for polymerization. Investigation of the possibility of organic monomer polymerization on icy moons could provide good constraints on the origin and early evolution of extraterrestrial life.

The forced libration of Europa’s deformable shell and its dependence on interior parameters

One of the most important goals of future missions to jovian moon Europa will be to unambiguously determine and characterize the putative subsurface ocean, as well as characterizing the overlying ice shell. In addition to magnetic, altimetry and gravity measurements, observations of Europa’s librations are expected to contribute to the realization of such an important goal. The longitudinal libration of Europa’s shell in the presence of a subsurface ocean has been previously studied assuming that Europa’s internal solid layers behave rigidly. However, at the frequency of the acting diurnal tides and external gravitational torques, the response of Europa’s interior is not rigid but rather viscoelastic. In this paper, we develop a differential libration model that takes into account the effect of diurnal deformation on the forced longitudinal libration of Europa’s internal solid layers. We apply our libration model to a rather large range of possible interior models of Europa to investigate the dependence of the shell libration amplitude on the geophysical parameters that characterize the interior of Europa, in particular the shell. From all analyzed interior parameters, we find that the poorly constrained rigidity of the shell has the largest effect on the libration amplitude. This results suggests thus that future libration observations could be very useful to constrain the value of this parameter for Europa. However, we also notice that the effect of the viscosity of a dissipating ice sublayer at the bottom of the shell (if present) on the libration amplitude of the shell would not allow for an unambiguous determination of the shell rigidity. Furthermore, the dependence of the shell libration amplitude on the shell thickness and density is rigidity-dependent and weak in comparison to the dependence on the rigidity. As a result, libration observations would not be able to provide any information on the thickness and/or density of the shell without previous determination of the rigidity.

Implantation of multiply charged sulfur ions in water ice

Sulfuric acid has been suggested to be present on the surface of the jovian moon Europa where it is mixed with the dominant water ice. The formation mechanism of sulfuric acid is still under discussion.We present new experimental results on the implantation of Sq+ (q=7, 9, 11) ions at an energy range between 35 and 176keV in water ice at 80K. Previous results on 200keV S+ implantation in H2O at 80K have also been included in the data analysis. Experiments with multiply-charged ions at different energies are particularly relevant to simulate the complexity of the irradiation environment to which the surface of Europa is exposed being embedded in the jovian magnetosphere.The experiments were performed at the low energy ion beam facility ARIBE of GANIL in Caen (France). 35–176keVSq+ (q=7, 9, 11) ions were implanted in solid H2O layers which were frozen at 80K. Fourier Transform Infrared Spectrometry (FTIR) was used to analyze the sample in the 5000–600cm−1 (2–16.7μm) region with a spectral resolution of 1cm−1.The results of our experiments indicate that implantation produces hydrated sulfuric acid with yields that increase with ion energy, from 0.12moleculesion−1 for 35keV ions to 0.64moleculesion−1 for 200keV ions. We have also searched for the production of SO2 and H2S, but we were not able to find any evidence for their synthesis.We conclude that sulfur ion implantation is the dominant formation mechanism of hydrated sulfuric acid at Europa. The suggestion that the observed distribution of sulfuric acid on the surface is well correlated with the local flux of sulfur ions find a full explanation by present experimental data.

Parametric excitation of gyroscopic waves in Thomson-Delaney cells of the ocean of Jovian moon Europa

A possible mechanism (of parametric resonance type) of excitation of planetary gyroscopic waves in Thomson-Delaney cells of the ocean of Jovian moon Europa is considered. It is assumed that the basis of this mechanism is a variation of liquid depth in a cell caused by tidal oscillations under the action of gravitational perturbing influence of Galilean satellites of Jupiter. Such a model leads to a system of linear differential equations with periodic coefficients of the Hill’s type. Under some additional assumptions it changes over into a system of independent Mathieu equations. The regions of parametric resonance of this system are constructed.

Gravitational and hydroelastic waves in thomson-delaney cells of the ocean on Jovian Moon Europa

The possibility of appearance of hydrothermal cells in the system “Ocean of Jovian Moon Europaits ice crust” was theoretically proved by R. Thomson and J. Delaney [8]. We formulated in [11] boundary value problems for planetary gyroscopic waves and gravitationally elastic waves in these cells, and the spectrum of nondivergent gyroscopic waves was numerically investigated. The present paper is a continuation of [11]. It considers the spectra of independent, purely gravitational and hydroelastic waves. The possibility of resonant excitation in Thomson-Delaney cells of gravitational and hydroelastic waves is considered.

On the problem of rotation of the ocean and icy crust of Jovian moon Europa

A mathematical model of the non-steady-state magnetosphere of the ocean of Jupiter’s moon Europa is described. This magnetosphere is induced by the magnetic field of Jupiter, and its interaction with the Europa ocean is considered. Rotation of the ocean and its icy crust with respect to the hard core is described by a system of integro-differential equations with singular kernels, the values of the Reynolds number and magnetic Reynolds number being large for the system. The solution to these equations obtained by the methods of operational calculus is analyzed quantitatively and compared with a model experiment.

Nuclear electric ion propulsion for three deep space missions

Nuclear electric ion propulsion is considered for three sample deep space missions starting from a 500 km low Earth orbit encompassing the transfer of a 100 MT payload into a 1500 km orbit around Mars, the rendezvous of a 10 MT payload with the Jovian moon Europa and the rendezvous of a similar payload with Saturn's moon Titan. Near term ion engine and space nuclear reactor technology are assumed. It is shown that nuclear electric ion propulsion offers more than twice the payload for the Mars mission relative to the case when a nuclear thermal rocket is used for the trans-Mars injection maneuver at Earth, and about the same payload advantage relative to the case when solar electric propulsion is used for the Mars heliocentric transfer. For missions to the outer planets nuclear electric ion propulsion increases the payload mass fraction by a factor of two or more compared with high thrust systems that utilize gravity assist trajectories.