Introduction: In October 2024, NASA’s Europa Clipper will launch on a journey to explore the habitability of Jupiter’s moon Europa. At the beginning of the next decade, the spacecraft will orbit Jupiter, flying by Europa nearly 50 times over a four-year period to: (1) Characterize the ice shell and any subsurface water, including their heterogeneity, ocean properties, and the nature of surface-ice- exchange; (2) understand the habitability of Europa’s ocean through composition and chemistry; and (3) understand the formation of surface features, including sites of recent or current activity, and characterize high science interest localities. In addition, the search for current activity cross-cuts these three principal science objectives.

Flight System: The Flight System (Figure 1) comprises the Propulsion Module, which provides the thermally-controlled spacecraft structure, propulsion subsystem, and solar array (not shown); the Avionics Module, which enables spacecraft guidance, navigation, and control operations, provides power conditioning and computer resources which stores and prioritizes science data for downlink; the Radio-Frequency (RF) Module, which provides telemetry uplink and science data downlink capabilities; and a highly capable suite of remote-sensing and in-situ instruments (Figure 2) to achieve the science objectives of the mission. The spacecraft is solar-powered with large batteries for use during the flyby and playback periods, uses reaction wheels for precise attitude control, and has a bipropellant system for propulsion and coarse attitude control. The Avionics Module consists of a radiation vault, nadir platform, and secondary structures. The module supports multiple instruments, components, and elements from other modules. The vault provides radiation protection and thermal interface control to internal electronics, and mounting support for various external instruments and spacecraft components. The telecom system includes a high-gain antenna for communication at X- or Ka-band, a co-aligned medium-gain, three low-gain antennas, and three fan beam antennas.

Figure 1: Europa Clipper flight system

Instrumentation: The remote sensing payload consists of the Europa Ultraviolet Spectrograph (Europa-UVS), the Europa Imaging System (EIS), the Mapping Imaging Spectrometer for Europa (MISE), the Europa Thermal Imaging System (E-THEMIS), and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON). The in-situ instruments are the Europa Clipper Magnetometer (ECM), the Plasma Instrument for Magnetic Sounding (PIMS), the SUrface Dust Analyzer (SUDA), and the MAss Spectrometer for Planetary Exploration (MASPEX). Gravity and radio science will be achieved using the spacecraft’s telecommunication system, and valuable scientific data will be acquired by the spacecraft’s radiation monitoring system. All instruments are body mounted, and the spacecraft points the remote sensing instruments toward nadir during most of the flybys, while the in-situ instruments face the ram direction at closest approach. The MISE instrument has an internal mirror that further allows it to scan along-track to compensate for target motion when close to Europa. The narrow-angle camera has a two-axis gimbal to allow for acquisition of stereo coverage and to extend its field of regard to off-nadir targets.

Figure 2: Europa Clipper suite of instruments

Mission Status: The assembly, testing, and launch operations (ATLO) phase is in its final stage. System-level testing has been completed at the Jet Propulsion Laboratory, and the flight system has been integrated with the solar array at Kennedy Space Center. The Operations and Mission Readiness Reviews have been successfully completed, and the Europa Clipper will be encapsulated in the Falcon 9 Heavy rocket in early October. The launch period begins on 10 October 2024, and once lifted off, the Europa Clipper will be cruising to the Jupiter System with gravity assists by Mars followed by Earth on the way. Go Europa Clipper!

How to cite: Korth, H., Pappalardo, R., Buratti, B., Craft, K., Daubar, I., Howell, S., Klima, R., Leonard, E., Matiella Novak, A., and Phillips, C.: Europa Clipper: The Final Countdown, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-120, https://doi.org/10.5194/epsc2024-120, 2024.

The JUICE mission has been launched by an Ariane 5 launcher on April 14, 2023 and is now on its way to reach Jupiter and its icy moons in 2031. The focus of JUICE is to characterise the conditions that may have led to the emergence of habitable environments among the Jovian icy satellites, with special emphasis on the internally active ocean-bearing worlds, Ganymede and Europa. Following a Jupiter Touring phase of 4 years, JUICE will become the first orbiter of a moon that is not our own, entering Ganymede orbit in 2034.

The spacecraft passed its commissioning review successfully on July 19, 2023, following the Near Earth Commissioning Phase (NECP), and, despite a few hickups, the ESA and multi-national instruments teams are now operating our interplanetary ship successfully. 

We report on the interplanetary cruise so far, and in particular on the first-ever Moon-Earth double gravity assist manouver performed on August 19-20. 

How to cite: Altobelli, N., Witasse, O., Vallat, C., Tanco, I., Dietz, A., and Erd, C.: JUICE mission news , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-405, https://doi.org/10.5194/epsc2024-405, 2024.

Knowledge of (near-)surface properties and their spatial heterogeneity will be critical to understanding the evolution of the top few-to-tens of meters of icy worlds, resulting in changes to its structure, density, and composition. On icy worlds, landform modification processes dominated by mass wasting and impact erosion (e.g., Ganymede) as well as potential plume fallout and refrozen brine infiltration (e.g., Europa), could leave behind layered deposits of varying density and thickness. Therefore, characterizing such heterogeneity (layering) can reveal much about the different processes acting on the near-surface environment. These can be studied with a multi-frequency/bandwidth approach applied to surface radar reflectometry measurements.

Radar reflectometry has been demonstrated to be a valuable technique for characterizing near-surface ice on Earth and Mars with mature plans for it to be applied to future observations of the Jovian icy moons, collected by the Europa Clipper and JUICE missions. Both missions host nadir-pointing ice-penetrating radar instruments: the Radar for Europa Assessment and Sounding: Ocean to Near- surface (REASON) on Europa Clipper operating at center frequencies of 60 MHz and 9 MHz, with bandwidths of 10 MHz and 1 MHz, respectively, and the Radar for Icy Moons Exploration (RIME) on JUICE at a single 9 MHz center frequency but bandwidths of 1 and 2.8 MHz.

Reflectometry is related to the measurement of the radar surface echo strength that is sensitive to surface and near-surface properties with a footprint-size spatial resolution of several kilometers (depending on the spacecraft altitude). The near-surface depth is bounded by the vertical resolution defined by the bandwidth of the transmitted signal. In void, [60/10 MHz], [9/2.8 MHz] and [9/1 MHz] radar systems have a vertical resolution of 15 m, 54 m and 150 m, respectively. The reflection strength of a homogeneous icy near-surface combined with a smooth surface is nearly independent of the radar system across the RIME-REASON frequency range. However, a sharp dielectric gradient from a horizontal discontinuity in the near-surface (e.g., layers) affects the effective surface reflection coefficient because of the phase shift between the surface and the near-surface reflections that are coherently integrated when measured at the receiver.

Previous applications of reflectometry rested on the assumptions implicit in the Radar Statistical Reconnaissance (RSR) technique, which has been regularly used to characterize bulk near-surface properties (e.g., porosity) and surface roughness, each predominantly dependent on the coherent and incoherent components of the total surface return, respectively. However, these previous applications of RSR utilized observations collected at near constant altitude. Europa Clipper and JUICE will both perform flybys of their targets of interest where altitude rapidly changes across the observation window. Thus, an understanding of how altitude (convolved with changes in the surface geology) can affect the balance between observed coherent and incoherent backscattered energy is necessary to confidently apply reflectometry to detect near-surface layering on Europa and Ganymede. 

Here, we simulate the radar surface echo from synthetic terrains, using a version of the multilayer Stratton-Chu coherent simulator with rough facets. We assess the coherent content of the total surface power to changes in altitude, by comparing the power derived from simulated surface echoes at the REASON/RIME shared center frequency (9 MHz) but different bandwidths (1 vs. 2.8 MHz). Coherent and incoherent geometric power falls off at different rates with altitude. Thus, the coherent content of the total return at a particular altitude over the target of interest could affect our ability to invert for the properties of near-surface layers. Note in particular that any landform modification of different terrain types (e.g., chaos terrain versus ridged plains on Europa, and dark versus bright terrains on Ganymede) may be better observed at different altitudes. In addition, our results provide valuable insight into targets and altitudes suitable for cross calibrating RIME and REASON [9/1 MHz] for comparative radar studies across the Jovian icy moons.

How to cite: Chan, K., Grima, C., Gerekos, C., and Blankenship, D.: RIME-REASON synergistic opportunities for detecting near-surface layering on icy moons, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-691, https://doi.org/10.5194/epsc2024-691, 2024.

Introduction

Jupiter’s icy moons are the focus of two upcoming planetary missions: JUICE (JUpiter ICy moons Explorer), developed by the European Space Agency (ESA), which is primarily dedicated to Ganymede but will also conduct flybys over Callisto and Europa; Europa Clipper, developed by NASA, which is primarily dedicated to studying Europa. Understanding the surface topography of these moons is crucial for comprehending their evolutionary history. Adequate preparation for the observations these probes will conduct requires preliminary knowledge of the characteristics of these planetary bodies. Integrating information from past missions with data to be acquired by these probes will enhance our understanding of these moons and the processes that have shaped their forms. This approach will allow us to achieve a more comprehensive knowledge and a better understanding of the icy satellites.

In preparation of these missions, we tested the approach of using images acquired by different spacecrafts under unfavourable conditions to generate topographic models through two photogrammetric techniques: stereogrammetry and shape-from-shading (SfS). The focus of this study is on three areas of Europa: the region of Thera Macula and Thrace Macula (Area A; Figure 1, top), the southern extension of Thrace Macula (Area B; Figure 1, bottom), and the band Libya Linea (Area C; Figure 2). The first two areas, in particular, were long-standing targets of the JUICE mission, but subsequent orbital reconfigurations have excluded their observation.

Dataset

The data used in the topographic modelling of the three regions include image data from the Voyager II ISS (Image Science Subsystem) [1] and the Galileo SSI (Solid-State Imaging) [2], acquired between 1979 and 2002. The Voyager II ISS dataset consists of 8 images with global resolutions (GR) of 1.6 – 1.9 km/pxl, acquired in colour, UV, and clear filters. The Galileo SSI dataset comprises 5 GR images at 1.4 – 1.7 km/pxl, acquired in colour, IR, and clear filters; 7 images with regional resolutions (RR) of 187-428 m/pxl, in the clear filter; and 3 high-resolution (HR) images at 44 m/pxl in the clear filter.

Images were processed using the Integrated Software for Imagers and Spectrometers (ISIS) [3]. We used updated SPICE kernels from [4], which provide the most accurate coordinate reference frame currently available for Europa. Processing included radiometric calibration, edge trimming, noise filtering, removal of Reseau points (for Voyager data), manual filtering of punctual artifacts, brightness equalization, and map-projection.

Topographic Modelling

We assessed the suitability of single images and 83 potential stereo pairs for stereo analysis by extracting geometric and spatial information from the common surface shared by each image pair. The suitability for stereo analysis and estimation of vertical precision were conducted using well-established criteria from the literature [5, 6, 7] .

We used the stereo algorithm implemented in the Ames Stereo Pipeline (ASP) [6] to generate stereo DEMs of areas A and B, testing various pre-processing operations and algorithm configurations. Galileo and Voyager GR images were employed in the stereo analysis of Area A, resulting in 55 DEMs. These include combinations of images acquired by the same spacecraft, either Galileo (G-G) or Voyager (V-V), and combinations formed by a Galileo and a Voyager frame (G-V). For Area B, 5 stereo DEMs were generated from combinations of Galileo LR and HR images.

SfS refinement of stereo DEMs was performed employing Galileo LR images in Area A, and Galileo HR images in Area B. In a sub-area of A, we were also able to exploit a multi-view approach, though under extreme illumination conditions. Topography in Area C was obtained from SfS only, without a stereo initial DEM, using all 5 Galileo LR frames. Various settings of SfS constraints were explored to optimize DEM quality.

Results

The quality of the resulting stereo DEMs was assessed using criteria from the literature [5-8] and additional factors such as elevation range, noise, inter-DEM consistency, amount of height variation, and ruggedness. Based on these criteria, a blended mosaic of three G-V DEMs provided the most plausible terrain model of Area A, possibly representing the first stereo topography of this Europan region documented in the literature. The vertical precision’s RMS in the blended DEM is 0.47 km. Further analyses are underway to assess significant topographic differences. The G-G stereo DEM in Area B offers a more reliable vertical extent of the chaotic terrain, measured at 494 ± 84 m. In general, topographic features appear poorly related to mappable geologic structures and more akin to topographic swells observed on other Europan regions [9].

Local topography was assessed by comparing SfS DEMs obtained under various configurations of constraints and using shadow measurements as a reference for accuracy. The obtained solutions favoured either height accuracy or successful filtering of typical SfS artifacts, such as sun-aligned streaks or artificial, albedo-driven reliefs, but not both. The highest local reliefs, up to ~850 m, are observed in the steep blocks of Thera Macula

Acknowledgments

G.C. and G.M. acknowledge support from the Italian Space Agency (2023-6-HH.0).

References

[1]          B. A. Smith et al., Space Sci Rev, 1977

[2]          M. J. S. Belton et al., Space Sci Rev, 1992

[3]          J. Laura et al., 2023

[4]          M. T. Bland et al., Earth and Space Science, 2021

[5]          K. J. Becker et al., 2015

[6]          R. A. Beyer et al., Earth and Space Science, 2018

[7]          R. L. Kirk et al., Remote Sensing, 2021

[8]          M. T. Bland et al., Remote Sensing, 2021

[9]          P. M. Schenk et al., 2001

Figure 1 Top: Stereo-SfS DEM of Thera Macula and Thrace Macula (Area A). Bottom: Stereo-SfS DEM of Thrace Macula’s southern extension (Area B)

Figure 2 Panel I: SfS-DEM of Libya Linea (Area C). Panels II-IV: Zoomed-in views of selected features.

How to cite: Chiarolanza, G. and Mitri, G.: Unveiling the Topography of Europa's Thera Macula, Thrace Macula, and Libya Linea Using Stereogrammetry and Shape-from-Shading, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-99, https://doi.org/10.5194/epsc2024-99, 2024.

Nowhere in the solar system are impact morphologies observed in greater variety than on the icy Galilean satellites.  This is very likely a consequence of the structural and thermal state of the crust at the time of impact, and perhaps impact velocity.  We have conducted a multi-disciplinary investigation to study how these features formed. The particular combination of geophysical factors and impactor characteristics that is shared by these satellites is likely responsible for these features. We have derived Digital Terrain Models for a number of these features, which have been used as a tool for producing facies maps. 

Impact features on Ganymede and Callisto are relatable to impact features on Europa, with implications for its ice shell history.  We have identified two broad classes of these impact features: (a) Crater forms, which variously include pits, central domes, and crater rims; and (b) Palimpsests (including pene-palimpsests), which all have smooth enclosed central plains, an extensive outer undulating plains unit, concentric arcuate ridges (in the case of pene-palimpsests), and no recognizable rim as in the example of Buto (Fig. 1).   The measured volume of Buto’s Undulating Plains unit (Fig. 2) is a factor of two smaller  than that of ejecta from an impact into a cold (T=120 K) target, as found from hydrocode modeling using the iSALE code, or ejecta scaling relations (Housen and Holsapple 2011).

We hypothesize that the undulating plains material may have been derived from a fluid or “slushy” layer (if present) in the near-surface target at the time of impact.  In the next section of this abstract we report the latest modelling of this possibility  The recognition of undulating plains as the dominant unit in Memphis and Nidaba, two other palimpsests on Ganymede that we have mapped (which display highly variable crater counts, with Buto being the least cratered), leads us to a working hypothesis that these plains only form where and when the target has substantial near surface fluid or “slush.”  By extension, all other impact features on Ganymede and Callisto, such as those with central pits and central domes, formed in targets with no pre-existing, or at most inconsequential, amounts of fluid or “slush.”  Supporting this conclusion, modeling by members of our group indicates that the present topography of impact features with central pits and domes can be largely explained by the behavior of ice alone, with no or little contribution from fluids (Caussi et al., 2024).  Our study supports explanations for the formation and final appearance of palimpsests (and pene-palimpsests) that are entirely applicable to this feature class, and sharply distinct from explanations for other impact features (crater forms) on Ganymede and Callisto.

Modelling of fluid ejecta emplacement

Recently performed Impact simulations incorporating a subsurface fluid layer (or low strength layer) at shallow depth 5-10 km produce a nearly flat surface profile with excavated material that may match the observed layer (Fig. 3).

Two sample iSALE simulations produced near-surface melt volumes of 3600 and 5000 cubic km within radii of 60 km from the impact point, amounts very close to the inferred volume of the UP layer. Bear in mind that using formulation from Kraus et al. (2011) that impact events of the scale of Buto may produce  at least 10% melt in the ejecta as well.   The role of the pre-existing 5-10 fluid layer in our iSALE models most profoundly affects the final profile of the impact feature.   In contrast, no fluid layer, and fluid layers >40 km deep, result in ”final” impact feature profiles with resemble classic craters.

Implications for Europa

Tyre (fig. 4) and Callanish, if analogues to pene-palimpsests, are much smaller than the Ganymede and Callisto features.  The pene-palimpsests of Ganymede and Callisto formed within their ice crusts (and upper mantles) in the first ~15% (?) of Solar System history, whereas Europa’s features and crust represent events of the last ~1% of Solar System history.  Is Europa’s Tegid really analogous to dome craters on Ganymede and Callisto?  Central domes are brighter and inferred to represent frozen impact-melt beneath the center of the craters.  Can we see and map the extent of frozen sub crater impact-melt pools with GPR? Callisto and Ganymede’s upper non-ice crust composition and thicknesses may be different: we need more lab work with respect to fundamental radar properties.  REASON “non-targeted” observations of pene-palimpsests and dome craters on Ganymede and Calllisto combined with RIME profiles of these features may be useful in determining how analogous Tyre, Callanish, and Tegid really are to the Ganymede and Calllisto features.

References

Caussi, M.L. et al. (2024) Dome craters on Ganymede and Callisto may form by topographic relaxation of pit craters aided by remnant impact heat. J. Geophys. Res., in press.

Housen, K. R. and Holsapple, K. A. (2011) Ejecta from impact craters. Icarus  211, 856-875.

Kraus , R. G., Senft,  L. E., Stewart, S. T. (2011) Impacts onto H₂O ice: Scaling laws for melting, vaporization, excavation, and final crater size. Icarus 214, 724-738.

Moore, J. M. et al. (2001) Impact Features on Europa: Results of the Galileo Europa Mission (GEM). Icarus, 151, 93–111.

References for iSALE

Amsden, A., Rupple, H., and Hirt, C. (1980) SALE: A simplified ALE computer program for fluid flow at all speeds. Los Alamos Nat. Lab. Rept. LA-8095.

Collins, G. S., Melosh, H. J., and Ivanov, B. A. (2004) Modeling damage and deformation in impact simulations. Met. Plan. Sci. vol 39, 217-231.

Wünnemann, K., Collins, G., and Melosh, H. (2006) A strain-based porosity model for use in hydrocode simulations of impact and implications for transient crater growth in porous targets. Icarus vol 180, 514-527. 

How to cite: Moore, J., Korycansky, D., McKinnon, W., White, O., Schenk, P., Caussi, M., and Dombard, A.: Large Impact Features on Icy Galilean Satellites: New Modelling of fluid ejecta emplacement, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-103, https://doi.org/10.5194/epsc2024-103, 2024.

Europa’s surface is one of the youngest in the solar system. The Jovian moon is believed to hide a global liquid water ocean under its icy crust [1] and is exposed to intense space weathering due to the continuous bombardment by electrons and ions from Jupiter’s magnetosphere [2]. To understand the processes governing the evolution of the surface it is necessary to finely characterize the microphysics of the ice (composition via endmember volume abundance, grain size and surface roughness). Many recent studies have identified the presence of a large number of chemical compounds on the surface and mapped their distribution [3,4,5]. However, other microphysical properties are generally not integrated or not precisely constrained, and the low spatial resolution of the data used implies that these properties cannot be linked to surface morphologies (ridges, lineaments, chaos, craters).

Here we report an integrated work in which we aim to estimate the microphysical properties of Europa’s surface at the scale of a particular morphology: the dark lineaments of the trailing hemisphere. To do so, we used the Near-Infrared Mapping Spectrometer (NIMS) data [9] as they provide hyperspectral images of the surface at a resolution of a few kilometers per pixel, thus allowing to resolve these morphologies. Using a radiative transfer model [6] in a Bayesian MCMC inference framework [7,8] we produce maps of the microphysical properties at the NIMS spatial resolution. The work we present here is the results of three successives studies we have carried out.

We first analyzed a calibrated spectrum of a dark lineament from the trailing Anti-jovian hemisphere (fig. 1). The estimated signal-to-noise ratio (SNR) is between 5 and 50, we mainly focus on the 1.0-2.5 µm region for which the SNR is higher with an uncertainty on the absolute calibration up to 10% [8]. With this spectrum, we tested all combinations of 3, 4 and 5 endmembers from a list of 15 relevant compounds [10]. With more than 5000 tested combinations we showed that some compounds appear necessary to reproduce the observation, such as water ice and sulfuric acid octahydrate, in agreement with previous studies [3,4,11]. However, adding either hydrated sulfates or chlorine salts produces results substantially similar [10].

In a follow-up study [12], we focused on the few acceptable combinations identified by our Bayesian inversions and we analyzed the results in terms of grain size and surface roughness. We showed that the grain size of the essential end-members is well constrained and similar from one combination to another (fig. 2). The macroscopic roughness is however poorly constrained [12], as expected from a single geometry observation. Such results imply that it is possible to describe the surface in terms of volume abundance and grain size for the essential compounds (water ice and hydrated sulfates). 

Using the previous results and thanks to numerical optimizations we are able to independently invert every spectrum of a NIMS hyperspectral cube with this Bayesian MCMC algorithm. We selected a mixture of 5 relevant end-members from our previous studies and mapped, for the first time, the volume abundance and grain sizes of water ice, sulfuric acid hydrate and hydrated sulfates at the scale of a NIMS footprint (fig 3). We propose a geological interpretation in which lineaments are a preferential location for material exchange between Europa’s surface and interior [13]. 

Figure 1 - (Top): Map of Europa with a zoom on Harmonia Linea observation. The colors represent the reflectance at 0.7 µm normalized using the Lommel-Seeliger law. (Bottom): All spectra from the corresponding cube and the selected spectrum (in black) used in this study.

Figure 2 - Posterior Probability Density Functions (PDF) of the grain size for all combinations in which crystalline water ice is used (in black) and averaged PDF (in red).

Figure 3 - Maps of the crystalline water ice volume abundance estimated from our Bayesian inversions of an entire NIMS hyperspectral image.

References: [1] Pappalardo, R. et al. (1999) JGR. [2] Carlson, R. W. et al. (2005) Icar. [3] Ligier, N. Et al. (2016) The Astr. Jour. [4] King, O. Et al. (2022) PSS. [5] Villanueva, G. et al. (2023), Science. [6] Hapke, B. (2012). Cambridge Univ. Press. [7] Cubillos, P. et al. (2016), The Astr. Jour. [8] Braak, C. J. F. (2008), Stat & Comp. [9] Carlson, R. et al. (1992) ed. C. T. Russell. [10] Cruz-Mermy, G. (2022) Icarus. [11] Mishra, I. et al. (2021) Planet. Sci. [12] Cruz-Mermy, G. (2024a) In prep. [13] Cruz-Mermy, G. (2024b) In prep.

How to cite: Cruz Mermy, G., Schmidt, F., Andrieu, F., Cornet, T., and Belgacem, I.: Microphysical properties of Europa's dark lineaments, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-752, https://doi.org/10.5194/epsc2024-752, 2024.

Introduction

Ganymede, the largest moon in the solar system, presents a compelling subject for studying icy moons due to its unique geological processes compared to its neighbors. As a result, it is now the primary target of ESA's JUICE mission [1, 2]. Predominantly, impact craters dominate its terrains, each exhibiting a distinctive morphology and ejecta pattern. These features help to understand the vertical stratigraphy of Ganymede's crust at a local scale. Additionally, our study investigates the role of spreading and rifting processes in transporting dark terrain material to the subsurface. 

Data and Methodology

We identified four crater types for our study: Bright Ray Craters (BRCs) with bright ray ejecta, Dark Ray Craters (DRCs) with dark ray ejecta, Craters with both bright and dark ejecta (DBRCs), and Dark Halo Craters (DHCs) with circular to subcircular dark ejecta. We utilized the global mosaic from [3]. We examined several craters also observed by Galileo NIMS, with spatial resolutions up to ~2 km/pxl [4, 5]. We mapped the band depth (BD) of one of the major water ice absorptions at 1.5 or 2 µm to derive information on the relative abundance of water ice versus dark material following the approach presented in [6]. Morphologic details of the craters, including inner features and the extension of their halos or ray systems, were mapped.

To calculate excavation depth, we use equation from [7],

D_e = 1/10* D_t                                                                                     (Eq. 1),

where D_e is the maximum depth of excavation, and D_t is the diameter of the transient crater cavity.

To determine the transient diameter (D_t) of a crater with known final crater diameter (D), we employ the equation by [8].

D = D_t^ε ⋅ D_c^1−ε [8]                                                                  (Eq. 2)

D_c = 2.5 km for Ganymede, ε ~ 1.13, which accounts for crater slumping [8]

Results

Distribution of crater types

In this study, we analyzed 36 impact craters on Ganymede (Fig. 1), categorizing 3 as DHCs and the rest as ray craters. Among these, the ray crater Tammuz (DBRC) exhibits half bright and dark ejecta. In detail, 20 craters are situated in the light terrain, 8 in the dark terrain, and 8 were emplaced along the boundary between light and dark terrain.

Excavation depths of crater types

Craters with dark ejecta predominate below 40 km diameter, while beyond 40 km, BRCs prevail (Fig. 2 a and b). Dark terrain material originates from excavation depths of up to 3 km in light terrain. Larger ray craters located at the borders of light and dark terrain are observed to be BRCs (Fig. 2c).

Individual Craters

1. Antum

Antum, a DRC ~ 15 km in diameter, is situated at around 5.5°N/141.1°E on dark terrain, with a maximum excavation depth of ~ 1.2 km (Fig. 3). Its main geological features include a crater with a distinct floor and rim, as well as extended dark rays. This is supported by the BD map of water ice absorption at 1.5 µm, derived from NIMS observations, indicating enrichment of water ice in the crater (smaller ice grains) and dark non-ice material in the ray material (larger ice grains).

2. Kittu

Kittu, a 15 km DRC, is situated at ~ 0.5°N/25°E on light terrain (Fig. 4). The maximum excavation depth is ~ 1.2 km. The main geological units are a central peak, a bright rim and floor, a continuous bright ejecta and discontinuous dark ejecta. NIMS confirms richness in water ice and smaller ice grains in colder water ice while low water ice abundance and larger ice grains in warmer dark ejecta.

Discussion

To explain craters like Antum, the depth of excavation for dark ejecta should match the thickness of the top dark terrain material (Fig. 5a). In the Antum region, dark terrain thickness reaches ~1.2 km. Conversely, for craters like Kittu (Fig. 5b) on light terrain, dark ejecta suggest that dark material is relatively thin and comes from a layer just beneath the surface, ~1.2 km. Dark material reaching such depths could be attributed to tectonic rifting, necessitating mechanisms for material transport. These findings provide constraints for future assessments of light terrain formation processes and dark material transport mechanisms.

Figure 1: The global distribution of BRCs (blue), DRCs (black), BDRCs (green), and DHCs (yellow) on Ganymede.

Figure 2: Crater diameter vs. excavation depth (km) for various crater types and terrains: a) craters located in light terrain, b) in dark terrain and c) at border between light and dark terrains. 

 Figure 3: Dark ray crater (DRC) Antum: a) Voyager image, b) geologic map, c) NIMS derived BD map of the water ice absorption at 1.5 µm and d) grain size of water ice as derived by the NIMS BDR map of the water ice absorptions at 2 and 1.5 µm after [6].

Figure 4: Dark ray crater (DRC) Kittu: a) High resolution Galileo observation, b) geologic map of Kittu crater, c) Voyager + Galileo mosaic, d) geologic map, e) NIMS derived BD map of the water ice absorption at 1.5 µm  and f) grain size of water ice as derived by the NIMS derived BDR map of the water ice absorptions at 2 and 1.5 µm after [6].

Figure 5: Schematic illustration of the subsurface layers required to explain various ejecta pattern. In the left part is the transient cavity illustrated and how the different layers are involved in the ejecta curtain. The larger right side shows the ejecta blanket and the position of different ice layers of the target prior to impact. The shown scenarios are applicable to craters a) DRC Antum (on dark terrain) b) DRC Kittu (on light terrain) .

References:

[1] Grasset, O., et al., Planetary and Space Science, 78, 1-21, 2013. [2] Stephan, K., et al., Planetary and space science, 208, 105324, 2021. [3] Kersten, E., et al., EPSC2022-450, 2022 [4] Stephan, K., et al., ScSSI, abstr. 9060, 2008. [5] Hibbitts, C. A., Icarus, 394, 115400, 2023. [6] Stephan, K., Icarus, v. 337, p. 113440, 2020. [7] Melosh, H. J., ISBN 0 19 504284 0, 245 pp, 1989. [8] Schenk, P. M., Cambridge University Press, p. 427 – 456, 2004

How to cite: Baby, N. R., Kenkmann, T., Stephan, K., Wagner, R., and Hauber, E.: Ray and Halo Impact craters on Ganymede: Insights into stratification of icy crust, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-633, https://doi.org/10.5194/epsc2024-633, 2024.

Jupiter’s moon Europa is one of the prime targets of planetary exploration given its high astrobiological potential. The exchange of material between its surface and subsurface ocean is a key aspect in assessing the ocean habitability, as it can lead to a chemical disequilibrium, which is thought to be an important process that drives the development of habitable environments (Hand et al., 2007). Moreover, material from the subsurface ocean that may be exposed at the surface can provide valuable information about the ocean chemistry and its organic content. 

The interaction between the surface and the subsurface ocean on Europa may have been largely facilitated by impacts whose basins’ shape provide important information about the ice shell’s thermal state, thickness, and dynamics (conductive vs. convective ice shell). On Europa, a change in the basin morphology, observed for basin diameters larger than ∼8 km, is thought to indicate the presence of a weak layer at a depth of ∼7 km (e.g., Bray et al., 2014).  This layer could potentially represent warm convecting ice or the presence of the liquid ocean (e.g., Silber and Johnson, 2017). A recent study that investigated multiring forming basins on Europa suggests an ice shell thickness larger than 20 km consisting of a 6-8 km conductive layer overlying a warm convecting region (Wakita et al., 2024). In addition to impact basins, evidence for surface-ocean interaction is documented by the surface features and geologic activity. In particular, the young surface age of ∼40 – 90 Myr (Bierhaus et al., 2009) suggests that some form of resurfacing has occurred in the past.

In a recent study, Carnahan et al., (2022) investigated the surface-to-ocean transport of water generated by impacts on Europa. Here, we assume that the water produced as a consequence of the impact process rapidly recrystallizes, but leaves behind a compositional and thermal anomaly in the shallow layers of the ice shell. We use the geodynamic code GAIA (Hüttig et al., 2013) in a 2D geometry to model the effects of impacts on the large-scale dynamics of Europa’s ice shell. The thermal anomaly introduced by impacts is calculated using scaling laws (Melosh, 1989). We chose different impactor sizes and velocities, and compositional anomalies to test various scenarios. As local weak zones may develop at the impact location, we monitor if impacts can promote surface mobilization. Our models include a variable thermal conductivity (Petrenko & Whitworth, 1999) and expansivity (Feistel & Wagner, 2006), and consider a mixed diffusion, dislocation, and basal slip/grain boundary sliding rheology (Goldsby & Kohlstedt, 2001). Since the grain size is the parameter largely controlling the viscosity inside the ice shell and thus driving the ice shell dynamics, we test grain sizes between 1 cm and 1 mm, a range observed in polar ice sheets on the Earth (Faria et al., 2014; Montagnat & Duval, 2000; Ng & Jacka, 2014). 

In Fig. 1 we show the effects of impacts on the convective velocity of a 40-km-thick ice shell using a grain size of 1cm (Fig. 1a) and 1mm (Fig. 1b). Our models show that impacts can lead to an increase in the convection vigor. However, this increase decays within a few 100 kyr and the dynamics within the ice shell become similar to those of ice shells that did not experience the effects of impacts. Dense chemical heterogeneities introduced by impacts in the top part of the ice shell, while initial driving convection due to an unstable density gradient, sink towards the ice-ocean boundary where they accumulate and hinder thermal convection (Fig. 1b, cyan curve).

Surface mobilization is facilitated by impacts, in particular if they introduce both thermal and compositional anomalies (Fig. 2a). As the impacts create local weak zones, cold and dense surface material sinks into the ice shell, if the viscosity of material is sufficiently low to allow for surface mobilization to occur. Some of this surface material is able to reach the ice-ocean boundary, while some becomes mixed with the ice shell material (Fig. 2b).

The efficiency of material exchange between the surface and the ocean on Europa both at the time of the impact and during the subsequent evolution will provide important implications for the ocean composition and the ice shell structure that can be tested with future measurements of the JUICE and Europa Clipper missions.

References:

Bierhaus, E. B., Zahnle, K., Chapman, C. R., Pappalardo, R. T., McKinnon, W. R., & Khurana, K. K. (2009). Europa, 161.

Carnahan, E., Vance, S. D., Cox, R., & Hesse, M. A. (2022). GRL, https://doi.org/10.1029/2022GL100287.

Faria, S. H., Weikusat, I., & Azuma, N. (2014). Journal of Structural Geology, https://doi.org/10.1016/j.jsg.2013.09.010.

Feistel, R., & Wagner, W. (2006). Journal of Physical and Chemical Reference Data, https://doi.org/10.1063/1.2183324.

Goldsby, D. L., & Kohlstedt, D. L. (2001). JGR: Solid Earth,  https://doi.org/10.1029/2000JB900336.

Hand, K. P., Carlson, R. W., & Chyba, C. F. (2007). Astrobiology, https://doi.org/10.1089/ast.2007.0156.

Hüttig, C., Tosi, N., & Moore, W. B. (2013). PEPI, https://doi.org/10.1016/j.pepi.2013.04.002.

Melosh, H. J. (1989). New York: Oxford University Press; Oxford: Clarendon Press.

Montagnat, M., & Duval, P. (2000). EPSL,
https://doi.org/10.1016/S0012-821X(00)00262-4.

Ng, F., & Jacka, T. H. (2014). Journal of Glaciology, https://doi.org/10.3189/2014JoG13J173

Petrenko, V. F., & Whitworth, R. W. (1999). Physics of ice. OUP Oxford.

Schenk, P. M., Chapman, C. R., Zahnle, K., & Moore, J. M. (2004). Jupiter: The planet, satellites and magnetosphere, 2, 427.

Wakita, S., Johnson, B. C., Silber, E. A., & Singer, K. N. (2024). Science Advances, https://doi.org/10.1126/sciadv.adj8455.

How to cite: Plesa, A.-C., Rückriemen-Bez, T., and Wünnemann, K.: The role of impacts on ice shell dynamics and surface-to-ocean exchange on Europa, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-701, https://doi.org/10.5194/epsc2024-701, 2024.

The NASA Europa Clipper mission [1] will explore Jupiter’s icy moon Europa via multiple flybys in the early 2030s. The ocean world Europa is one of the most promising locations to search for life elsewhere in the Solar System [2, 3, 4, 5] and thus, Europa Clipper’s main goal is to characterize Europa’s habitability. In the future, a follow-on landed mission may possibly explore Europa from its surface. In the Europa Clipper science team, the work of the Reconnaissance Focus Group has been to assess the trajectory for opportunities [6]. Here we show that 12 of the 49 currently planned flybys for the prime mission of Europa Clipper [7] survey at least one area where the fundamental requirements for terrain relative navigation (TRN) are fulfilled. We use the term ‘reconable’ to refer to these 12 flybys.

What is a “reconable” flyby? 

A ‘reconable’ flyby contains at least one section where the following fundamental (i.e., architecture independent) requirements for terrain relative navigation (TRN) are fulfilled, as based on current technology [8, 9]:

• Time of day: the data must be taken during the Europan day (i.e. lit by sunlight)
• Incidence angle of data: 30°- 60° for TRN process to be successful,  20°- 30° may be usable in some instances, but there is a rapid degradation above 60°.
• Altitude: 50-100 km for sufficiently high spatial resolution data with minimal smearing. 100-105 km altitude may be useable in some instances.

Thus, only 12 of the 49 flybys are reconable.

Fig. 1: Global map of Europa with the footprint of all 49 flybys currently planned for Europa Clipper’s prime mission. Reconable and supporting flybys are in solid colored lines. 

Most promising flybys

Using data from the Galileo mission, we study what is currently known about our reconable areas, and rank these flybys based on scientific criteria. The current rankings reflect our present-day knowledge and are highly likely to change with Europa Clipper’s in-depth study of the moon’s geological, geochemical and geophysical characteristics. 

Three of them have exceptional scientific interest. They all show signs of recent activity and have been covered significantly enough by past missions to make a comprehensive assessment of their potential. Some highlights include 

Fig. 2: Ground track for planned E5 flyby. Reconable area highlighted by white box. 

Fig. 3: Ground track for planned E19 flyby. Reconable area highlighted by white box. 

Fig. 4: Ground track for planned E22 flyby. Reconable area highlighted in white box. 

Conclusion

Reconable flybys are relatively rare and important in the prime mission of Europa Clipper: only 12 out of 49. It’s important to recognize that this current assessment is limited by the data available. Rankings will probably evolve as the data from Europa Clipper, and particularly from the Europa Imaging System (EIS) [10] gets processed. This work demonstrates the process that can be used by the Europa Clipper team to assess potential landing sites. 

Great potential landing sites do exist: there are places on Europa’s surface that are both particularly scientifically interesting (e.g., rank 1* reconable flybys) AND where we will collect data that is suitable for enabling safe and successful landing based on current technologies.

The results of this work have been shared with the Geology Working Group (Pappalardo et al. 2024 (under review), Daubar et al. 2024) / the broader science team of Europa Clipper, and will be integrated in the science input provided for observation planning. 

Acknowledgements

This work was supported by the Europa Clipper Project. The research described in this manuscript was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Our team would also like to acknowledge the work from the Galileo instrumental teams and the PDS imaging node for making this data publicly available.

References

[1] Howell & Pappalardo, Nature Comm, 2020.

[2] Pappalardo et al. JGR: Planets, 1999.

[3] Carr et al. Nature 1998.

[4] Vance et al. JGR: Planets, 2018.

[5] Cockrell et al. Astrobiology, 2016.

[6] Phillips et al. PSJ, in revision.

[7] Pappalardo et al. SSR, accepted.

[8] Johnson et al. AIAA Guidance, Navigation, and Control Conference, 2015.

[9] Neleseen et al. IEEE Aerospace Conference, 2019.

[10] Turtle et al. SSR under review.

How to cite: Belgacem, I., Scully, J. E. C., Parekh, R. A., Phillips, C. B., Grima, C., Collins, G. C., Craft, K., Detelich, C., Leonard, E., Mishra, I., Patterson, W., Prockter, L. M., Sutton, S. S., Stickle, A. M., and Wyrick, D. Y.: Potential landing sites to be surveyed by Europa Clipper, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-132, https://doi.org/10.5194/epsc2024-132, 2024.

How to cite: Boccelli, S. and Tucker, O. J.: Simulations of Europa’s vapor plumes and ice-grain fallout, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-584, https://doi.org/10.5194/epsc2024-584, 2024.

Introduction:  The surfaces of Europa and Ganymede contain hydrated material(s) that are continually altered by bombardment with Jovian magnetospheric ions, electrons, and micrometeroids. This bombardment may affect the structures of the surface materials, altering their spectral signatures in the visible and infrared [1]. The physical and chemical alteration by ultraviolet (UV), particle, and micrometeroid impacts has been used to explain the spectral features observed on the surfaces of ocean worlds [ e.g. 2], as well as provide insight into discrepancies between the infrared (IR) characteristics of these surfaces with analogs [ e.g. 3]. Regardless of the challenges, some robust identifications have been made. NaCl has been identified on Europa through radiation-induced absorption bands near 460 nm and 230 nm [4] and MgSO₄ has been tentatively identified through an infrared band near 3.7 microns [5].  The hydrated sulfate is also hypothesized to exist on Ganymede [6]. We further investigate the spectral nature of irradiated materials to understand the compositions of Europa and Ganymede, including effects of high-energy (1 to 40 keV) electron irradiation and laser-simulated micrometeroid bombardment on the visible (Vis) through infrared (IR) reflectance spectra of these materials and other cryogenic hydrated materials with implications for interpreting the surface compositions of Europa.

We present visible – infrared spectra of compositional analogs (mainly hydrated salts) to the icy Galilean satellites that have been exposed to high energy electron irradiation (1 to 40keV) and/or subjected to simulation of micrometeroid bombardment by lasing at 1064 nm, 212 milliJoules, with a ~9 ns duration. Experiments were conducted in the ultrahigh vacuum chamber in the APL Laboratory for Spectroscopy under Planetary Environmental Conditions (LabSPEC). Bidirectional reflectance spectra at an incidence angle of 15 degree and emission angle of 45 degree were collected once vacuum reached < 1 microtorr during which (including during pump-down) the sample temperature was maintained between 130 K and 150 K during irradiation and sample collection. Reflectance spectra at wavelengths from 400 nm to 2400 nm spectra were collected with an SVC point spectrometer; spectra from 1500 nm to 8000 nm were collected using a Bruker Vertex 70 FTIR.  Electron irradiation was produced with a Kimball-Physics model 6104 gun operated at a flux on the sample of 16  to 500 nanoamps/cm², which ranges from a few times Europa conditions to about 2 orders of magnitude higher than electron flux on the equatorial portion of the leading hemisphere Europa. Total fluence equated to only days to several months of exposure on Europa’s leading hemisphere.

Hydrated halides, sulfates, and hydrated sulfuric acid were irradiated. Despite the relatively short Europa-surface equivalent time of electron irradiation, every irradiated sample exhibited significant spectral changes. Electron irradiation causes darkening in the visible to near-IR for all samples, with an F-type color center forming in NaCl and a possible M-center forming in partially hydrated MgCl₂. In the infrared, the irradiation affects the ~ 3-mm Fresnel reflection peak and generally increases the reflectance in the 4-micron transparency region, likely by increasing porosity through microscopic damage/stimulated desorption. Additionally, in the one experiment that contained organics in a matrix of NaCl with adsorbed waters (see 7), the organic bands in the visible and infrared diminished with increasing irradiation and after the final irradiation, CO₂ was present, likely a result of chemistry between the degraded organics and adsorbed water on the NaCl. Laser simulation of micrometeroids physically damages and disrupts the surface, altering the reflectance at 3-micron and also increasing reflectance in the 4-micron region. No other spectral changes were noted, including no evidence for desiccation of the hydrated material. Neither electron irradiation nor damage by simulated micrometeroid bombardment otherwise alters the infrared spectra of hydrated cryogenic materials.

Acknowledgments: We would like to acknowledge the support of SSW Grant # 80NSSC20K1044.

References: [1] Hibbitts, C. A. et al. (2019), Icarus, 326, 37-47; [2] Hand, K. and B. Carlson, GRL, 42, 3174-3178, ,2015; [3] Hibbitts, C. A., and C. Paranicas. In Space Weathering of Airless Bodies Workshop, 1878, p. 2062. 2015.; [4] Trumbo, S.K., Brown, M.E., Hand, K.P., (2019) Science Adv., 5; eaaw7123.; [5] Trumbo, S. K., et al. (2017). The Astron. J., 153(6), 250.; [6] McCord et al., (1998), J. Geophys. Res.; [7] Price, T., C.A. Hibbitts, K. Craft, (2024) AbSciCon, abstract.

How to cite: Hibbitts, C., Stockstill-Cahill, K., Lloyd, E., Pascale, L., Clayton, H., and Wagoner, C.: Investigating cryogenic space weathering processes for Europa and Ganymede, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-675, https://doi.org/10.5194/epsc2024-675, 2024.

Introduction

The surface of icy moons has a microstructure shaped by a complex interplay of physical processes. Among them, ice sintering, also known as annealing or metamorphism,  transports material from ice grains into their bond region, resulting in changes in the thermal, optical and mechanical properties of the ice. 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. 

A multiphysics simulation model “LunaIcy” incorporating the sintering process coupled with the MultIHeaTS thermal solver, was developed to study the evolution of ice microstructure on Galilean satellites, specifically tracking the changes in the ice grain and bond radii over time. LunaIcy was applied to the evolution of Europa's ice microstructure over a million years 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.

Methods

To accurately simulate the temperature-dependent sintering process, the sintering model is coupled with the thermal solver  MultIHeaTS (Mergny 2024a) creating our multiphysics simulation model, LunaIcy (Mergny 2024b) as shown in Figure 1.

MultIHeaTS - Thermal solver

We have developed an efficient open-source fully implicit algorithm called MultIHeaTS, which uses finite differences to solve the heat equation on 1D heterogeneous media with an irregular grid. Due  to its stability, MultIHeaTS can compute the same temperature for a given time up to 100 times faster than the explicit method. This is particularly advantageous for simulating processes that occur on large timescales. Thanks to such fast computation, MultIHeaTS is used to estimate the temperature profile on Europa during one million years with varying orbital parameters, allowing us to obtain the full temperature history (Mergny et al, 2024a).

LunaIcy - Sintering model 

Previous sintering models used in planetary science, directly compute the mass flux from the grain surface to the bond surface, as it was suggested by (Swinkels 1981). However this difference of curvature between the grains and bonds, does not appear in any of the analytical models of sintering used on Earth (Miller 2003, Flin 2003, Flanner 2006). The evaporation-condensation laws  may have been incorrectly used for planetary cases, as they do not seem to properly describe the exchange of matter between the ice and the gas in the pore space. To address this, in our sintering model, we  calculate both mass flux separately, the flux of matter from the grain surface to the pore space and the flux between the pore space and the bond. As changes in ice microstructure affect the thermal properties this leads to a two-way coupling between sintering and heat transfer.

Fig.1 Block diagram of the proposed multi-physics simulation model LunaIcy.

Results

Figure 2 shows the sintering heatmap of the top layer for  this set of simulations at the equator; for each pair of the porosity and albedo parameters, is represented the ratio of the final bond radius to the initial one. As expected, lowering the albedo or increasing the porosity, which yields to a less conductive material, increases the surface temperature thereby enhancing sintering. Only situations of very high albedo A > 0.8, do not see any effect of sintering during the one million year period. For the warmest regions, the top layer bond radius can grow by almost two orders of magnitude its initial size, leading to significant changes in the thermal conductivity of the ice.

Fig. 2 Sintering efficiency heatmap for the top layer as a function of porosity and albedo at the equator. Colors indicate the ratio of the top layer bond radius after one million years to the initial bond radius. In the majority of scenarios, the bond radius more than doubles after one million years, except for very high albedo.

In Figure 3, we present the evolution of the relative bond radius for the different grain sizes, focusing on the top layer, at x=0. As anticipated, the sintering timescale shows a significant dependence on the grain size, with smaller grains undergoing faster sintering due to their higher surface curvature (Blackford 2007, Molaro 2019). While the exact grain size of Europa's surface remains unknown, thanks to such parameter exploration, we can predict the evolution of the various possible surface configurations.

Fig.3 Evolution of the top layer relative bond radius for different initial grain radius. Smaller grains show more efficient sintering due to their higher surface curvature. 

Discussion and conclusion

Our simulations spanned a million years, allowing us to thoroughly explore the evolution of Europa's icy surface microstructure. Results show that the hottest regions experience significant sintering, even if high temperatures are only reached during a brief portion of the day. This process takes place on timescales shorter than Europa's ice crust age, suggesting that these regions should currently have surface ice composed of interconnected grains. Simulating highly coupled processes like sintering offers valuable insights into Europa's ice microstructure, enhancing surface measurements like spectroscopy and constraints on grain size, which will significantly benefit the upcoming missions JUICE and Europa Clipper.

References 

• Mergny, C., & Schmidt, F. 2024a, MultIHeaTS: a Fast and Stable Thermal Solver for Multilayered Planetary Surfaces, PSJ in revision
• Mergny, C., & Schmidt, F. 2024b, LunaIcy: Exploring Europa's Icy Surface Microstructure through Multiphysics Simulations, PSJ in revision
• Swinkels, F., & Ashby, M. 1981, Acta Metallurgica
• Miller D, et al, 2003, CRST
• Flin F., et al, 2003, Journal of Physics D: Applied Physics
• Flanner M., et al 2006, JGR
• Blackford, J, et al., 2007, Journal of Physics D: Applied Physics
• Molaro J., et al, 2019, JGR: Planets

How to cite: Mergny, C. and Schmidt, F.: Active Sintering on Europa Over a Million-Year Timescale, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-863, https://doi.org/10.5194/epsc2024-863, 2024.

The surfaces of the icy satellites in the outer Solar System show landforms and features that testify an intense geologic activity. Kilometric-scale tectonic structures shape the majority of their icy crusts showing complex patterns that mostly refer to extensional and strike-slip regimes. Such stress-related structures (i.e., fractures/faults) represent weakness zones that provides insights to the dynamics and the mechanical properties of the crusts. Additionally, these structures are potential conduits for fluid propagation within the icy crusts that can connect the surface with the sub-crustal layers, including the internal liquid ocean. Therefore, tectonic structures of the icy satellites are pivotal for the understanding of both internal processes and also for astrobiology research. In view of upcoming missions, such as JUICE and Europa Clipper, the knowledge about such icy bodies requires support to better understand their geology and tectonics, whose investigations are constrained to regional-scale remote sensing detection.

The contribute of terrestrial analogues has always represented a strong aid to unravel planetary surfaces issues. On Earth, glaciers and ice sheets are optimal analogues, since they show deformation styles similar to the shear zones in the icy satellite surfaces. Although the formation processes differ, the similarity of their structures allows to unravel and predict the state of deformation in icy satellites at different scales. Moreover, the comparison with glacier deformation allows to better understand the local-scale environment and target areas for future exploration.

In this contribution we propose a multiscale analysis in four glaciers in the Argentinean Southern Patagonia Ice Field. Deformation structures of glaciers in the Tierra del Fuego province, named Martial, Vinciguerra, Ojo del Albino and Alvear gl., have been detected at both local- and regional-scale, through fieldwork and remote sensing investigation, respectively. The investigations allowed to achieve two datasets of the structures attribute measurements (i.e., azimuth, dip, length, spacing, width and sinuosity). Therefore, the datasets have been compared to acquire knowledge that has been then transferred to the surfaces of the icy satellites.

The data have been stored on GIS and Digital Outcrop Modeling tool to better unravel the structural settings and their relation at both surface and subsurface, in the third dimension. In this way, we produced 3D models that show the glacier structures development from the surface to the subsurface and have been used to infer models of the deep structural setting of shear zones in the icy satellites. Such models show the occurrence and development of deep low-angle structures, including the compressional ones, which are challenging to detect on icy satellite surfaces by remote sensing investigation.

Therefore, this work attempts i) to identify scaling laws between deformation structures measured at local-scale in the glacier outcrops and those mapped at regional-scale on satellite images; ii) to relate and compare such scaling laws with structures of shear zones of the icy satellites and in turn iii) to infer their local and deep structural setting.

Acknowledgments: This work is part of the EVIDENCE project that has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 871149. The activity has been realized under the ASI-INAF contract 2023-6-HH.0. We gratefully acknowledge funding from INAF through the Mini Grant DISCOVERIES. 

How to cite: Rossi, C., Pozzobon, R., Martini, M., Flores, E., Lucchetti, A., Pajola, M., Penasa, L., Munaretto, G., Tusberti, F., and Beccarelli, J.: Interpolation of the subsurface structural pattern of the icy satellites through multiscale comparison with terrestrial analogues, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-539, https://doi.org/10.5194/epsc2024-539, 2024.

JWST observations of Europa’s leading hemisphere show excess CO₂ over chaos terrains where the ocean is likely to have breached the ice shell. Analysis of the 3.5 µm peroxide absorption reveals elevated amounts of H₂O₂ in these chaos regions. Laboratory experiments motivated by these observations show that trace inclusions of CO₂ can substantially inflate the radiolytic peroxide yield, more so than in pure water ice. These experiments suggest that endogenic CO₂ amplifies H₂O₂ synthesis in the chaos terrain when processed by Jupiter’s magnetospheric particles. We combine the CO₂-enhanced H₂O₂ yields with the energy dose delivered by the magnetospheric particles onto the leading hemisphere to generate a peroxide distribution map to compare with the observed distribution. Our results suggest that geologic activity at Europa’s chaos conspires with precipitating radiation to contribute to the excess peroxide observed by Webb. The rapid transport of the peroxide to the underlying ocean via brine-percolated conduits underscores strong implications for Europa’s habitability. Additionally, these JWST observations combined with laboratory measurements set the stage for detailed mapping of CO2 (via its 2.7 and ~4.23-4.29 µm absorptions) and H2O2 (via its 3.5 µm absorption) with MISE (Europa Clipper) and MAJIS (JUICE) at unprecedented spatial resolution.

How to cite: Raut, U., Protopapa, S., Mamo, B. D., Teolis, B. D., Villanueva, G., Cartwright, R., Nordheim, T., Retherford, K. D., Blaney, D. L., and Kammer, J.: Ocean-sourced CO2 inflates radiolytic H2O2 at Europa’s Chaos, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-269, https://doi.org/10.5194/epsc2024-269, 2024.

We present the first observations of Ganymede by the JWST (ERS #1373) (Bockelée-Morvan et al., 2024). NIRSpec IFU (2.9–5.3 μm) and MIRI MRS (4.9–28.5 μm) observations were performed on the Leading and Trailing hemispheres, with a spectral resolution of ∼ 2700 and a spatial sampling of 0.1 to 0.17". We analysed the spectral signatures and brightness temperatures, and their spatial distribution on the surface. Reflectance spectra show signatures of water ice, CO₂ and H₂O₂. An absorption feature at 5.9 μm, with a shoulder at 6.5 μm, is revealed, and tentatively assigned to sulfuric acid hydrates. The CO₂ 4.26-μm absorption band shows latitudinal and longitudinal variations in depth, shape and position over the two hemispheres, unveiling different CO₂ physical states. In the ice-rich polar regions, which are the most exposed to Jupiter's plasma irradiation, the CO₂ band is redshifted with respect to other terrains. In the boreal region of the leading hemisphere, the CO₂ band is dominated by a high wavelength component at ∼ 4.27 μm, consistent with CO₂ trapped in amorphous water ice. At equatorial latitudes (and especially on dark terrains) the observed band is broader and shifted towards the blue, suggesting CO2 adsorbed on non-icy materials, such as minerals or salts. Maps of the H₂O Fresnel peak area correlate with Bond albedo maps and follow the distribution of water ice inferred from H₂O absorption bands. Amorphous surficial ice is detected in the ice-rich polar regions, and is especially abundant on the northern polar cap of the leading hemisphere. Leading and trailing polar regions exhibit different H₂O, CO₂ and H₂O₂ spectral properties. On both hemispheres the north polar cap ice appears to be more processed than on the south polar cap. A longitudinal metamorphization of the ice at (sub)-micrometer-scale is observed on both hemispheres. Ice frost is tentatively observed on the morning limb of the trailing hemisphere, possibly formed during the night from the recondensation of water subliming from the warmer subsurface. Latitude and local time variations of the brightness temperatures indicate a rough surface and a low thermal inertia of 20–40 SI, consistent with a porous surface, with no obvious difference between the leading and trailing sides.

• Bockelée-Morvan et al., Composition and thermal properties of Ganymede’s surface from JWST/NIRSpec and MIRI observations. A&A 681, 27 (2024) https://doi.org/10.1051/0004-6361/202347326

How to cite: Poch, O., Bockelée-Morvan, D., Lellouch, E., Quirico, E., Cazaux, S., de Pater, I., and Fouchet, T. and the JWST ERS #1373 team: Composition and thermal properties of Ganymede's surface from JWST/NIRSpec and MIRI observations, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-70, https://doi.org/10.5194/epsc2024-70, 2024.

Jupiter's icy moon Ganymede has been seen to have a tenuous exosphere produced by sputtering and possibly sublimation of water ice. To date, only hydrogen and oxygen have been detected in this exosphere (Barth et al., 1997; Hall et al. 1998; Roth et al. 2021). Here, we report on the first detection of CO2 in Ganymede's exosphere through observations of the 4.3 microns band with the NIRSPec/IFU instrument of the James Webb Space Telescope. Maps of the CO2 gas distribution over the leading and trailing hemispheres reveal an unexpected patchy CO2  exosphere. CO2 gas is observed over different terrain types, principally those exposed to intense Jovian irradiation, but also over some bright or dark terrains. The highest CO2 column density is found over the north polar cap of the leading hemisphere, which also has unique surface ice properties. The distribution and production mechanisms of CO2 vapor will be discussed using simulations of Ganymede's exosphere (Leblanc et al. 2017) and through comparison with maps of surface ice properties obtained with JWST (Bockelée-Morvan et al. 2024).

References:

*Barth, C.A., Hord, C.W., Stewart, A.I.F., Pryor, W.R., Simmons, K.E., McClintock, W.E., Ajello, J.M., Naviaux, K.L., Aiello, J.J.: Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede. GRL 24(17), 2147–2150 (1997)
*Bockelée-Morvan, D., Lellouch, E., Poch, O., Quirico, E., Cazaux, S., de Pater, I., Fouchet, T., Fry, P.M., Rodriguez-Ovalle, P., Tosi, F., Wong, M.H., Boshuizen, I., de Kleer, K., Fletcher, L.N., Meunier, L., Mura, A., Roth, L., Saur, J., Schmitt, B., Trumbo, S.K., Brown, M.E., O’Donoghue, J., Orton, G.S., Showalter, M.R.: Composition and thermal properties of Ganymede’s surface from JWST/NIRSpec and MIRI observations. A\&A 681, 27 (2024)
*Hall, D.T., Feldman, P.D., McGrath, M.A., Strobel, D.F.: The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede. The Astrophysics Journal 499(1), 475–481 (1998) https://doi.org/10.1086/ 305604
*Leblanc, F., Oza, A.V., Leclercq, L., Schmidt, C., Cassidy, T., Modolo, R., Chaufray, J.Y., Johnson, R.E.: On the orbital variability of Ganymede’s atmosphere. Icarus 293, 185–198 (2017)
*Roth, L., Ivchenko, N., Gladstone, G.R., Saur, J., Grodent, D., Bonfond, B., Molyneux, P.M., Rether- ford, K.D.: A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations. Nature Astronomy 5, 1043–1051 (2021)

How to cite: Bockelee-Morvan, D., Poch, O., Leblanc, F., Zakharov, V., and Lellouch, E. and the ERS Ganymede team: CO2 in Ganymede’s exosphere revealed by JWST, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-141, https://doi.org/10.5194/epsc2024-141, 2024.

We analyze the emission of energetic neutral atom (ENA) flux from Callisto and Europa as a tool to probe moon-plasma interactions on a global scale. In situ ENA detectors sample a two-dimensional snapshot of the entire interaction region, as opposed to observations that provide magnetic field and plasma data only along one-dimensional trajectories. Charge exchange between energetic magnetospheric ions and cold atmospheric neutrals results in ENAs that propagate along rectilinear trajectories. Since the distribution of ENA flux is resultant from the interaction between the ambient plasma, the magnetospheric field configuration and the neutral gas distribution, ENA images can contextualize and quantitatively constrain these aspects of the moon-magnetosphere interaction on a local as well as a global scale. We combine the perturbed electromagnetic fields from a hybrid plasma model with a particle tracing tool to model ENA generation for the energetic ions interacting with Europa's and Callisto's neutral envelopes. By taking into account the point-like size (on scales of the plasma interaction) and limited field of view of a spacecraft detector, we apply our model to investigate which features of the emitted ENA flux will be observable by the JUICE spacecraft during its close flybys of both moons.

How to cite: Haynes, C. M., Tippens, T., Simon, S., and Liuzzo, L.: Observability of ENA emissions at Europa and Callisto: predictions for the JUICE mission, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-16, https://doi.org/10.5194/epsc2024-16, 2024.

How to cite: Beth, A., Galand, M., Modolo, R., Leblanc, F., and Jia, X.: Impact of ion-neutral chemistry on Ganymede's ionosphere, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-447, https://doi.org/10.5194/epsc2024-447, 2024.

Jupiter’s icy Galilean satellites Europa, Ganymede, and Callisto possess tenuous atmospheres that are mainly sourced from the surface. However, the extent of the collisional atmosphere of the icy moons is still subject to ongoing debate. In this study, we model the atmospheres of the three icy satellites using the Direct Simulation Monte Carlo (DSMC) method [1] to locate the exobases at the icy moon atmospheres and to investigate the influences of collisions among the different atmospheric species.

Our model [2, 3] includes the main physical and chemical processes that create the atmospheres of the icy moons, such as the sublimation of surface ice (H₂O) and the radiolytic production and sputtering of molecular oxygen (O₂) and hydrogen (H₂). Furthermore, we include photochemical reactions and electron impacts, that can ionise and dissociate species in the atmosphere. To determine the location of the exobase, we use the Knudsen number (Kn), defined as the ratio of the mean free path (λ) to the scale height (H). The exobase, marking the boundary between collision-dominated and collision-free atmospheric regions, is typically considered to be at Kn = 1. In addition, we compare the results of the collisional DSMC model with a ballistic model, where the particles do not interact with each other, to investigate the influences of collisions on the abundances and escape rates of the included species. 

In this effort of comparative selenology [4], we find that the exobases of all three icy moon atmospheres are located above the surface (see Figure 1). For Ganymede and Callisto, our model shows that the collisionality is significantly affected by the abundance of sublimated H₂O near the subsolar point. In contrast, for Europa, the abundance of recycled O₂ results in the exobase being located above the surface, which differs from the assumptions made in previous exospheric models. Furthermore, we find that a collisional atmosphere leads to reduced escape rates for most species, as particles that would have escaped on ballistic trajectories lose their energies via collisions. However, collisions with dissociation products can also significantly increase the escape of heavy species, such as sublimated H₂O and recycled O₂, that would not be expected to escape in a non-collisional atmosphere, as their thermal velocities are significantly smaller than the escape velocities of the icy moons.

Figure 1: Knudsen number as a function of the solar zenith angle and altitude (in km and satellite radii) for Europa (left), Ganymede (middle), and Callisto (right). A solar zenith angle of 0° corresponds to the subsolar point. The exobase (Kn = 1, solid line), and the boundary between the quasi- and fully collisional regime (Kn=0.1, dashed line) are also shown.

In the 2030s, both ESA's Jupiter Icy Moons Explorer (JUICE) and NASA's Europa Clipper mission are set to conduct close-up explorations of the three satellites. Equipped with high-resolution mass spectrometers, more specifically the Neutral gas and Ion Mass spectrometer (NIM) aboard JUICE [2] and the MAss Spectrometer for Planetary EXploration (MASPEX) on Europa Clipper [3], they will measure the atmospheric composition. In this study, we show that the collisionality of the icy Galilean moon atmospheres affects the abundances and escape rates of the different species that will be measured, consequently impacting the determination of the moons' underlying surface composition.

Acknowledgements:
This work has been carried out within the framework of the National Centre of Competence in Research PlanetS supported by the Swiss National Science Foundation under grant 51NF40_205606. The authors acknowledge the financial support of the SNSF. Calculations were performed on UBELIX (http://www.id.unibe.ch/hpc), the HPC cluster at the University of Bern.

References:
[1] Bird, G. A. (1994). Molecular gas dynamics and the direct simulation of gas flows.
[2] Carberry Mogan, S. R., et al. (2021). Icarus, 368, 114597.
[3] Carberry Mogan, S. R., et al. (2022). Journal of Geophysical Research: Planets, 127(11).
[4] Schlarmann, L., et al. (2024), in preparation.
[5] Föhn, M., et al. (2021), IEEE Aerospace Conference (50100). IEEE, 1-14.
[6] Waite Jr, J. H., et al. (2024). Space Science Reviews, 220.3, 30.

How to cite: Schlarmann, L., Vorburger, A., Carberry Mogan, S. R., and Wurz, P.: Investigating influences of collisions on the icy Galilean moon atmospheres, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-509, https://doi.org/10.5194/epsc2024-509, 2024.

The neutral particles that escape the surface and exosphere of Europa populate a neutral gas torus around Jupiter. The detection and characterization of this neutral structure can not only shed light on the escape processes at work at Europa, but can also help to determine if Europa has a global influence on the Jovian plasma and magnetosphere.

We will present a new Monte Carlo model of the Europa neutral gas torus developed in the frame of the FACOM project funded by ANR (French national agency for research). Neutral ionization rates in the Jovian magnetosphere are computed based on Voyager plasma densities and temperatures, including suprathermal electrons. The source rate of neutrals escaping Europa’s gravity is constrained with the three-dimensional Exosphere Global Model EGM [Oza, Leblanc, et al., 2019]. In particular, we will present the first simulations of the H2O water torus of Europa, with a source rate of H2O at Europa constrained with EGM.

Finally, we will use our new model to reconsider the observability of the neutral torus in UV observations [Roth et al., 2023], energetic ion pitch angle distributions [Sarkango et al., 2023], ENA imaging [Smith et al., 2019], and pick-up ion observations [e.g. Szalay et al., 2022]. The impact of our results on future JUICE and Clipper observations of the Europa neutral gas torus will be presented.

This research holds as part of the project FACOM (ANR-22-CE49-0005-01_ACT) and has benefited from a funding provided by l’Agence Nationale de la Recherche (ANR) under the Generic Call for Proposals 2022. 

References:

Oza et al. 2019: 10.1016/j.pss.2019.01.006

Roth et al. 2023: 10.3847/PSJ/accddd

Sarkango et al. 2023: 10.1029/2023GL104374

Smith et al. 2019: 10.3847/1538-4357/aaed38

Szalay et al. 2022: 10.1029/2022GL098111

Smith, H. T., Mitchell, D. G., Johnson, R. E., Mauk, B. H., & Smith, J. E. (2019). Europa neutral torus confirmation and characterization based on observations and modeling. The Astrophysical Journal, 871(1), 69.

How to cite: Nenon, Q. and Leblanc, F.: A new model of the neutral gas torus of Europa, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-461, https://doi.org/10.5194/epsc2024-461, 2024.

The term “microsignatures” refers to localized decreases in trapped particle flux caused by particle absorption by moons or rings that orbit within a planetary magnetosphere. Microsignatures can survive for hours and thus propagate far from the parent moon and its magnetospheric interaction region. They thus evolve under the influence of the background magnetospheric environment, meaning that their profile and location can trace processes in the magnetosphere itself, such as particle transport. Microsignatures are most common in electrons from few keV and up to the ultra-relativistic range (>10 MeV). A large number of microsignature detections at Saturn has revolutionized our understanding of energetic particle transport processes in the planet’s radiation belts. However, to this date, we lack a comprehensive survey of satellite microsignatures at Jupiter, even though few single case studies demonstrate their detectability in both the Galileo and Juno datasets. To proceed with their detailed study at Jupiter, it is necessary to first develop a framework that allows us to predict their timing and verify under which conditions, geometries and energies such predictions are most successful. The complex magnetospheric configuration at Jupiter (compared to the spin-aligned magnetic field of Saturn) introduces several challenges. In this work, we present the first steps towards developing this framework and contrast our initial predictions against energetic particle observations by Juno. In particular, we identify over thirty microsignatures from Jupiter’s innermost moons (Metis, Adrastea, Amalthea and Thebe), which orbit in a region where the effects of magnetic field asymmetries are greatly enhanced. By testing the predicted against the observed timing of those events, we optimize our model and our predictions for the JUICE and Europa Clipper missions at the Galilean moons. The long and continuous residence of both missions in the inner magnetosphere, particularly close to the orbits of Europa, Ganymede and Callisto, would result in unique geometries and opportunities to probe Jupiter’s inner and middle magnetosphere with numerous microsignature observations.

How to cite: Roussos, E., Krupp, N., Hao, Y., Fraenz, M., Paranicas, C., Kollmann, P., Clark, G., Betty Pei-Chun Tsai, B. P.-C., and Long, M.: A model for energetic electron microsignatures by Jupiter's moons and application for JUICE and Europa Clipper missions, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1079, https://doi.org/10.5194/epsc2024-1079, 2024.

Ocean worlds seem to offer a very suitable environment for life appearance and sustainability and studying their internal dynamic is a key topic for exobiology. However, the ocean of large icy moons, such as Ganymede, has long been rejected as a potential habitat for life to develop. Due to pressure, a high-pressure (HP) ice layer should exist between the core and the ocean, preventing direct interactions and mass exchanges between both. Therefore, the HP ice layer dynamics has long been a relatively unexplored topic. However, several studies have been carried out in recent years ([1],[2],[3],[4],[5],[6]), each of them exploring one or several aspects of the problem, to characterize the possibility and efficiency of mass transfer between the core and the ocean. They all suggest that, under certain conditions, exchanges could happen by solid and/or liquid convection through this layer. In addition to helping us understand better the internal dynamics of such planetary bodies, these studies are of the utmost importance, as they concern bodies that could join the list of potentially habitable planets, but also because this layer of ice is likely to exist on large ocean exoplanets, which are highly interesting bodies for exobiology.

Previous studies ([1],[2],[3],[4],[5]) only considered pure water ice for the HP ice layer. However, the ocean of these moons is composed of salty water, which could originate from various processes, including core/ocean interactions. Therefore, in our last paper [6], we introduced salts to explore their effects on the overall dynamics. This work aims to characterise better the effectiveness of salt transfers between the core and the ocean and study ocean composition's evolution over time. Previous studies concluded that, in certain conditions, the heat flux from the core is sufficiently high for the temperature to reach the melting temperature at the HP ice/core boundary. As a result, pockets of melt could appear at the bottom of the HP ice layer, and water could seep into the core through fractures, enriching it with salts. (Fig1.a). If the salty water is less dense than pure ice, it should rise through the ice and could refreeze on the way, enriching the ice with salts. We considered a 2D HP ice layer with a flux of salts coming from the core (Fig1.b) and studied the exchanges of salts between the core and the ocean by solid convection. In this study, the salts are represented by Lagrangian tracers. This work gives a large panel of results, such as the influence of salts on the HP ice dynamics and their effect over time, as well as the evolution of the composition of salts in the ocean. One important result is that two distinct dynamic regimes can be observed depending on the buoyancy number. For what can be considered as low values of the buoyancy number (Fig2.a) the salts have nearly no effect on the ice density and they are passively transported to the ocean through upwelling hot plumes. Conversely, for high values of the buoyancy number (Fig2.b), salty ice is much denser than pure ice and accumulates at the bottom of the ice shell, and only a small fraction of salts reach the ocean. The switch between both regimes mainly depends on the Rayleigh number. For Ganymede, we obtained a radial velocity of the order of a meter per year at the HP ice/ocean interface, which is rather efficient and means that salts reaching the boundary should easily be dissolved in the ocean.

One of the dynamic outputs of study [6] is the possible existence of a thin and highly salted ocean between the HP ice layer and the core. This ocean could exist under specific conditions, in particular a sufficient amount of salts entering the HP ice layer from the core for the salty ice to accumulate at the bottom of the shell and melt it completely. We studied this particular case (Fig3) to better understand its implications on the HP ice layer dynamic and the efficiency of mass transfer between the core and the ocean. However, validating the feasibility of this hypothesis would require more information about the core composition in terms of chemical elements capable of forming salts. The results of this study (Fig4) show a very different flow pattern from the one we obtain for a model without a basal ocean (Fig2). It is dominated by a low-degree mode (fluctuating between one or two) with large cold downwelling and hot upwelling currents (Fig4). This could imply an important heat transfer through the layer and, if this basal ocean has existed at least punctually on such a moon as Ganymede, it could have significant implications for the ocean composition and tectonic evolution of the outer icy layer.

Fig1: a) Illustration of the enrichment of HP ice in salts thanks to interactions with the core. b) Model illustration.

Fig2 : Snapshots of 1/12 of the HP ice layer for a buoyancy number of a) 0 and b) 10.

Fig3 : Model illustration with a basal ocean.

Fig4 : Snapshots of the evolution of the HP ice layer in time.

[1]: Choblet, G et al., 2017, Icarus. http://dx.doi.org/10.1016/j.icarus.2016.12.002.

[2]: Kalousová, K., Sotin, C., 2018, Geophys. Res. Lett. http://dx.doi.org/10.1029/2018GL078889.

[3]: Kalousová, K., et al., 2018, Icarus. http://dx.doi.org/10.1016/j.icarus.2017.07.018.

[4]: Kalousová, K., Sotin, C., 2020, Earth Planet. Sci. Lett. http://dx.doi.org/10.1016/j.epsl.2020.116416.

[5]: Lebec, L. et al., 2023, Icarus. http://dx.doi.org/10.1016/j.icarus.2023.115494.

[6]: Lebec, L. et al., 2024, Icarus. https://doi.org/10.1016/j.icarus.2024.115966.

Acknowledgments: We would also like to thank Adrien Morison, Daniela P. Bolrão and Paul J. Tackley for their contribution to this work.

How to cite: Lebec, L. and Labrosse, S.: Dynamics of high-pressure ice layers in large ocean worlds, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-30, https://doi.org/10.5194/epsc2024-30, 2024.

A better understanding of the interior structure of icy moons in our Solar System is necessary to answer fundamental questions about their formation, evolution, and habitability. Until now, insights on the interior layering, core structure, and density distribution have been solely derived from global-scale gravity measurements, such as the mass, the moment of inertia (from the degree-2 static gravity field and/or obliquity), and the tidal gravity response. With the upcoming NASA Europa Clipper and ESA JUICE missions, we will obtain reliable estimates of the gravity field of Jupiter’s icy moons to higher degrees (shorter wavelengths).

In this work, we present a new methodology and investigate the efficacy of utilizing the measured gravity field amplitude spectrum as an additional constraint in the inversion of the interior structure of differentiated icy bodies. After introducing and discussing the general methodology, we focus on Europa by considering the anticipated measurement accuracy of the Europa Clipper gravity and radio science investigation. We show that a Bayesian inversion of Europa’s interior that incorporates the measured gravity field spectrum offers much stronger determination of key geophysical parameters related to the interior structure of the body. In particular, it allows reliable constraints on the hydrosphere (ice shell and ocean) thickness, to within 10-20 km uncertainty, while at the same time reliably estimating key characteristics of the core, the silicate mantle, and the ocean floor, which is not possible with the traditional approach. The approach we present offers new sensitivity to the seafloor topography and elastic thickness, and it gives a way of probing the heat flow from the silicate interior. Ultimately, these fundamental constraints will help evaluate the potential for habitability of Europa and its subsurface ocean.

Updating our previous work (Cascioli et al., 2024), we augment the inverse problem with the expected tidal constraints that Europa Clipper will derive, and we assess their beneficial role on the interior structure determination.

Cascioli, G., et al. "Leveraging the Gravity Field Spectrum for Icy Satellite Interior Structure Determination: The Case of Europa with the Europa Clipper Mission." The Planetary Science Journal 5.2 (2024): 45.

How to cite: Cascioli, G., Mazarico, E., Nimmo, F., and Dombard, A.: The importance of gravity field spectrum for icy satellite interior structure determination: the case of Europa with the Europa Clipper mission, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-583, https://doi.org/10.5194/epsc2024-583, 2024.

Introduction: The fundamental thermodynamic concept of the eutectic (from Greek εὐ-τῆξῐς, “easy-melt”), defines an invariant point in temperature-concentration (T-X) space at which the liquid will transition entirely into a mixture of solid phases. In planetary science eutectics play a crucial role in defining minimum temperature of stability of liquids (i.e. water, silicate melt). Characterization of eutectics are central in understanding the behavior of aqueous systems in various icy ocean worlds processes such as cryo-volcanism, crustal melt transport, or long term oceanic and high-pressure ice mantle evolution [1].

The canonical definition of the eutectic is often considered at a fixed pressure (typically 1 atmosphere for aqueous systems); however, increased states of compression substantially affect the melting curves of solids, resulting in a change of eutectic T-X coordinates. When pressure is varied, the eutectic becomes a univariant line in the P-T-X space, the trajectory of which depends upon the geometry of the liquidus curves of the contributing solid phases.

In this work we focus on the effect of pressure on eutectics coordinates using a novel isochoric (i.e. constant volume) experimental technique, and determine the absolute limit of liquid stability of several aqueous systems: Na₂CO₃, KCl, MgSO₄, Na₂SO₄, Urea, NaCl, MgCl₂, and NaHCO₃

Methods: We use isochoric freezing chambers made of Al7075 with an internal volume of approximately 5.5 mL, designed to withstand up to 300 MPa [2]. The loaded solutions are cooled slowly (1-0.5K/hour) as low as 180K. The growth of ice Ih of greater volume than the aqueous fluid within the isochoric chamber results in a significant pressure increase, up to 260 MPa. Once the sample fully frozen, the temperature is then slowly increased up to full melting and the resulting pressure change is monitored. At constant volume and with variable temperature the Gibbs’ Phase rule [3] implies that the state of system will naturally follow the triple equilibrium univariant line (the eutectic). Using this principle we are able to determine with unmatched accuracy and high throughput the eutectic lines pressures and temperatures coordinates of many crucial aqueous systems for icy worlds.

Results and Conclusions: Our experimental result show that most systems eutectics follow a similar behavior driven by the negative Clapeyron slope of ice Ih melting surface. We also report that all systems reach a minimum liquid stability temperature around 213 MPa, roughly 22K below the known 1 bar eutectic. This pressure corresponds to the quadruple point with ice III or ice II (and the aqueous brine, ice Ih and the salt containing phase). This quadruple point in a binary system represents the lowest possible temperature of stability of a liquid in the system and is located at the end of the eutectic line. Surprisingly this invariant thermodynamic point of crucial importance has not yet been described or defined in the literature. We propose a definition and a name for this new fundamental concept: the cenotectic, from Greek “κοινός-τῆξῐς”, “universal-melt”). We also explore in this work the implication of the cenotectic on the long term evolution of large icy moons and exoplanets [4].

.

References:

[1] Journaux, B., et al., (2020). Space Science Reviews 216:7.

[2] Chang, B., et al. (2022) Cell Reports Physical Science 3, 100856

[3] Gibbs, J. W., (1874) On the equilibrium of heterogeneous substances, New Haven: Published by the Academy.

[4] Zarriz, A., et al. (2024), arXiv:2403.01028

How to cite: Journaux, B., Powell-Palm, M., and Zarriz, A.: Eutectics under pressures: definition of a universal limit of liquid stability, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-724, https://doi.org/10.5194/epsc2024-724, 2024.

The question of the habitable environments within the Galilean moons is raised by the presence of hydrospheres in those worlds. The study of the current volatiles inventory on those moons is indicative of their formation processes and how they could affect this inventory. However, for the ability to disentangle between the possible scenarios, it is necessary to examine the post-accretion processes that could impact the volatile inventory of the hydrospheres. Especially, an “open-ocean” phase that occurred shortly after accretion, before the icy crust formation, must be considered, in view of its influence on the volatile inventory. More specifically, the abundance of ammonia in Europa’s building blocks is a key constrain, both on the habitability conditions of the ocean and the composition of the primordial thick atmosphere of the moon.

Our work focuses on modelling the ocean-atmosphere equilibrium which took place over this period, based on the type of material accreted by Europa at its formation. To do so, we compute the vapour-liquid equilibrium between water and volatiles, as well as the chemical equilibria happening within the ocean to model the primitive hydrosphere of Europa. Our model allows for an assessment of the impact of the initial distribution of volatiles resulting from the thermodynamic equilibrium between Europa’s primordial atmosphere and ocean. In particular, we show the correlation between the ratio of dissolved CO₂ and NH₃ and the distribution of partial pressures in the primordial atmosphere of Europa.

Fig1: Evolution at T = 300 K of the distribution of species in both the atmosphere and the ocean as a function of the mole fraction of initially NH₃ incorporated into the atmosphere. All the other species’ initial mole fractions remain constant.

Navigating between different compositions of accreted material and varying the proportion of ammonia incorporated into the ocean after accretion, we obtain a range of primordial volatile distributions in the hydrosphere, to be linked to nowadays inventory. We also find ammonia abundance thresholds above which CO₂ content is significantly depleted by NH₂COO^-  formation (fig 1).

How to cite: Amsler Moulanier, A., Mousis, O., and Bouquet, A.: The influence of ammonia on the primordial distribution of volatiles in the hydrosphere of Europa, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-221, https://doi.org/10.5194/epsc2024-221, 2024.

The extent of differentiation within the interiors of the Galilean moons remains inadequately determined. Analysis of Callisto's moments of inertia, derived from Galileo's gravity data, suggests its structure lacks full differentiation. Furthermore, a recent reevaluation of the Galileo data casts doubt on the long-standing assumption of Europa possessing a metallic core. Our objective is to elucidate the accretion conditions necessary for the Galilean moons to develop  in a manner consistent with their current state of differentiation. To achieve this, we employ a numerical model simulating the thermal evolution of icy moon interiors throughout their accretion and post-accretion phases. Each moon's embryo undergoes various heating mechanisms, including tidal heating, radiogenic heating, accretional heating from multiple impacts, and heat from the surrounding circumplanetary disk during growth. The magnitude of each heating process is contingent upon the presumed formation trajectory of each moon within the Jovian circumplanetary disk. Consequently, we investigate the accretion scenarios that explain the presence of a partially differentiated moon similar to Callisto and a fully differentiated moon akin to Ganymede. Additionally, we examine the conditions conducive to iron melting and the formation of a metallic core within Europa's context.

Figure 1- Temperature distribution inside Callisto, at the end of the accretion. t_start is the time when accretion begins relative to the formation of CAI’s. To avoid melting during accretion, the temperature profile (in black) must remain below the pressure-dependant melting curve for water ice (in red) with 5% ammoniac. For Callisto to accrete partially differentiated, the fraction of fusion φ_fus in radius must not exceed the 15-20% threshold.

How to cite: Bennacer, Y., Mousis, O., Monnereau, M., and Hue, V.: Probing the Formation Conditions of the Galilean Moons Through Their Differentiation States, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-905, https://doi.org/10.5194/epsc2024-905, 2024.

Introduction
Icy moon Europa possesses one of the youngest surfaces in the Solar System. Overall smooth, yet rich in unique tectonic features, it records mostly extensional processes. Nonetheless, reconstructions of tectonic motions at Europa’s surface identified ca 20,000 km² that have disappeared over the visible tectonic history (1, 2). As an analogy to Earth, subduction has been proposed as a process accommodating the convergent tectonic motions (1). Later models, however, demonstrated that the limited density contrast in the ice shell prevents spontaneous sinking, implying that subduction has to be driven by external forces (3, 4). Here we model i) lateral compression of an ice shell, investigating the transport of near-surface ice and ii) stresses in a spherical ice shell thinning due to enhanced tidal heat production as a possible mechanism for triggering global compression. Finally, we evaluate the amount of material that could be recycled during such an event.

Method

In order to model tectonic deformation of ice, we developed a 2D Cartesian model with visco-elasto-plastic rheology, capturing both ductile flow and brittle failure of the material (5, 6). Fixed temperature is prescribed on the surface and ice-water interface (90 and 270 K, respectively), while the vertical boundaries are adiabatic. The top and bottom boundaries can evolve freely, however, we assume that the topography at the ice-ocean interface is immediately erased by melting and freezing (7). Tectonic deformation is triggered by prescribing convergent velocity 10 mm/yr on the vertical boundaries. To evaluate the efficiency of subduction, we monitor the position of the near-surface ice, taken as the topmost 500 m of the shell at the beginning of the simulation. For evaluation of global stresses, we modified the approach of ref. (8) by accounting for tidal and radiogenic heating from Europa’s silicate mantle (9). Starting from a 40-km-thick shell, we impose eccentricity variations of various magnitudes and evaluate the radial profile of lateral compressional stress.

Results

First, we performed series of simulations, varying the ice shell thickness D in the range 5 - 30 km. Figure 1a shows the evolution of near-surface ice layer position. Within the simulation, the near-surface ice reaches the subsurface ocean only for thin shells (D = 5 and 10 km, see Fig. 1b). In these cases, only one pair of faults develops, focusing the tectonic deformation into a single narrow site. Downward transport then proceeds with the velocity equal to the compression velocity (compare the slope of the corresponding solid and dashed lines). However, for thicker shells, a cluster of several pairs of faults emerges, yielding wider convergence sites, resulting in slower downward transport of near- surface ice. Moreover, the shell fractures at multiple locations, further decreasing the efficiency of the transport (see Fig. 1c).

Figure 1. Compression of ice shell after 2 Myr. a) Solid lines show evolution of relative delivery depth of near-surface ice (orange layer in panels b) and c)). Dashed lines represent sinking with the same speed as the compression, color numbers denote final depth of the near-surface ice. b) Thickness 5 and 10 km (delivery to the subsurface ocean) c) Thickness 15 – 30 km (no delivery).

Figure 2 shows evolution of a conductive ice shell in reaction to eccentricity variations. When the eccentricity starts to increase from its current value e = 0.0094, the ice shell immediately thins and accumulates compressional stress. Figure 2a shows that already at four times higher eccentricity, the ice shell melts down to 10 km. Figure 2b then shows the stress profile for such a case. The maximum of compressional stress is reached in the top rigid part of the shell at the moment of its minimum thickness. As the eccentricity starts to decrease, compressional stress gradually transforms into extensional. Difference in the ice shell radii shown in Fig. 2b yields surface area contraction of 90,000 km², which, based on the surface radiolytic production estimated by ref. (10), might deliver up to 2.6 × 10⁹ mol O₂ yr ⁻¹ into the subsurface ocean.

Figure 2. Ice shell’s response to eccentricity variations. a) Eccentricity variations (dashed line, left axis) and resulting shell thickness variations (solid line, right axis). b) Radial profile of compressive lateral stress for maximum eccentricity of e_max = 0.04.

Conclusions
Lateral compression of an ice shell can result in subduction-like process if the ice shell is ≲ 10 km thin, delivering the near-surface ice into the subsurface ocean. For thicker shells, the downward transport proceeds more slowly. Moreover, we show that variations in orbital eccentricity provide a way to simultaneously reduce the ice shell thickness and evoke global compressional stress, conditions plausible for the subduction-like process. Carrying radiolytic oxidants during the period of strong hydrothermal activity, subduction-like process might facilitate ignition of complex chemical reaction in the subsurface ocean.

Acknowledgments
This research is supported by Agence Nationale de la Recherche ANR-2020-CE49-0010, Czech Science Foundation project 22-20388S, Charles University project SVV 260709 and Charles University project GA UK 26624.

References
1. Kattenhorn and Prockter (2014), Nat. Geosci. 7, 762–767.
2. Collins et al. (2022), J. Geophys. Res. Planets 127, e2022JE007492
3. Johnson et al. (2017), J. Geophys. Res. Planets 122, 2765–2778.
4. Howell and Pappalardo (2019), Icarus 322, 69–79.
5. Goldsby and Kohlstedt (2001), J. Geophys. Res.: Sol. Ea. 106, 11017–11030.
6. Buck et al. (1998), Faulting and Magmatism at Mid-Ocean Ridges, AGU, pp. 305–323.
7. Kvorka and Čadek (2024), Icarus 412, 115985.
8. Rudolph et al. (2022), Geophys. Res. Lett. 49, e2021GL094421.
9. Běhounková et al. (2021), Geophys. Res. Lett. 48, e2020GL090077.
10. Szalay et al. (2024), Nat. Astron.

How to cite: Kihoulou, M., Choblet, G., Tobie, G., Kalousová, K., and Čadek, O.: Eccentricity variations trigger “subduction” in Europa’s ice shell, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-311, https://doi.org/10.5194/epsc2024-311, 2024.

Tidal dissipation is fundamental to provides the energy that sustain natural satellites’ internal oceans, as identified in Europa and Enceladus, or Io’s volcanic activity. A proper modeling of the dissipative phenomena is crucial to correctly infer internal structure properties and composition.

Tidal dissipation is responsible for orbital expansion of satellites. From the orbital expansion it is possible to estimate the amount of dissipation in the planet or the satellite, exploiting astrometric and radiometric observations.

The two most used tidal models of satellites parametrize the tidal dissipation using a time lag (e.g. Mignard, 1980) or a phase lag (e.g. Eanes et al., 1983), respectively. Applying the models to ephemerides propagation, we found that they produce slightly different orbital effects for the satellite, with a difference of 3% in  semimajor axis and 15% in eccentricity. In this work we provide a possible strategy to reconciliate the two models.

Moreover, regarding synchronous satellites tides, classical theoretical formulas from Yoder et al. (1981), Segatz et al. (1988), which assume that the prime meridian of the moon points to the empty focus of its orbit, predict an orbital energy dissipation which is equivalent to the following evolution of semimajor axis and eccentricity of the moon’s orbit:

where Eq. (2) is found from conservation of the angular momentum.

However, computing the secular orbital evolutions using Gauss planetary equations and considering the same rotational and tidal models, we found a difference in the derivative of the semi-major axis, which is directly related to the orbital energy, by almost a factor 3. This may bias the estimation of dissipation parameters from the orbital evolution by the same factor. In addition, during the orbital evolution the angular momentum is not conserved. In this work we analyze these inconsistencies between the energetic approach and the direct propagation of the dynamical equations from tidal potential. We argue that the difference may originate from the effect of the tidal dissipation on the moons rotational state, not considered in the classical approach were the prime meridian is imposed to point the empty focus.  Finally, we identify possible strategies to reconcile the propagation with theoretical predictions, ensuring the conservation of angular momentum.

This may have important consequences in the estimation of the satellite ephemerides and tidal dissipation of the Jupiter system with the upcoming JUICE and Europa Clipper missions, whose unprecedented accuracy will require high-accuracy models of the dynamics inside the system.

How to cite: Magnanini, A. and Zannoni, M.: Consistent satellites tidal and rotational models for ephemerides estimation, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1109, https://doi.org/10.5194/epsc2024-1109, 2024.