Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021
Icy worlds: Past and future explorations


Icy worlds: Past and future explorations
Co-organized by MITM
Convener: Gabriel Tobie | Co-conveners: Carly Howett, Alice Lucchetti, Frank Postberg, Federico Tosi
Tue, 21 Sep, 15:10–17:00 (CEST)

Session assets

Discussion on Slack

Oral and Poster presentations and abstracts

Chairpersons: Alice Lucchetti, Gabriel Tobie, Federico Tosi
Luis Gomez Casajus, Marco Zannoni, Paolo Tortora, Ryan Park, Dustin Buccino, Marzia Parisi, Steven Levin, and Scott Bolton


The Juno Extended Mission (EM) presents the first opportunity to acquire gravity measurements of the Galilean satellites after the Galileo mission, ended almost 20 years ago. On June 7th, 2021, Juno will flyby Ganymede with a closest approach altitude of ~1050km and a relative velocity of ~18.5 km/sec. This will be the first time that a probe acquires gravity measurements of Ganymede since December 28th, 2000, when Galileo performed G29, its last flyby of this Galilean moon.

In total, the Galileo probe performed 6 flybys of Ganymede, among which only 4 had coherent two-way S-band Doppler tracking during the closest approach, and could be used to estimate the gravity field of Ganymede. The very first analysis of these data, used only two flybys, G1 and G2, and estimated J2 and C22 applying the hydrostatic equilibrium constraint (J2/C22 = 10/3) (Anderson et al., 1996). The analysis concluded that Ganymede’s internal structure is likely formed by a metallic core surrounded by a silicate mantle enclosed by an ice shell. A subsequent gravity field analysis was unable to fit all tracking data (G1, G2, G7 and G29) without including a high degree gravity field nor obtain a physical interpretation of the results (Schubert et al., 2004). These problems could be solved only by adding mass anomalies to the dynamical model of Ganymede’s interior (Anderson et al., 2004; Palguta et al., 2006).

This new encounter, (G34), during Juno’s EM, offers the possibility of improving the knowledge on the gravity field of Ganymede. The gravity field of a body can be estimated through the reconstruction of the probe’s trajectory during a close encounter, exploiting the dynamical Doppler shift of a highly stable microwave carrier, induced by the relative motion between the DSN stations on the Earth and the probe. During G34, through which Juno will also perform a radio occultation experiment, the spacecraft will use simultaneously the X/X and X/Ka radio links. An accurate range-rate noise budget led us to the conclusion that we can expect an accuracy of 0.025 mm/s, at 60 s integration time, compatible to the accuracy acquired during PJ13 and PJ27, where the same radio link configuration was adopted. By comparison, the Galileo range-rate data had an accuracy of 0.34 mm/s, because of the use of S-band link and the onboard Low Gain Antenna. Moreover, this link configuration allows to use the multi-frequency link calibration technique to remove the downlink plasma contribution, preventing biases from local dispersive noises as the possible Ganymede’s ionosphere or the Io plasma torus.

Nevertheless, our results from a covariance analysis indicate that, by itself, G34 cannot provide an improvement to the current knowledge on Ganymede’s interior structure, being only able to constraint the hydrostatic ratio J2/C22 to the ~45%. This is mainly due to the flyby characteristics, and in particular the high relative velocity. However, a joint analysis with the coherent S-band radio tracking data of the Galileo spacecraft (Figure 1), acquired more than 2 decades ago, represents an opportunity to shed some light on the gravity field of this Galilean satellite.

Preliminary results from numerical simulations, performed using JPL’s orbit determination program, MONTE (Evans et al., 2018), using a setup similar to the one used for Jupiter’s gravity determination (Durante et al., 2020), indicates that the hydrostatic ratio J2/C22 could be constrained to within ~10% (1-sigma).

Figure 1: Juno and Galileo ground-tracks over Ganymede during the different encounters. The ticks are separated by 60 s.

This work will present an updated gravity field of Ganymede, showing the possible implications in terms of interior modelling. This will be the outcome of a joint analysis of all the available real data acquired during G34 and the Galileo flybys, applying modern orbit determination techniques used in the past in the Cassini gravity analyses (Durante et al., 2019, Zannoni et al., 2020). The real data used in this analysis will be the last gravity measurements of Ganymede acquirable until future flybys, of Juice and Europa Clipper missions, in the next decade.


LGC, MZ and PT are grateful to the Italian Space Agency (ASI) for financial support through Agreement No. 2017-40-H.0, and its extension 2017-40-H.1-2020, for ESA’s BepiColombo and NASA’s Juno radio science experiments. The work of RP, DB, MP, and SL was carried out at the Jet Propulsion Lab, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Government sponsorship acknowledged.


  • Anderson, J. D., Lau, E. L., Sjogren, W. L., Schubert, G., & Moore, W. B. (1996). Gravitational constraints on the internal structure of Ganymede. Nature, 384(6609), 541-543.
  • Anderson, J. D., Schubert, G., Jacobson, R. A., Lau, E. L., Moore, W. B., & Palguta, J. L. (2004). Discovery of mass anomalies on Ganymede. Science, 305(5686), 989-991.
  • Durante, D., Hemingway, D. J., Racioppa, P., Iess, L., & Stevenson, D. J. (2019). Titan's gravity field and interior structure after Cassini. Icarus, 326, 123-132.
  • Durante, D., Parisi, M., Serra, D., Zannoni, M., Notaro, V., Racioppa, P., ... & Bolton, S. J. (2020). Jupiter's gravity field halfway through the Juno mission. Geophysical Research Letters, 47(4), e2019GL086572.
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  • Schubert, G., Anderson, J. D., Spohn, T., & McKinnon, W. B. (2004). Interior composition, structure and dynamics of the Galilean satellites. Jupiter: The planet, satellites and magnetosphere, 1, 281-306.
  • Zannoni, M., Hemingway, D., Casajus, L. G., & Tortora, P. (2020). The gravity field and interior structure of Dione. Icarus, 345, 113713.

How to cite: Gomez Casajus, L., Zannoni, M., Tortora, P., Park, R., Buccino, D., Parisi, M., Levin, S., and Bolton, S.: The gravity field of Ganymede after the Juno Extended Mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-537,, 2021.

Gianluca Chiarolanza, Giuseppe Mitri, and Monica Pondrelli

Introduction: Chaotic terrains on the icy moon Europa are among the youngest surface features within the satellite’s visible geological history [1]. These regions appear as highly disrupted surfaces formed by irregular groups of isolated plates surrounded by a lumpy matrix material [2]. They are often covered in a reddish-brown material that is interpreted to consist of hydrated sulfates [3] or sulfuric acid hydrate [4]. Current models of chaos formation include a melt-through of the surface provoked by an internal heat source (i.e. a hydrothermal plume) [5], or the mobilization of brines trapped near the surface, in response to partial melting of the outer shell induced by icy diapirs rising through the crust [6]. We investigated two chaotic terrains named Thrace Macula and Thera Macula, both located on the southern hemisphere of Europa. Among the hypotheses proposed to explain their origin are the upwarping of the surface followed by an extrusion of low-viscosity liquids [7, 8], the collapse of large-scale domes [9], or interactions between the ice and shallow subsurface water lenses [10]. Here we provide a geomorphological and stratigraphic analysis of Thrace and Thera Macula, derived from an extensive geological mapping based on the highest-resolution images of the area acquired throughout the Galileo space mission. We also provide preliminary results of a topographic analysis performed on photoclinometric DEMs obtained using Ames Stereo Pipeline [11, 12].


Figure 1. Geological Map of Thrace (right) and Thera Macula (left).


Results: The mapped area includes plains dominated by ridge complexes, bands, linear features (double ridges, troughs), craters (for less than 1%), and chaotic terrains (Fig. 1). Local displacements of band margins and double ridges suggest the occurrence of crustal movements along tectonic faults.

Thera Macula is characterized by a distinctive dichotomy between its northern, partly fractured icy plain, and the southern complex of low-albedo chaotic terrains. The margins of the macula, particularly to the north, appear heavily fractured and forming a complex of steep scarps faced towards the macula, with elevations ranging from -30 to -390 m (Fig. 2). The southern lobe is the only one displaying a positive relief up to 360 m in height. The dark, chaotic terrain consists for 85% of matrix material, and for 15% of large plates that show signs of displacement. On average, plates rise up to 320 m from the surrounding matrix, and some can exceed 700 m in height. Apparently, the matrix has replaced a pre-existing terrain which underwent a strong degradation process.

Thrace Macula exhibits a larger proportion of matrix material, which makes up to 98% of the macula’s surface, while the remaining 2% is composed of blocks. The latter are represented either by large plates not completely detached from the margins (only identified in the northern sector), or by small sub-kilometer, often tilted blocks found in the center of the macula. In contrast to Thera, the boundaries are not marked by steep scarps, and the matrix looks domed up above the surrounding plain. The macula’s northern and central sectors are separated by a bright, roughly linear stripe that could be an intersecting double ridge postdating the formation of the macula. The high-resolution images of the southern lobe show the presence of a higher-albedo matrix, where any pre-existing structure is no longer recognizable, and a lower-albedo matrix, where the pre-existing features are still preserved and appear mostly unaltered.


Figure 2. DEM of Thera Macula obtained through photoclinometry


Discussion: Geological mapping has revealed that around the “non-chaotic” plains are structures intersected by linear features (troughs, double ridges, bands), that appear displaced along two opposite directions. A graphic reconstruction of the original placement of surface units has confirmed that lateral and extensional motion of the icy crust has occurred at some point during Europa’s geological history, generating linear displacements up to 7.50 km in length. Furthermore, the morphology and the orientation of the plates surrounded by the chaotic matrix in Thera Macula indicates they must have undergone shifting, rotation and tilting upon their formation. A reconstruction of the original placement of 17 plates has confirmed that 47% of them have undergone horizontal translation, moving between 0.7 and 9.2 km (Fig. 3). Also, 30% of the plates have rotated by an average of 10.6°, either clockwise or counterclockwise, whereas motion of the plates has prevalently occurred from the inward-facing scarps towards the innermost areas.


Figure 3. Reconstruction of plates in Thera Macula to their estimated original positions


Finally, we inferred the relative age of the two maculae and the variety of geological features that comprise the surrounding plains by performing a detailed analysis of cross cutting relationships. The resulting stratigraphic column (Fig. 4) can be summarized in a four-stage sequence of events: 1) Formation of plains, including ridge complexes and smooth plains; 2) Formation of bright bands and isolated double ridges; 3) Formation of dark bands and additional linear features; 4) Formation of impact craters and chaos regions, with Thrace Macula possibly having formed earlier than Thera.


Figure 3. Stratigraphic chart of surface features. Unit abbreviations: high albedo plains (hap), bright band (bb), bilaterally symmetric band (bsb), dark band (db), double ridge (dr), raised-flank trough (rftr), trough (tr), chaos (ch), crater (cr).


Acknowledgments: GM acknowledges support from the Italian Space Agency (2018-25-HH.0).

References: [1] Prockter L. M. et al. (1999), JGR, 104, 16531-16540. [2] Collins G. and Nimmo F. (2009), in Europa, U. Arizona Press, 259-281. [3] Dalton J. et al. (2005), Icarus, 177, 472-490. [4] Carlson R. W. et al. (2002), Icarus, 157, 456-463. [5] Greenberg R. et al. (1999), Icarus, 141, 263–286. [6] Head J. W. and Pappalardo R. T. (1999), JGR, 104, 1999. [7] Fagents S. A. (2003), JGR, 108, 2003. [8] Miyamoto H. et al. (2005), Icarus, 177, 413-424. [9] Mével L. and Mercier E. (2007), PSS, 55, 915-927. [10] Schmidt B. et al. (2011), Nature, 479, 502–505. [11] Beyer R. et al. (2018), Earth Space Sci, 5. [12] Lesage E. et al. (2021), Icarus, 361, 114373.

How to cite: Chiarolanza, G., Mitri, G., and Pondrelli, M.: Geological Mapping, Topography and Stratigraphy of Thrace and Thera Macula, Europa, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-329,, 2021.

Oliver King and Leigh Fletcher

Introduction: The Galilean satellites’ surfaces and subsurfaces will be explored by robotic spacecraft in the late 2020s and early 2030s, but significant advances from ground-based astronomical facilities will be possible in the coming decade. Ganymede’s surface is composed of regions of brighter young terrain with similar composition to Europa’s young surface, and older dark terrain that has a higher abundance of silicate-rich material [11,16]. Infrared spectra from the Galileo orbiter Near-Infrared Mapping Spectrometer (NIMS) provided high-spatial-resolution IR spectra of Ganymede but with limited spatial coverage in many locations. In recent years, ground-based adaptive optics observations in the infrared with Keck/OSIRIS and VLT/SINFONI have provided new insights into the distributions of surface materials on the Galilean satellites [2,4,10,11].

Datasets: Near-IR observations of Ganymede were taken during VLT/SPHERE [1] science verification in 2014 and 2015, observing Ganymede’s leading/anti-jovian hemisphere (sub-observer longitude of 116°W). Data were taken in IRDIFS_EXT mode, allowing simultaneous imaging with the Integral Field Spectrograph (IFS) and Infrared Differential Imaging Spectrometer (IRDIS) sub-systems of the SPHERE instrument. IFS [5,13] produces image cubes with spectra from 0.95 to 1.65 μm (R∼30). It has a high spatial resolution, with a pixel size of 7.46 mas/px, corresponding to ∼25 km/px at Jupiter. Accounting for diffraction, this allows features ∼150 km across to be resolved. IRDIS produced simultaneous imaging through two filters, with transmissions centred on 2.11 and 2.25 μm that enables measurement of the strength of water ice absorption around 2 µm (Figure 1). We have also analysed a series of Galileo/NIMS observations with similar spectral and spatial coverage to our SPHERE data. The NIMS detector covering the 0.99 to 1.26 µm wavelength range failed early in the Galileo mission at Jupiter, meaning the NIMS coverage of the SPHERE spectral range is limited.

The dataset has been reduced and cleaned to produce mapped spectral cubes of Ganymede’s observed hemisphere. Images are photometrically corrected to remove the variation in brightness towards the edge of the observed disc caused by varying viewing angles and illumination levels of the surface. Our photometric correction uses the Oren-Nayar model, which generalizes the Lambertian model to more accurately represent rough surfaces [15]. This enables regions at large emission angles to be mapped accurately, providing significant improvements over the Lambertian model that overcorrects the brightness towards the edge of the disc. The Oren-Nayar correction allows our mapping to reach emission angles of ∼70°, higher than previous studies that extend to 50° to 60° [2,7,10].

Figure 1: Two-colour IRDIS observation of Ganymede (left), where yellow is 2.11µm and blue is 2.25µm, compared to simulated visible light image (right). Water ice has a broad absorption bad around 2µm, so the blue areas have higher water ice abundance.

Spectral modelling: We analyse the mapped cubes by fitting to laboratory spectra from reference cryogenic libraries. These reference spectra include water ice, sulfuric acid, and hydrated salts [3,8,12]. We have developed an implementation of the Hapke bidirectional reflectance model [9], which we use to model a range of ice grain sizes. We also include ‘synthetic’ black and white spectra which are used to model spectrally-flat (e.g., silicate) material.

Our fitting routine is run for each observed location to produce compositional maps of Ganymede’s observed hemisphere. We treat each observed spectrum as a linear combination of discrete endmembers Ei(λ) with respective abundances ai, where the modelled spectrum is calculated as M(λ)=ΣiaiEi(λ). Our routine uses Markov Chain Monte Carlo techniques [6] to model an observed spectrum, producing a posterior distribution of fitted abundance values for each endmember, and combinations of different endmembers (Figure 2).

Figure 2:  Example fitting result using our Monte Carlo fitting routine on a spectrum from Galileo Regio. The black dots give the best estimate abundance and the lines show the 1-sigma range. The width of the coloured area is representative of the posterior distribution of that endmember's abundance.

The median of each distribution is used as the best estimate abundance for each endmember, and the width of the distribution is used to estimate the uncertainty on that central value. The posterior distributions of combinations of endmembers can likewise be calculated, accounting for correlations in the uncertainties of the summed endmembers. Whilst the uncertainty on a specific endmember’s abundance may be large (e.g., a specific ice grain size), the uncertainty of the abundance of a combination of endmembers is often much smaller (e.g., the total abundance of all ice endmembers). The use of Monte Carlo techniques allows better exploration of the endmember parameter space than a simple linear fit, and the uncertainty estimates allow more detailed understanding of potential detections.

Modelling results from the SPHERE and NIMS datasets show strong similarities and are consistent with previously observed compositional features (Figure 3). These include Ganymede’s younger brighter terrain having higher water ice abundances and the older terrain has a high abundance of spectrally-flat (i.e., grey) material. This spectrally-flat material has a uniformly low albedo and is consistent with higher silicate abundance in Ganymede’s old dark terrain.

Additional SPHERE observations were planned to achieve full longitude coverage of Ganymede in 2020, however this observing campaign has been delayed due to the COVID-19 pandemic. This full observing campaign will allow ~95% of Ganymede’s surface areas to be mapped using SPHERE.

Figure 3: Example compositional maps showing fitted water ice and synthetic (spectrally-flat grey material) abundances for VLT/SPHERE and Galileo/NIMS datasets.

Acknowledgments: We would like to thank the Royal Society for supporting this work.

References: [1] Beuzit et al., 2019, arXiv:1902.04080. [2] Brown & Hand, 2013, AJ. [3] Carlson, Johnson, & Anderson, 1999, Science. [4] Carlson et al., 1992, Galileo Mission. [5] Claudi et al., 2008, SPIE. [6] Foreman-Mackey et al., 2013, Astronomical Society of the Pacific. [7] Grundy et al., 2007, Science. [8] Hanley et al., 2014, JGR-Planets. [9] Hapke, 1993, Theory of reflectance and emittance spectroscopy. [10] Ligier et al., 2016, AJ. [11] Ligier et al., 2019, Icarus. [12] McCord et al., 1999, JGR-Planets. [13] Mesa et al., 2015, A&A [15] Oren & Nayar, 1994. [16] Pappalardo et al., 2004, Jupiter: The Planet, Satellites and Magnetosphere.

How to cite: King, O. and Fletcher, L.: Compositional Mapping of Ganymede with VLT/SPHERE and Galileo/NIMS using Markov Chain Monte Carlo Spectral Analysis, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-279,, 2021.

Cecilia Tubiana, Alice Lucchetti, Tilmann Denk, Ricardo Hueso, Luisa Maria Lara, Thomas Roatsch, Katrin Stephan, Federico Tosi, Alessio Abudan, Thomas Bilotta, Vincenzo Della Corte, Alessandro Dattolo, Stubbe Hviid, Volker Martens, Klaus-Dieter Matz, Romolo Politi, Rolf Schroedter, Frank Trauthan, Michele Zusi, and Pasquale Palumbo and the JANUS team

The JUICE (JUpiter ICy moons Explorer) mission was selected in May 2012 as the first Large mission (L1) in the frame of the ESA Cosmic Vision 2015-2025 program and it will be launched in 2022. The mission aims to perform an in-depth characterization of the Jovian system, with an operational phase of about 3.5 years [1]. Main targets for this mission will be the vast Jovian system, including Jupiter itself, its magnetosphere, satellites, rings, neutral gas tori and the complex interplays among all those system components. Detailed investigations of three of Jupiter's Galilean icy satellites (Ganymede, Europa, and Callisto) will be achieved thanks to a large number of fly-bys and 9 months in orbit around Ganymede.

JANUS (Jovis, Amorum ac Natorum Undique Scrutator) is the scientific camera system onboard JUICE [2]. Despite the resource limitations, and the environmental constraints, the instrument architecture and design will be able to satisfy the great variability of observing conditions for its different targets, benefiting from the spacecraft and orbit design to its maximum. The JANUS design has to cope with a wide range of targets, from Jupiter’s atmosphere, to solid satellite surfaces and their exospheres, rings, and transient phenomena like lightning. In order to obtain multispectral observations of scientific targets as well as specific observations in narrow bands, JANUS is equipped with a filter wheel mechanism with 13 wide and narrow-band filters, allowing wavelength coverage in the 340 - 1080 nm range. JANUS will greatly improve spatial coverage, resolution and time coverage on many targets in the Jupiter system. JANUS ground sampling ranges from 400 m/pixel to < 3 m/pixel for the three main Galilean satellites, and from few to few tens of km/pixel for Jupiter and other targets in the Jovian system, such as Io, the minor inner and outer irregular moons, and Jupiter’s rings. JANUS observations of Jupiter’s atmosphere will range from full mapping to regional imaging at spatial resolutions down to 10 km/pix. Global wind fields with accuracies better than 1.0 m/s will be obtained several times during the mission.

Assuming the availability of scientific data volume (during operations about 20% of 1.4 Gbit/day is allocated to JANUS), JANUS observations will fully cover Ganymede in 4 colours with a resolution of about 100 m/pix as a goal, also providing regional DTMs. About 3% of the surface of Ganymede will be covered with a resolution of 10 - 30 m/pix for selected Regions of Interest, using both panchromatic and colour filters, and providing stereo images for the 3D reconstruction of the surface. This will represent dramatic improvements in imaging with respect to Galileo coverage in all the science targets covered by JUICE/JANUS.

In addition to presenting the science goals that we are aiming to achieve during the JUICE science phase, we will show examples of a case study of operations, to highlight how the achievement of science goals is strictly related to the resources available to the instrument.

References: [1] Grasset et al., (2013), PSS, 78, 1-21. [2] Palumbo et al., (2014), EGU conference

Acknowledgements: The activity has been realized under the ASI-INAF contract 2018-25-HH.0. LML acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa” award to the Instituto de Astrofısica de Andalucia (SEV-2017-0709) and from project PGC2018-099425-B-I00 (MCI/AEI/FEDER, UE).

How to cite: Tubiana, C., Lucchetti, A., Denk, T., Hueso, R., Lara, L. M., Roatsch, T., Stephan, K., Tosi, F., Abudan, A., Bilotta, T., Della Corte, V., Dattolo, A., Hviid, S., Martens, V., Matz, K.-D., Politi, R., Schroedter, R., Trauthan, F., Zusi, M., and Palumbo, P. and the JANUS team: JANUS: the camera system onboard JUICE. Operational approach and scientific capabilities from operations case studies., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-499,, 2021.

Oriane Gassot, Alain Herique, Wlodek Kofman, and Olivier Witasse

Studying the presence of water and characterizing the tectonic structure in the firsts tens of kilometers of the crust of the Galilean icy moons are crucial to understand the formation and evolution of these bodies and could provide an insight into the setting for extra-terrestrial life within our solar system (Nimmo and Pappalardo, 2016). The most promising technique for directly detecting subsurface oceans is a penetrating radar and the use of low frequencies (< 30 MHz) is preferred to probe the moons' subsurfaces since the losses due to the surface roughness and absorption of the ice are reduced (Bruzzone et al., 2011). However, Jupiter has a loud radio environment for frequencies < 40 MHz  (Cecconi et al., 2012), and the use of a passive mode, which would exploit Jupiter’s decametric radio emissions, is considered to operate the radar with low frequencies in the sub-Jovian hemispheres. However, the passive radar operates in a complex bistatic 3D geometry where Doppler and delay information are not separable. This justifies the use of simulations with realistic orbitographies to identify which configurations will lead to the best and worst performances, which is necessary to establish the scenarios of observation of the radar. In this paper, we compute the impact of the geometry on the final bistatic performances, using planned Juice orbits.

In order to study the influence of the geometry only, we do not take into consideration the stochastic character of Jupiter’s noise. The emission is then a simple impulsion located in the Jupiter auroral coronas:  we regard four sources, at the Eastern and Western borders of Jupiter’s North and South auroral coronas, as presented in Figure 1.

In this geometry, the passive radar is orbiting around Ganymede and studies the reflection of the Jovian signal by a point target at the nadir in a monostatic geometry, or a point making a specular reflection in the bistatic case. To recognize weak, delayed, and Doppler-shifted contributions of the Jovian signal (also called reference signal), cross-correlation is carried out between the registered Jovian signal and the measured reflection. The next signal processing is the computation of the SAR synthesis to retrieve a Range-Doppler map of the surface probed.


Figure 1. Considered location for the four sources of emission of Jupiter’s radio emissions, and Jupiter. The sources will then be referred to as red source, blue source, yellow source, and green source.

To compute the performances of the radar, we simulate the signal scattered in monostatic and bistatic cases, for three different scenarios (figure 2, table 1) using different integration times, perform SAR synthesis and compute on the images the size of the resolution cell.  We depict in table 2 the area of the resolution cell for each scenario, as well as the size of the Fresnel zone.

The area of the resolution cell is always better than the size of the Fresnel zone, but table 2 depicts that with an integration time of one minute, probing close to the center of the sub-Jovian hemisphere provides the best resolution (1.33 km²). However, this conclusion changes when the integration times are increased to two minutes, and probing with an incidence angle of 38° becomes more favorable than along the nadir (0.28 km²). Indeed, in this geometry, the Range and Doppler sidelobes are not separated, and this non-separability increases with the integration.

Thus, while the along-track resolution is predictably worsened in the bistatic case compared to the monostatic scenario and improves with increasing integration time, the resolution in across-track varies as well with the integration time. For orbit 2, it improves with an increasing integration time, while, for orbit 1 and 3, which are aligned with nadir, the integration time does not seem to largely impact it. This means that, while probing along the center of the sub-Jovian hemisphere seems more efficient with small integration times, probing different parts of the sub-Jovian hemisphere might become of interest if increasing the integration time is possible.

This study showed that, as expected, the resolutions obtained are largely dependent on the geometry, but as well on the integration time. These conclusions will have to be kept in mind will preparing the probing strategy of Jupiter’s moons.

Figure 2.
Left: JUICE orbits as visualized on Cosmographia (
Right: Position of the three considered orbits ( orbit n°1 in red, orbit n°2 in green, and orbit n°3 in cyan), with the bistatic specular point in bright color, and the nadir point with an opacity.

Table 1 Description of each scenario.

Orbit Associated color  Date Center Time Incidence
Orbit n°1 Red 18/03/2033 15:41 2
Orbit n°2 Green 21/03/2033 09:46 38
Orbit n°3 Cyan 27/04/2033 08:02 27


Table 2 Performances resulting from each orbit. ρ2 is the area of the resolution cell while Fresnel depicts the area of the Fresnel zone.

      ρ2 (km2)
Fresnel (km2)
Orbit n°1 1 mn Monostatic 0.503 22
Bistatic 1.33 38
2 mn Monostatic 0.25 22
Bistatic 0.655 38
Orbit n°2 1 mn Monostatic 0.518 22
Bistatic 1.78 55
2 mn Monostatic 0.25 22
Bistatic 0.28 55
Orbit n°3 1 mn Monostatic 0.518 22
Bistatic 1.92 46
2 mn Monostatic 0.28 22
Bistatic 0.953 46


How to cite: Gassot, O., Herique, A., Kofman, W., and Witasse, O.: Passive radar probing of the Galilean Moons, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-521,, 2021.

Mathilde Kervazo, Marie Běhounková, Gabriel Tobie, Gaël Choblet, and Caroline Dumoulin

The presence of a subsurface ocean on Europa [1] in direct contact with the silicate mantle, along with the spectacular volcanic activity exhibited by its neighboring satellite Io [2] raise the possibility of seafloor volcanic activity [3,4], which has significant implications for Europa’s ocean habitability. 

Observational constraints concerning the heat budget of Jupiter’s moons exist only for Io. The total power emitted at its surface is estimated to about 100 TW at present [5], which is several orders of magnitude higher than can be explained by radiogenic heating alone. Tidal dissipation in a partially molten layer at the top of the silicate mantle appears consistent with the prodigious heat flux emitted from Io [6,7], but it is still unclear how Io reached such a highly dissipative state. 

Unlike Io, the heat flux at the surface of Europa’s rock mantle is unknown. Due to a larger distance from Jupiter and a smaller size, dissipation in Europa's mantle is expected to be considerably smaller than on Io [8], but still could be comparable to present-day radiogenic heating [4]. From 3D models, we recently showed that combined radiogenic heating and tidal heating can sustain partial melting in Europa's mantle during billions of years [4], especially during periods of enhanced eccentricity, which may lead to melt accumulation in the asthenosphere. Even if the melt production in Europa’s mantle is much smaller than Io’s, evaluating the coupling between melt generation and tidal heat production is essential to assess the extent of seafloor volcanism on Europa. 

In the present study, we follow the approach developed to model the solid tides in Io’s partially molten interior [7], and adapt it to the context of Europa, corresponding to a deeper asthenosphere than on Io with presumably a smaller amount of melt present. We test the influence of partially molten zones on the tidal dissipation rate in Europa’s mantle (Figure 1), assuming rheological laws including the effect of melt on anelastic properties of rocks. Using the 3D model prediction of Běhounková et al. [4] for melt production, we estimate the effect of melt accumulation in Europa’s mantle on local dissipation rate and re-assess the consequences in terms of melt production rate. We investigate different scenarios for the eccentricity evolution and provide upper limits in terms of global power generated by tidal heating in Europa’s mantle through its evolution.

Figure 1: Spatial distribution of the averaged volumetric heating rate in Europa’s mantle including a 25 (a), a 50 (b) and a 100-km thick (c) partially molten layer with a melt fraction of 20%. 


[1] Khurana, KK., et al., Nature, Vol. 395, pp. 777 (1998).
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Acknowledgements: This research received funding from the French “Agence Nationale de Recherche” A.N.R. (OASIS project, ANR-16-CE31-0023-01,( G.T., G.C., M.K., C.D.), from CNES (JUICE and Europa Clipper missions, G.T., G.C., M.K., C.D.) and from Czech Science Foun- dation through project No. 19-10809S (M.B.)


How to cite: Kervazo, M., Běhounková, M., Tobie, G., Choblet, G., and Dumoulin, C.: Impact of partial melting on tidal dissipation in Europa's silicate mantle, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-274,, 2021.

Marissa Cameron, Kevin Hand, Cynthia Phillips, Earl Maize, Jo Eliza Pitesky, Shawn Brooks, Kate Craft, Ray Crum, Jason Hofgartner, Amy Hofmann, Kenneth Hurst, Brett Kennedy, Emily Klonicki, Glenn Reeves, Eric Roberts, Alejandro Miguel San Martin, Jennifer Scully, Lori Shiraishi, and Grace Tan-Wang and the Project Science and Engineering Teams

Introduction: Jupiter’s moon Europa is a prime target in our exploration of potentially habitable worlds beyond Earth, and of ocean worlds in the outer solar system. The exploration of Europa presents an important target for both astrobiology and comparative oceanography, i.e., the opportunity to study liquid water oceans as a planetary process. Europa’s icy shell also offers the opportunity to study tectonics and geologic cycles across a range of mechanisms (e.g., Earth’s cooling versus Europa’s tidal dissipation) and compositions (silicate in the case of the Earth, versus ice in the case of Europa). Europa is a scientifically important and strategic target for both planetary science and astrobiology.

Critically, Europa’s subsurface ocean has likely existed for much of the history of the solar system, potentially providing a persistent, stable environment in which a second, independent origin of life may have arisen. Observations and models indicate that the ocean is likely in contact with a rocky, silicate seafloor, and the ice shell may have tectonic activity that could allow reductant-oxidant cycling. This scenario could lead to an ocean rich in the elements and energy needed for the emergence of life, and for potentially sustaining life through time. The persistence of Europa’s ocean means that life could be alive there today – i.e., signs of extant life could be found within the ice and ocean of Europa. The discovery of signs of extant life is critical if we are to understand biology as a universal process: Does it contain DNA or does it function on some other large biomolecules for information storage, replication, and repair? Are there many separate ‘trees of life’ within our solar system, or is the tree of life on Earth the only one? The search for past life on worlds like Mars is very important, but the search for extant life is how we will truly revolutionize biology (if life exists beyond Earth).

Figure 1. Science goals and objectives of the Europa Lander mission concept. Within each Goal are the high level Objectives, represented as a “fan” across each Goal.

The high-level science goals of the Europa Lander Mission Concept are:

  • Search for evidence of biosignatures on Europa.
  • Assess the habitability of Europa via in situ techniques uniquely available to a lander.
  • Characterize surface and subsurface properties at the scale of the lander to support future exploration.

These goals are achieved by employing a lander on the surface that collects and processes a minimum of three separate samples, each of at least seven cubic centimeter in volume, and acquired from a depth of at least 10 cm.

Overview of Mission Concept: Here we provide an overview of significant milestones, developments, and technology advancements that have been made, or are ongoing, to retire science, technology, cost, and schedule risks associated with the mission concept.

  • The mission concept achieves high value science without requiring an excessive number of engineering ‘miracles’; this mission aims to be the right ‘first’ mission to the surface of Europa and balances technical risk with science return and cost.
  • The technology and instrumentation investments made to date (which exceed $300M) could enable a new era of planetary exploration. Many of the technologies that have, or are, being developed for the Europa Lander Mission Concept can be utilized for landing on other ocean
  • The Europa Lander builds on the investment in Europa Clipper, using data from that mission for landing site There would be at least five years of time between the end of Clipper’s prime mission and the landing site selection date. Importantly, data from Clipper would be unlikely to dramatically change our approach to de-orbit, descent, and landing (DDL). The mission concept team examined a variety of mechanical configurations and concluded that even after the acquisition of the Clipper data, the DDL and mechanical architectures would not significantly change. Uncertainty about parameters such as porosity and structure at the sub-meter scale would still require the intelligent landing system, with terrain relative navigation and hazard avoidance. Furthermore, the lander would still need to employ the ‘snowshoe belly pan’ and ‘grasshopper’ adaptive stabilizer legs to accommodate soft and variable surfaces at the sub-meter scale.
  • The Lander concept uses primary batteries and could survive for many weeks to >60 days on the surface, depending on sampling and idle power usage The choice of primary batteries was, in part, to save on cost and complexity. A longer-lived mission concept with a radioisotope power system was studied, but planetary protection, thermal management, and mass were found to contribute to increasing cost and technical risk.

How to cite: Cameron, M., Hand, K., Phillips, C., Maize, E., Pitesky, J. E., Brooks, S., Craft, K., Crum, R., Hofgartner, J., Hofmann, A., Hurst, K., Kennedy, B., Klonicki, E., Reeves, G., Roberts, E., San Martin, A. M., Scully, J., Shiraishi, L., and Tan-Wang, G. and the Project Science and Engineering Teams: The Europa Lander Mission Concept: In Situ Exploration of an Ocean World, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-386,, 2021.

Cristóbal González Díaz, Guillermo M. Muñoz Caro, Héctor Carrascosa de Lucas, Sofía Aparicio Secanellas, Margarita González Hernández, José J. Anaya Velayos, Guillermo Anaya Catalán, Olga Prieto-Ballesteros, Victoria Muñoz-Iglesias, Oscar Ercilla Herrero, Rosario Lorente, Nicolas Altobelli, and Olivier Witasse

The upcoming JUpiter ICy moons Explorer (JUICE) (ESA) and Europa Clipper (NASA) missions will perform detailed observations of the giant gaseous planet Jupiter and three of its largest moons (Ganymede, Callisto, and Europa).

A series of experiments was performed to measure the thermal conductivity and calorimetry of macroscopic frozen salt solutions of particular interest in Jovian icy moons. The following salts were investigated: Na-chloride (NaCl), Mg-sulphate (MgSO4), sodium sulphate (Na2SO4), and Magnesium chloride (MgCl2). Measurements were performed at atmospheric pressure and temperatures from 0 to -70ºC in a climatic chamber. Temperature and thermal conductivity were measured during the course of the experiments. A small sample of the liquid salt-water solution was set aside for the calorimetry measurements. A side effect of the measurements is that they served to spot phase changes in the ice mixtures with high sensitivity. An important result is that, the phase changes observed in the standard calorimetric tests, could be monitored in situ with high sensitivity in the thermal conductivity measurements. Indeed, when a phase change occurs, a large peak appeared in the thermal conductivity values as the result of the natural heat release that accompanied the phase change. 

How to cite: González Díaz, C., Muñoz Caro, G. M., Carrascosa de Lucas, H., Aparicio Secanellas, S., González Hernández, M., Anaya Velayos, J. J., Anaya Catalán, G., Prieto-Ballesteros, O., Muñoz-Iglesias, V., Ercilla Herrero, O., Lorente, R., Altobelli, N., and Witasse, O.: Thermal conductivity measurements of frozen salt solutions in Jovian moons to support future JUICE mission., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-714,, 2021.

Olga Prieto-Ballesteros, Oscar Ercilla Herrero, Javier Sánchez Benítez, Victoria Muñoz-Iglesias, Alberto Rivera-Calzada, Guillermo Muñoz-Caro, Cristobal González-Díaz, Rosario Lorente, Nicolas Altobelli, and Olivier Witasse

ESA and NASA will launch two missions to explore the main Galilean moons of Jupiter in the coming years, JUpiter ICy moons Explorer-JUICE and Europa Clipper, respectively. Both missions will be able to determine the distribution of the potential habitable zone in the upper dozens of kilometers of the ice bodies by using onboard radar instruments [Bruzzone et al. 2013, Grasset et al., 2013, Phillips and Pappalardo 2014, Aglyamov et al. 2017]. Considering the possible presence of mixtures of water with salts, volatiles, and clays in the ice and liquid layers, we performed laboratory experiments to obtain the electrical properties of these chemical systems at solid and liquid state, and at different pressure conditions (Table 1). The results that we present at the conference will facilitate the interpretation of the future data received from the radar sensors.

We measured the dielectric properties of these samples with a BDS80 Broadband Dielectric Spectroscopy system (Novocontrol), which allows working in a frequency range from 1 Hz to 10 MHz and temperatures from 143 to 323 K. Both, real permittivity (ε´) and electric conductivity (σ) were measured at 0.1 MPa while cooling the samples in temperature steps of 10 K. From these data, we calculated the activation energy and the attenuation of the radar wave depending on the chemical composition, the temperature of the sample, and the frequency of the applied electric field [Petrenko and Whitworth 1999, Pettinelli et al. 2015].

Conductivity measurements at high pressure were carried out using a modification of a chamber used previously for planetary simulation experiments (Muñoz-Iglesias et al. 2019). It is based on a stainless steel cylinder, which has different access on the bases and along the main body. Pressurization of the sample is carried out by water or gas from one access at the chamber base, while a sapphire window is at the opposite side for visual control and spectral analysis. Connections throughout the body of the cylinder are for the thermocouple, the pressure gauge and a special one to plug in the electrical sensor. The key feature is the plug for in-situ conductivity measurements with feed through wires connected to the probe, made of two Pt electrodes placed in a PTFE structure in order to maintain the configuration and to ensure the distance between electrodes (around 500 micrometers). Both electrodes have connections to isolated copper wires, which pass through the cell body to be connected to the signal transducer and the computer to record the data. 



This work was supported by the ESA contract number 4000126441/19/ES/CM. We thank Anezina Solomonidou for assistance in the project proposal.



Aglyamov et al. (2017) Bright prospects for radar detection of Europa’s ocean, Icarus, 281, 334-337.

Bruzzone et al. (2013) RIME: Radar for Icy moon Exploration, IEEE International Geoscience and Remote Sensing Symposium - IGARSS, Melbourne, 3907-3910.

Grasset et al. (2013). Jupiter Icy moons explorer (JUICE): an ESA mission to orbit Ganymede and to characterise the Jupiter system. Planet. Space Sci. 78, 1-21

Muñoz-Iglesias et al. (2019). Experimental Petrology to Understand Europa's Crust. JGR-Planets 124, 2660-2678

Phillips and Pappalardo (2014). Europa Clipper mission concept: exploring Jupiter’s ocean moon. Eos Trans. AGU 95, 165-167.

Pettinelli et al. (2015) Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review. Reviews of Geophysics, 53, 593-641.

How to cite: Prieto-Ballesteros, O., Ercilla Herrero, O., Sánchez Benítez, J., Muñoz-Iglesias, V., Rivera-Calzada, A., Muñoz-Caro, G., González-Díaz, C., Lorente, R., Altobelli, N., and Witasse, O.: Dielectric properties of binary aqueous systems with sulfates, chlorides, and volatiles for subsurface radar sounding, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-467,, 2021.

Maryse Napoleoni, Fabian Klenner, Jon K. Hillier, Nozair Khawaja, Kevin P. Hand, and Frank Postberg

Jupiter’s moon Europa is predicted to harbor a global liquid water ocean beneath its icy crust [1,2].  Like Saturn’s moon Enceladus, Europa could be cryovolcanically active, with evidence for plumes of water recently reported (e.g. [3,4]). Such plumes could eject gas and water ice grains from the subsurface ocean into space. Sputtering and micrometeorite bombardment may also eject icy surface particles to high altitudes [5].

Ice grains ejected from icy moons can be analyzed during spacecraft flybys by impact ionization mass spectrometers [6], such as the Cosmic Dust Analyzer (CDA) onboard the Cassini spacecraft or the Surface Dust Analyzer (SUDA) that will be onboard the upcoming Europa Clipper mission [7]. These instruments can determine the composition of the ice grains and potentially indirectly sample subsurface oceans. In the Saturnian system, data collected by the CDA instrument showed that Enceladus’ interior ocean is salt-rich [8], sustains water-rock hydrothermal interactions [9], and contains a variety of organic material, including complex macromolecules [10] and low mass volatile compounds, potentially acting as amino acid precursors [11]. On Europa, the subsurface ocean is predicted to be in direct contact with silicates and possible seafloor magmatic activity, enhancing the potential habitability of the moon [12]. The moon’s surface is also exposed to the harsh radiation environment of Jupiter, which may induce oxidation and potentially other chemical reactions involving both ice and non-ice compounds [13].

Interpreting mass spectra acquired in space requires terrestrial calibration by analogue experiments. The Laser Induced Liquid Beam Ion Desorption (LILBID) technique reproduces the impact ionization mass spectra of ice grains recorded in space [14]. Previous LILBID experiments have shown that bioessential molecules, such as amino acids and fatty acids, can be detected in ice grains at concentrations as low as the µM or nM level [15], and that the abiotic and biotic formation processes of these molecules can be distinguished from each other based on spectral features [16]. Microbial biosignatures were also investigated recently, showing that nucleobases, fatty acids and other bacterial fragments can be clearly identified [17].

Here we investigate whether Europa-relevant organic compounds encased in ice grains can also be detected and characterized using impact ionization mass spectrometry in space. High-sensitivity LILBID experiments have been performed with formamide (CH3NO), farnesol (C15H26O), cholesteryl linoleate (C45H76O2) and N-dodecanoyl-L-homoserine lactone (C16H29NO3) to predict their spectral appearance in both anion and cation mass spectra. Formamide is expected to form by radiolytic reactions on Europa’s surface [18], while farnesol, cholesteryl linoleate and N-dodecanoyl-L-homoserine lactone are lipids, representing potential biosignatures and selected for their high resistance to degradation. These compounds were investigated in water matrices with varying NaCl concentrations designed to mimic Europa’s predicted salty ocean and surface composition [19,20]. Results show that the identification of molecular peaks as well as characteristic fragments is possible for both formamide and lipids. Additionally, the spectra of formamide in salt-rich matrices show that formamide can be detected via sodiated molecular clusters ([CH3NO+Na]+) and other Na-rich complexes.

The next steps will be to investigate these compounds in H2O2-rich matrices, designed to simulate the highly oxidizing surface environment of Europa [21]. Sulfate salts and sulfuric acid are also under consideration as important matrices relevant to the surface chemistry of Europa. Other potential biosignatures, as well as their irradiation products, will be investigated to study the likelihood of their survival and detection under Europa’s radiation environment. The recorded mass spectra will complement a comprehensive spectral reference library [22], which provides analogue data of a wide range of compounds applicable to impact ionization mass spectrometers onboard Europa Clipper or other future ocean world missions.




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[14] Klenner et al. (2019) Rapid Commun Mass Spectrom. 33.22: 1751-1760

[15] Klenner et al. (2020) Astrobiology 20:179–189

[16] Klenner et al. (2020) Astrobiology 20: 1168–1184

[17] Pavlista et al. (2021) EGU21-15475

[18] Hand (2007) PhD Thesis, Dpt of Geological and Environmental Sciences, Stanford University

[19] Zolotov et al. (2001) Journal of Geophysical Research: Planets 106.E12: 32815-32827

[20] Trumbo et al. (2019) Science advances 5.6: eaaw7123

[21] Johnson et al. (2003) Astrobiology 3, 823–850

[22] Klenner et al., in prep.

How to cite: Napoleoni, M., Klenner, F., Hillier, J. K., Khawaja, N., Hand, K. P., and Postberg, F.: Analogue experiments for the identification of organics in ice grains from Europa using mass spectrometry, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-494,, 2021.

Poster presentation
Pietro Matteoni, Tanner Hayes, Jon Hillier, and Frank Postberg

Events which emplace fresh material onto Europa’s surface may be irregular and catastrophic, such as large-scale impacts or localized, and potentially extant, processes in which faults, fractures, or brine transport bring subsurface liquid onto the surface. This liquid may originate from shallow reservoirs within the ice shell or directly from the ocean [1].

Emplacement may be a slow, extrusive process, or quicker, potentially with the formation of cryovolcanic plumes (e.g. [2,3,4]). Plume deposits or any other surface material can also be transported to high altitudes by another mechanism: when hypervelocity interplanetary micrometeoroids, or larger objects, impact the surface of an atmosphereless planetary body like Europa, these can generate impact ejecta with high enough velocities to reach altitudes of hundreds of kilometres [5]. Most of these ejecta particles are gravitationally bound, moving on ballistic trajectories lasting up to hundreds of seconds and producing an almost isotropic dust exosphere around Europa [6,7]. Subsurface oceans, in the Jovian and in other planetary systems, can therefore be characterized using the particles they emit, via either indirect or direct routes: 1) Detection and analysis of ejecta particles lofted by micrometeoritic impacts from those parts of the surface that recently interacted with subsurface waters (e.g certain landforms, such as chaos terrains on Europa, or plume deposits); 2) Direct sampling of plume particles in space (where plumes are present, as perhaps possible on Europa).

The time-of-flight mass spectrometer SUrface Dust Analyzer (SUDA), onboard the Europa Clipper spacecraft, is designed to measure the compositions and trajectories of such impact ejecta particles and/or plume material [8]. SUDA is an impact ionisation time-of-flight mass spectrometer, which uses the ions generated by hypervelocity impacts of dust grains onto the instrument target to generate mass spectra of the impinging particles. Depending on the altitude SUDA will be able to detect up to tens of ejected surface particles per second during each flyby, each likely to contain a wide variety of organic and/or inorganic compounds, and via trajectory reconstruction, map them to their origins on the surface [8].

The trajectories of impact ejecta, and potentially even those of directly emitted plume grains, are likely to be considerably affected by local surface properties, over a range of scales. Here we present our initial progress in producing new, and collating existing (e.g [9]), digital terrain models (DTMs) of the Europan surface, together with the development of Monte Carlo impact ejecta trajectory simulations which consider variations in ejecta mass and velocity distributions.

DTMs have been produced using both stereophotogrammetry and photoclinometry techniques, through the Integrated Software for Imagers and Spectrometers (ISIS) and the Ames Stereo Pipeline (ASP) Shape-from-Shading (SfS) tool [10], based on Galileo’s Solid-State Imager (SSI) images. In the case of Fig. 1a, through SfS we obtain a DTM of image 0526r with a resolution of ~35 m/px, following the methodology described in [11]. One of the most important parameters that can be calculated from DTMs is surface roughness (Fig. 1b), a measure that describes the height distribution of asperities. Its variations affect the trajectories and yield of ejecta particles generated by micrometeoroid impacts, along with the angles at which plume particles might be emitted. The DTMs of features of interest, with their associated roughness and slope information, will provide an important input for ejecta trajectory simulations.