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The main goal of the radiation monitor RADEM flying onboard the ESA JUICE mission is to provide continuous information on particle fluxes and their energy spectra. The monitor measures electrons up to 40 MeV and protons up to 250 MeV. Such a range of energies detected by RADEM enables covering the most hazardous regimes in terms of radiation damage. Spectroscopic information on particle energies is provided using eight quasi-logarithmic energy bins. RADEM has also a dedicated heavy-ion detector designed to measure a variety of heavy ion species with their LET between 0.1 and 10 MeV/cm/mg-1. Moreover, the monitor contains an additional detector sensitive to the direction of incoming radiation. It expands the instrument's angular coverage up to 35% of the sky. Apart from its spectroscopic and angular distribution functions, RADEM will continuously provide values of the radiation dose deposited by each particle species. Its telemetry data will be stored in the data center for the JUICE mission operated by the European Space Astronomy Centre. After preprocessing the higher-level data will become available to the JUICE scientific team. RADEM will be switched on shortly after the JUICE launch planned for April 2023 and after a short commissioning phase will start its nominal operation. Apart from regular and short tuning and calibration periods, it will remain operating for the rest of the mission i.e. almost 10 years. While its primary purpose is to monitor the mission levels for safety concerns of the spacecraft and its scientific payload, its measurements open a unique opportunity for conducting real-time, continuous observations during its full cruise to Jupiter. RADEM will study all aspects of the radiation phenomena characteristic to the Earth and Solar System. Correlations with other instruments will allow for advanced observations of particle event propagation and a better understanding of processes related to the dynamics of particle environments including their links with solar activity and magnetic fields across the solar system. In particular, during its first two years of the cruise to Jupiter, RADEM will precisely map the radiation environment between Venus and Mars, providing uninterrupted time-resolved spectroscopy and dosimetry data from Solar Energetic Particles and Cosmic Rays.

With MaQuIs we propose a mission to investigate the gravitational field of Mars. Observing the gravitational field over time yields information about the planets tectonic lithoshphere, mass distribution, and composition. Consequently, this mission allows to study static and dynamic processes on and under the surface of Mars, including phenomena such as melting cycles and tectonic activity.MaQuIs will deploy quantum mechanical means to measure Mars gravitational field with the highest precision yet. In addition, the nature of the proposed instrumentation achieves high sensitivities without needing more complex satellite constellations. As such, MaQuIs follows successful missions for the Earth and Moon, extending the technology to Mars.In this presentation we will outline the expected scientific merit and explain the underlying technology and planned configuration of the mission.

Electron density structure and its temporal/spatial variations is one of the critical quantities in understanding an upper atmosphere. Mars has currently several orbiters studying its upper atmosphere and in particular electron density profiles. The Mars Express (MEX), in the orbit since 2003 has been measuring Mars upper atmosphere through remote measurements by the MARSIS (Sub-Surface Sounding Radar Altimeter) sounder and radio occultations by MaRS (Radio Science Experiment - sounding of the internal structure, atmosphere and environment). The Mars Atmosphere and Volatile EvolutioN, MAVEN mission whose science objectives include exploration of the interactions of the Sun and the solar wind with the Mars magnetosphere and upper atmosphere has complete plasma package. Maven has been performing in-situ composition measurements since November 2014, in particular, the Langmuir Probe and Waves (LPW) instrument onboard MAVEN provide electron density using current-voltage sweeps. Here we present the comparison of multi-instrument spacecraft measurements of Mars upper atmosphere for selected dates, using remote sensing (MEX) and in-situ measurements (MAVEN). The mutual observations are selected from publicly available data on ESA Planetary Science Archive as well as MAVEN Science Data Center. The L2 level Radio Occultation data are analyzed to obtain the Mars upper Atmosphere Electron density profiles. The MARSIS and LPW provide top side electron profiles whereas Radio occultations provide the full electron density profile. We focuse on the photochemical equilibrium region of the Martian ionosphere–thermosphere below 200 km. The analysis also includesz the Deep Dip campaigns of MAVEN making in-situ measurements of the region near the ionospheric peak. The comparison of independent measurements using different techniques will be presented and the differences and similarities between the observations will be discussed to help to validate our understanding of the physical and chemical processes occurring in the atmosphere. Figure 1: Comparison of MEX Radio Occultations (RO) with the MAVEN Langmuir Probe and Waves (LPW) for 3 selected mutual observation in March 2015

Radars in orbit around Mars such as MARSIS on Mars Express and SHARAD on Mars Reconnaissance Orbiter provide evidence for the nature of the surface and some sub-surface layers. We have now extended our earlier work, which has indicated solar energetic particles are responsible for the attenuation of the reflected MARSIS and SHARAD radar signals from the surface of Mars, often resulting in radar blackouts. The MARSIS instrument also operates in a topside sounding mode, the Advanced Ionospheric Sounder AIS mode), which also receives surface reflections and we now investigate the impact of the solar energetic particles on the surface reflection during ionospheric sounding observations made by MARSIS. We present observations during two intervals in December 2014 and in September 2017. MARSIS AIS was making observations on the nightside as Mars Express moved towards periapsis and then towards the dayside. The nightside AIS observations clearly demonstrate a similar loss of the surface signal to that seen in the MARSIS sub-surface mode. In addition there is evidence of a reflection from the enhanced layer created by the electrons which is responsible for the attenuation in the signal. The critical frequency is of order 1 MHz, which is equivalent to a peak electron density of order 1010 m-3, at an altitude of about 100 km. These characteristics are similar to our previous modelling work of the impact of the solar energetic electrons.

JUICE is ESA’s first large class mission to the outer Solar System. The main objectives of JUICE are to study Jupiter and its space environment with a special focus on Jupiter’s moons Europa, Ganymede, and Callisto, and their potential habitability. In order to fulfil these objectives, the JUICE measurements need to be accurately corrected for any possible perturbations. Here, we present Spacecraft Plasma Interaction Software (SPIS) simulations of the surface charging of JUICE in the solar wind. The results will be used to correct the future JUICE measurements for the impact of the charging. We have used a solar wind environment model (i.e. a description of the environment covering typical values for parameters such as electron and ion densities, temperatures, and velocities, magnetic field strengths, and EUV flux) for the location where JUICE will perform its first measurements, between 1500 and 3000 RE from Earth. The typical values for the solar wind parameters and the minimum and maximum values from the expected parameter ranges have been used to simulate the interaction in both average and “extreme” solar wind conditions. Here we present the main results from the SPIS simulations: the surface potential of the spacecraft; the potentials at the locations of the particle and field instrumentation such as the RPWI Langmuir probes and the PEP plasma analysers; the electron and ion density at the locations of the RPWI instruments and the PEP plasma analysers; the characteristics of perturbing particle populations such as photoelectron and secondary electron populations produced by the spacecraft itself; and the properties of the ion wake of the spacecraft. The detailed knowledge of the listed parameters will be used to provide accurate analyses of the first in-situ particle and field measurements performed by JUICE.

JUICE is the first large mission chosen in the framework of ESA’s Cosmic Vision 2015-2025 program. The focus of JUICE is to characterize the conditions that might have led to the emergence of habitable environments among the Jovian icy satellites, in particular Europa, Callisto and Ganymede. In addition, JUICE will also perform a multidisciplinary investigation of the Jupiter system as an archetype for gas giants. To address those key science objectives, the spacecraft payload consists of 10 state-of-the-art instruments (and one experiment that use the spacecraft telecommunication system with ground-based instruments) that will perform remote and in-situ measurements of Jupiter, its moon and their environment. From a trajectory’s point of view, the mission calls for a three-year orbital survey of the Jupiter System designed as a sequence of 67 orbits around the planet of different periods and inclinations, and includes 35 flybys of Europa (2), Ganymede (12) and Callisto (21). The second part of the mission will devote an additional 9 months in orbit around Ganymede for an in-depth characterization of Ganymede as a planetary object and possible habitat. By its touring nature and due to the diversity of science targets and different disciplines involved in the mission, the development of an observation plan will often face conflicting observations opportunities, also limited in resources as a consequence of the large helio- and geo-centric distances at play. The Science Operations Center, together with the Project Scientist and the Science Working Team, are in charge of setting up and developing a science planning strategy for the mission to ensure that the prime science objectives of the mission are addressed while remaining compatible with the mission constraints. The JUICE Science Planning Process for the Jupiter tour consists of two main pillars: Segmentation of the trajectory. Based on the Cassini heritage, this activity relies on the analysis of all science opportunities along the tour to subsequently support the decision on selecting a prime scientific objective that will get priority in terms of resources (data volume, power and pointing) for a specific time window (a.k.a segment). Taking into account known operational activities’ constraints (navigation, wheel off-loading etc) and high-level resources’ estimate, an analysis of the expected scientific coverage and resource status along the tour is performed, and several iterations between the different disciplines experts (through the science working groups) take place to converge towards a plan that should ensure that prime science objectives of the mission are fulfilled within the operational constraints of the mission. Detailed sizing case analysis for science operations: For specific operationally of scientifically challenging sections of the segmentation, in-depth analysis is needed in order to refine resources assumptions (attitude, power and data volume) made at segmentation level. Those detailed analysis, performed down to observation level, is done in collaboration with the instrument teams. The present work will describe the different steps and groups involved in the science planning, the current status of the segmentation, and the detailed analysis of the first JUICE Europa flybys will be presented.

BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury. It was launched in October 2018 and it is due to arrive at Mercury in late 2025. It consists of two spacecraft, the Mercury Planetary Orbiter (MPO) built by ESA, and the Mercury Magnetospheric Orbiter (MMO) built by JAXA, as well as a Mercury Transfer Module (MTM) for propulsion built by ESA. The cruise phase to Mercury will last ~7 years and constitutes an exceptional opportunity for studying the evolution of the solar wind, solar transients, as well as for planetary science and planetary space weather. On this last topic, BepiColombo is proving to be an important upstream solar wind monitor for the terrestrial planets. This is especially important for case of Mars where we currently do not have many observations of the upstream interplanetary magnetic field or solar wind. As consequence, the analysis and interpretation of ionospheric observations is challenging, especially when coronal mass ejections and solar energetic particles hit Mars and we do not have Earth plasma observatories in good alignment with Mars to provide the space weather context. While Mars missions such as Mars Express or MAVEN are able to investigate the impact of the solar wind on the ionosphere, BepiColombo can be the solar wind sentinel for Mars, so we can see the cause and effect. Therefore, this work focuses on the response of the Martian ionosphere to solar transient events which have been previously detected by BepiColombo, and for which we can provide the right upstream solar wind context, such as during the last months of 2021. The objective is to evaluate the response of the Martian ionosphere and its dynamics with more accuracy knowing the exact energy inputs from the solar wind as detected in-situ in the solar wind by BepiColombo.

Space weather events are one of the many major factors affecting the escape and evolution of Martian atmosphere. The Sun-planet interaction is affected by several factors such as the heliocentric distance, chemical composition of the atmosphere, solar illumination and its intrinsic magnetic field. Mars has a varied environment of magnetic field. It has little or no intrinsic magnetic field, but there exist some strong, crustal magnetic field patches. Thus, it becomes one of the solar system's best models for understanding the interaction of weakly ionized plasma with a highly irregular magnetic field. Martian ionosphere owes its existence to the photoionization of neutral species by solar extreme ultraviolet (EUV) and X-ray photons. Martian ionosphere has a stratified structure with two main layers in its electron density profile (Ne). The primary layer (M2 layer) is formed by solar EUV radiation (~20-90 nm) and has a peak electron density at around 120-140 km altitude with a peak density of ~1011 m-3. The second layer (M1 layer) occurs at lower altitude with a peak electron density of ~109 m-3 and is formed by solar soft X-ray and electron impact ionization. The electron densities and the altitudes at which these peaks are very variable. Radio Occultation (RO) experiments are the first and major explorer of the Martian ionosphere and electron density profiles. RO experiments precisely measure the Doppler shift observed on the link between a planetary spacecraft and Earth, as the spacecraft passes into occultation behind the planet. The RO data can be processed to extract the vertical electron density profiles of the ionosphere. Apart from vertical electron density profiles, RO can also provide density, temperature and pressure profiles of the neutral atmosphere. Thus, it is also a powerful tool to understand the both the neutral atmosphere and ionosphere. Here we study the effect of solar flares and coronal mass ejections on the whole Martian atmosphere using the publicly available Mars Express radio occultation data from ESA Planetary Science Archive (PSA). For this, we selected two major solar disturbed periods occurred during 6th to 13th of June 2011 and 16th February to 31st March 2015. Using the Level 2 data we calculated the Ne profiles in the ionosphere as well as the temperature profiles of lower atmosphere. We also compare our results with numerical models and available measurements like MARSIS and Langmuir probe on NASA's MAVEN in the selected period. Figure 1: a. Electron density profiles observed by MEX on 2011 following a solar event on the 8th of June 2011.

Some of the atmospheric processes occurring in the Ice Giant planets Uranus and Neptune critically depend on the electrical properties of their respective atmospheres. Therefore, the modelling of the electromagnetic properties of the atmosphere provides valuable information to characterize several aspects of the atmosphere. For example, lightning phenomena is an indicator of strong convective movements, and the number densities of electrons and ions affect the growth of aerosols and cloud formation. In the pressure range where such process take place the solar radiation has been mainly absorbed and the main ionization source is galactic cosmic rays. Furthermore, the long distance to the Sun decreases the role of solar radiation on the atmospheric electrical properties, while the cosmic ray flux is similar or even larger compared to that at the inner solar system. This implies a proportionally greater role of the galactic cosmic rays induced ionosphere for the ice giant planets than for the rest of solar system planets. Protons and alpha particles, which mainly compound the cosmic rays, can reach the Uranus and Neptune lower stratosphere to ionize the atmospheric constituents and thus produce a low altitude ionospheric layer. The flux and composition of cosmic ray incident on a planetary atmosphere depend on the distance to the Sun and varies over the 11-year solar activity cycle. Furthermore, the planetary magnetic field deflects the incoming cosmic rays depending on their rigidity and angle of incidence. Therefore, both factors solar cycle and magnetic latitude must be considered to calculate the ionization rate by cosmic rays. The presence of aerosols also affects the ion-neutral chemistry by capturing electrons and positive ions depending on the aerosol size and concentration. Here we present the first results obtained with a numerical model, which is based on previous studies for Mars and Titan [1-2], capable to calculate the number density of electrons and positive ions, as well the aerosol charging between 0.1 and 15 bars in the atmosphere of Neptune and Uranus. We have found that, for the particle density and size vertical profiles obtained in previous works [3-4], aerosols are negatively charged, and the number density of electrons is lower than that of positive ions for pressures lower than around 1 bar. References [1] Cardnell, S., et al. (2016), A photochemical model of the dust-loaded ionosphere of Mars, J. Geophys. Res. Planets, 121, doi:10.1002/2016JE005077. [2] Molina-Cuberos, G.J., et al. (2018) Aerosols: The key to understanding Titan's lower ionosphere, Planet. Space Sci, 153, 157 – 162, doi: 10.1016/j.pss.2018.02.007. [3] Toledo et al. (2019), Constraints on Uranus's haze structure, formation and transport, Icarus 0019-1035, doi: 10.1016/j.icarus.2019.05.018 [4] Toledo et al. (2020) Constraints on Neptune’s haze structure and formation from VLT observations in the H-band, Icarus, doi: 10.1016/j.icarus.2020.113808

Planetary ionospheres undergo many changes at dawn and dusk due to both photochemical and transport processes. The relative importance of these different processes vary depending on a variety of factors, including the amount of solar radiation, the composition of the thermosphere, and the characteristics of any local magnetic fields. This study uses the similarities between the ionospheres on Mars and Earth to examine the behaviour of the ionosphere at dawn and dusk. It has notable implications for comparative aeronomy, as a solid understanding of ionospheric processes on planets with and without magnetic fields is important for characterising the environments of solar and exoplanets, as well as atmospheric evolution over long time scales. The amount of plasma present in the ionosphere was measuring using the total electron content (TEC), and grouped so that both solstices and different phases of the solar cycle could be examined. To allow comparisons between the ionospheres of Mars and Earth, which differ greatly in density, the rate of change of TEC as a function of solar zenith angle was used to compare the plasma production and losses in the main layer of each planetary ionosphere. Examination of the dawn and dusk TEC slopes shows that, to first order, the Martian slopes are symmetric while those at Earth are not. This symmetry is interpreted as an indicator of photochemical equilibrium, and different reasons for deviations from symmetry were explored. The presence or absence of a magnetic field played a large role in shaping plasma transport, with photochemical processes in both ionospheres behaving similarly in the absence of a magnetic field. At Mars, it was found that transport processes were most important at solar maximum, while at Earth transport processes were most important at solar minimum.

Icy moons of Jupiter and Saturn are privileged targets in currently developed space missions by several space agencies and in particular ESA, NASA and their partners. One of these missions is ESA’s first large mission in the Cosmic Vision Programme, JUICE [1], which is being developed to address questions regarding the Jupiter system and its satellites, with a focus on the largest moon, Ganymede. The overarching theme for JUICE is the emergence of habitable worlds around gas giants taking into account the requirements involving the presence of organic compounds, trace elements, water, energy sources and a relative stability of the environment over time.Among other, JUICE will determine the characteristics (composition and dynamics) of the exospheres of the icy moons [2], in particular Ganymede and Europa, with for instance coordinated observations among sets of instruments like UVS, PEP, RPWI, MAJIS, 3GM, J-MAG, JANUS and SWI. The JUICE mission is scheduled to be launched in spring 2023 and arrive at Jupiter in mid-2031 and is foreseen to last nominally for 3 and a half years. JUICE investigations will benefit from current observations by JUNO and will be synergistic to NASA’s Europa Clipper mission. I will describe the foreseen investigations of the tenuous atmospheres of the icy moons around Jupiter.Cassini explored the dense and organic-laden atmosphere of Titan during several flybys over 13 years [2,3] and also determined the characteristics of the Enceladus plumes. However, new questions have risen and several cold cases [4] remain that will constitute major science objectives for future space missions to the satellites around Saturn, like Dragonfly [5] or an orbiter in the Saturn system or a dedicated Enceladus mission…In particular, Titan’s atmosphere has not yet revealed all its secrets, in particular for the chemical composition, which should be much more complex than what was detected by Cassini-Huygens. Future in situ measurements will be extremely useful in unveiling this unique complex world. In the meantime, ground-based observations with large telescopes like ALMA, elsewhere in Chile or the ones in Hawaii can help complement the past discoveries.References:[1] Coustenis, A., Witasse, O., Erd, C., 2021. The JUICE mission: expectations and challenges. Fall issue of The Bridge on space exploration, Sept. 2021, Vol. 51, issue #3, pp. 41-50.[2] Coustenis, A., Tokano, T., Burger, M. H., Cassidy, T. A., Lopes, R. M., Lorenz, R. D., Retherford, K. D., Schubert G., 2010. Atmospheres/exospheres characteristics of icy satellites. Space Sci. Rev., 153, 155-184. https://www.nae.edu/260902/The-JUICE-Mission-Challenges-and-Expectations[3] Coustenis, A., 2021. “The Atmosphere of Titan”. In Read, P. (Ed.), Oxford Research Encyclopedia of Planetary Science. Oxford University Press (August 31). doi: https://doi.org/10.1093/acrefore/9780190647926.013.120[4] Nixon, C. A., Lorenz, R. D., Achterberg, R. K., et al. (2018). Titan's cold case files - Outstanding questions after Cassini-Huygens. Planetary and Space Science, 155, 50-72.[5] Barnes, J. et al. (2021). Science Goals and Objectives for the Dragonfly Titan Rotorcraft Relocatable Lander. The Plan. Sci. J., VoL. 2, Issue 4, id.130, 18 pp.

International audience; We present the first long-term characterization of ionization layers in the lower ionosphere of Mars (below ∼90 km), a region inaccessible to orbital in-situ observations, based on an analysis of radar echo blackouts observed on Mars Express and the Mars Reconnaissance Orbiter from 2006 to 2017. A blackout occurs when the expected surface reflection is partly or totally attenuated for portions of an observation. Enhanced ionization at altitudes of 60-90 km, below the main ionospheric electron density peak, leads to increased absorption of the radar signal, resulting in the blackouts. We find that (a) MARSIS, operating at frequencies between 1.8 and 5 MHz, suffered more blackouts than SHARAD, which has a higher carrier frequency (20 MHz), (b) there is a clear correlation of blackout occurrence with solar cycle, (c) there is no apparent relationship between blackout occurrence and crustal magnetic fields, and (d) blackouts occur during both nightside and dayside observations, although the peak occurrence is deep on the nightside. Analysis of Mars Atmosphere and Volatile EvolutioN Solar Energetic Particle electron counts between 20 and 200 keV demonstrates that these electrons are likely responsible for attenuating the radar signals. We investigate the minimum SEP electron fluxes required to ionize the lower atmosphere and produce measurable attenuation. When both radars experience a blackout, the SEP electron fluxes are at their highest. Based on several case studies, we find that the average SEP spectrum responsible for a blackout is particularly enhanced at its higher energy end, that is, above 70 keV.

Vernazza, P., et al., Until recently, the 3D shape, and therefore density (when combining the volume estimate with available mass estimates), and surface topography of the vast majority of the largest (D ≥ 100 km) main-belt asteroids have remained poorly constrained. The improved capabilities of the SPHERE/ZIMPOL instrument have opened new doors into ground-based asteroid exploration. Aims. To constrain the formation and evolution of a representative sample of large asteroids, we conducted a high-angular-resolution imaging survey of 42 large main-belt asteroids with VLT/SPHERE/ZIMPOL. Our asteroid sample comprises 39 bodies with D ≥ 100 km and in particular most D ≥ 200 km main-belt asteroids (20/23). Furthermore, it nicely reflects the compositional diversity present in the main belt as the sampled bodies belong to the following taxonomic classes: A, B, C, Ch/Cgh, E/M/X, K, P/T, S, and V. Methods. The SPHERE/ZIMPOL images were first used to reconstruct the 3D shape of all targets with both the ADAM and MPCD reconstruction methods.We subsequently performed a detailed shape analysis and constrained the density of each target using available mass estimates including our own mass estimates in the case of multiple systems. Results. The analysis of the reconstructed shapes allowed us to identify two families of objects as a function of their diameters, namely spherical and elongated bodies. A difference in rotation period appears to be the main origin of this bimodality. In addition, all but one object (216 Kleopatra) are located along the Maclaurin sequence with large volatile-rich bodies being the closest to the latter. Our results further reveal that the primaries of most multiple systems possess a rotation period of shorter than 6 h and an elongated shape (c=a ≤ 0.65). Densities in our sample range from ∼1.3 g cm-3 (87 Sylvia) to ∼4.3 g cm-3 (22 Kalliope). Furthermore, the density distribution appears to be strongly bimodal with volatile-poor (P ≥ 2.7 g cm-3) and volatile-rich (P ≤ 2.2 g cm-3) bodies. Finally, our survey along with previous observations provides evidence in support of the possibility that some C-complex bodies could be intrinsically related to IDP-like P- and D-type asteroids, representing different layers of a same body (C: core; P/D: outer shell). We therefore propose that P/ D-types and some C-types may have the same origin in the primordial trans-Neptunian disk., We warmly thank the ESO staff at Paranal and in particular Henri Boffin for providing us precious support and advice throughout the entire observing programme. P.V., A.D., M.F., L.J. and B.C. were supported by CNRS/INSU/PNP. The work of T.S-R. was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This work was supported by the MINECO (Spanish Ministry of Economy) through grant RTI2018-095076-B-C21 (MINECO/FEDER, UE). TRAPPIST project is funded by the Belgian Fund for Scientic Research (Fond National de la Recherche Scientique, FNRS) under the grant PDR T.0120.21. E.J. is a FNRS Senior Research Associate. F.M. is supported by the National Science Foundation under Grant No. 1743015.

In Huybrighs et al., 2020 we investigated energetic proton depletions along Galileo's Europa flyby E26. Based on a particle tracing analysis we proposed that depletions are caused by perturbed electrogmagnetic fields combined with atmospheric charge exchange and possible plumes. One depletion feature identified as a plume signature was shown to be an artefact Jia et al., 2021. Despite that, here we emphasize that Huybrighs et al., 2020 demonstrates that plumes can cause proton depletions and that these features should be sought after. Furthermore, the conclusions on the importance of perturbed electromagnetic fields and atmospheric charge exchange on the depletions are unaffected. We suggest that the artefact's cause is a mistagging of protons as heavier ions by EPD. The artefact prevents us from confirming or excluding that there is a plume associated depletion. We also address comments on the MHD simulations and demonstrate that 540-1040 keV losses are not necessarily inconsistent with 115-244 keV losses by plume associated charge exchange., Comment: accepted to journal of geophysical research letters

[Aims] To interpret adaptive-optics observations of (216) Kleopatra, we need to describe an evolution of multiple moons orbiting an extremely irregular body and include their mutual interactions. Such orbits are generally non-Keplerian and orbital elements are not constants. [Methods] Consequently, we used a modified N-body integrator, which was significantly extended to include the multipole expansion of the gravitational field up to the order ℓ = 10. Its convergence was verified against the 'brute-force' algorithm. We computed the coefficients Cℓ m, Sℓ m for Kleopatra's shape, assuming a constant bulk density. For Solar System applications, it was also necessary to implement a variable distance and geometry of observations. Our χ2 metric then accounts for the absolute astrometry, the relative astrometry (second moon with respect to the first), angular velocities, and silhouettes, constraining the pole orientation. This allowed us to derive the orbital elements of Kleopatra's two moons. [Results] Using both archival astrometric data and new VLT/SPHERE observations (ESO LP 199.C-0074), we were able to identify the true periods of the moons, P1 = (1.822359 ± 0.004156) d, P2 = (2.745820 ± 0.004820) d. They orbit very close to the 3:2 mean-motion resonance, but their osculating eccentricities are too small compared to other perturbations (multipole, mutual), meaning that regular librations of the critical argument are not present. The resulting mass of Kleopatra, m1 = (1.49 ± 0.16) × 10-12 M· or 2.97 × 1018 kg, is significantly lower than previously thought. An implication explained in the accompanying paper is that (216) Kleopatra is a critically rotating body., Broz, M. et al., This work has been supported by the Czech Science Foundation through grant 21-11058S (M. Brož, D. Vokrouhlický), 20-08218S (J. Hanuš, J. Ďurech), and by the Charles University Research program No. UNCE/SCI/023. This material is partially based upon work supported by the National Science Foundation under Grant No. 1743015. P.V., A.D., M.F. and B.C. were supported by CNRS/INSU/PNP. M.M. was supported by the National Aeronautics and Space Administration under grant No. 80NSSC18K0849 issued through the Planetary Astronomy Program. The work of TSR was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This work was supported by the MINECO (Spanish Ministry of Economy) through grant RTI2018-095076-B-C21 (MINECO/FEDER, UE). The research leading to these results has received funding from the ARC grant for Concerted Research Actions, financed by the Wallonia-Brussels Federation. TRAPPIST is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (F.R.S.-FNRS) under grant FRFC 2.5.594.09.F. TRAPPIST-North is a project funded by the University of Liège, and performed in collaboration with Cadi Ayyad University of Marrakesh. E. Jehin is a FNRS Senior Research Associate.

Introduction The exploration of Callisto is part of the extensive interest in the icy moons characterization. As other missions will focus on Ganymede and Europa, mission proposals devoted to the exploration and characterization of Callisto are currently emerging [2][6]. Compared to the other Galilean moons, the outermost moon has kept the best-preserved records of the Jovian system formation, which makes it a candidate of choice to understand how the Jupiter system formed, and how it works. Led by the National Space Science Center (NSSC), Chinese Academy of Science (CAS), the Gan De mission [2] aims to send an orbiter around Callisto in order to map its gravity and magnetic field, and to characterize its exosphere, surface and interior. Callisto’s degree of differentiation will also be investigated as well as the possible existence of an internal ocean, as Galileo measurements suggested. Because the mission is at a very early planning stage, there is no definitive mission plan yet. In this context, we selected a small representative set of potential science orbits around Callisto. These orbits are tested under different assumptions in a detailed simulation environment, to analyse the orbit recovery as well as the recovery of Callisto gravity field and k2 Love number using tracking data. The effect of non-gravitational forces exerting on the orbiter is carefully considered, especially the strategy to handle them through the whole estimation process. Exploring different orbits Flybys of Callisto from the JUICE and the Europa Clipper missions will already provide further information on this moon [4]. However, the global mapping of the outermost Galilean moon will be uniquely improved by means of an orbiter at a high inclination and a low altitude. A specific difficulty in the orbit design in the Jovian system is related to the orbit stability, which is highly impacted by the influence of Jupiter as a third body exerting strong perturbations on a Callisto orbiter. Additionally, the occultations of the Sun by Callisto should be minimized to ensure a maximum solar illumination for power, which is critical this far from the Sun. This implies a minimum angle between the orbital plane w.r.t the direction of the Sun (βSun), which is larger the lower the orbit is. Depending on the position of the Earth at that time, this might have a direct impact on the angle between the orbital plane w.r.t the direction of the Earth (βEarth), which is of importance for orbit determination and gravity field recovery using Doppler tracking data. In this presentation, we selected various low Callisto orbits, which are suitable as science orbits for Gan De. These orbits have different altitudes, Earth beta angles, and injection date into orbit. Although several of them can be used sequentially, they are considered separately. A detailed force model The set of different reference orbits is propagated using the planetary extension of the Bernese Software (BSW) [3]. All the planets of the solar system, as well as the Galilean satellites, are considered as point masses. In the case of Jupiter, the zonal harmonics of the gravity field are also part of the force model. Because our knowledge of Callisto’s gravity field is restricted to the results of the Galileo mission (degree 2) [1], we have derived a synthetic gravity field as ground truth for our simulation. The scaled gravity field of the Earth’s Moon is used from degree 3 to 100. An artificial value of k2 Love number is also adopted to provide a ground truth. In order to rely on solar power this far from the Sun, the size of the solar panels is significant, which makes the effect of the solar and planetary radiation pressure on them essential to account for. However, the modelling of these forces can be challenging, particularly because of their dependency on the geometry and the optical properties of the probe. Even with a precise macro model, the optical properties of each plate can evolve with time. Several strategies are tested to handle these non-conservative forces, such as the use of pseudo-stochastics parameters and the possibility of an on-board accelerometer. Orbit determination and gravity field recovery Using a detailed noise model [5] and a realistic tracking station schedule, realistic Doppler tracking data (2-way Ka-band Doppler range rate) is simulated as measurements from the Deep Space Network. The non-gravitational accelerations in action during the orbit propagation are used to generate realistic accelerometer measurements. These observations are then used to reconstruct the orbit along with dynamical parameters in a global least-squares adjustment. Within this closed-loop simulation, we investigate the impact of the non-gravitational accelerations and orbit geometry on the orbit determination and the recovery of the geodetic parameters. The focus of this presentation is on the quality of the retrieved gravity field parameters and tidal Love number k2. Acknowledgements This study has been funded with the support of the Swiss National Foundation (SNF). References [1] Anderson, J. D., et al. "Shape, mean radius, gravity field, and interior structure of Callisto." Icarus 153.1 (2001): 157-161. [2] Blanc, Michel, et al. "Gan De: Science Objectives and Mission Scenarios for China's Mission to the Jupiter System." EGU General Assembly Conference Abstracts. 2020. [3] Dach, R. et al. "Bernese GNSS software version 5.2." (2015). [4] Di Benedetto, Mauro, et al. "Analysis of 3GM Callisto Gravity Experiment of the JUICE Mission." arXiv preprint arXiv:2101.03401 (2021). [5] Iess, Luciano, et al. "Astra: Interdisciplinary study on enhancement of the end-to-end accuracy for spacecraft tracking techniques." Acta Astronautica 94.2 (2014): 699-707. [6] Smith, David E., et al. "MAGIC, A Discovery Proposal to the Icy Moon Callisto." AGU Fall Meeting Abstracts. Vol. 2019. 2019.

International audience; We study the influence of the solar extreme ultraviolet (EUV) flux intensity on the precipitating ion fluxes as seen by the Solar Wind Ion Analyzer, an energy and angular ion spectrometer aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We defined three periods with significantly different EUV flux intensity (1.6 and 3.2 times the lowest EUV intensity) and compare the precipitating ion flux measured by MAVEN/Solar Wind Ion Analyzer during each period. At low energy [30–650] eV, we find that the median (average) precipitating ion flux during the medium and low EUV periods are, respectively, 1.7 (2.1) and 3 (3.5) times more intense than the flux during the high EUV period. At high energy [650–25,000] eV, a similar trend in the intensity of the precipitating ion flux is observed but with an increase by 50% (46%) and 70% (79%), respectively. A larger EUV flux does therefore not seem to favor heavy ion precipitation into Mars's atmosphere, contrary to modeling prediction and overall expectations.

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express, which operates between 0.1 and 5.5 MHz, suffered from a complete blackout for 10 days in September 2017 when observing on the nightside (a rare occurrence). Moreover, the Shallow Radar (SHARAD) onboard the Mars Reconnaissance Orbiter, which operates at 20 MHz, also suffered a blackout for three days when operating on both dayside and nightside. We propose that these blackouts are caused by solar energetic particles of few tens of keV and above associated with an extreme space weather event between 10 and 22 September 2017, as recorded by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Numerical simulations of energetic electron precipitation predict that a lower O-2(+) nighttime ionospheric layer of magnitude similar to 10(10) m(-3) peaking at similar to 90-km altitude is produced. Consequently, such a layer would absorb radar signals at high frequencies and explain the blackouts. The peak absorption level is found to be at 70-km altitude. Plain Language Summary Several instrument operations, as well as communication systems with rovers at the surface, depend on radio signals that propagate throughout the atmosphere of Mars. This is the case also for two radars that are currently working in Mars' orbit, sounding the ionosphere, surface, and subsurface of the planet. In mid-September 2017, a powerful solar storm hit Mars, producing a large amount of energetic particle precipitation over a 10-day period. We have found that high-energy electrons ionized the atmosphere of Mars, creating a dense layer of ions and electrons at similar to 90 km on the Martian nightside. This layer attenuated radar signals continuously for 10 days, stopping the radars to receive any signal from the planetary surface. In this work, we assess the properties of this layer in order to understand the implications of this kind of phenomenon for radar performance and communications.

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.

Radio Occultation (RO) is a very powerful technique to probe a planetary atmosphere, in providing vertical density profiles of the neutral atmosphere and ionosphere. The standard method uses a radio link between a spacecraft and an Earth ground station. Nevertheless, the possibility to obtain information about the Martian atmosphere with mutual RO events, using data from NASA Mars Odyssey and Mars Reconnaissance Orbiters (MRO), has been demonstrated by Ao et al. (2015).Taking advantage of two European spacecraft in orbit around Mars, the European Space Agency is currently preparing experiments of mutual RO between Mars Express (MEX) and the ExoMars Trace Gas Orbiter (TGO). In preparation of MEX and TGO data inversion and analysis, a simulation-based strategy has been adopted and an algorithm able to retrieve vertical electron density profiles from Doppler shift measurements has been implemented and validated. Subsequently, in order to test the mentioned algorithm with experimental data, the same three RO events considered in the paper by Ao et al. (2015) have been processed. In particular, for each RO event, having the information about the satellites’ orbit, the (excess) Doppler shift values corresponding to the Mars Odyssey-MRO ray-paths have been converted to bending angles as a function of impact parameter. Then, assuming a spherical symmetry (Fjeldbo et al., 1971) for the ionosphere electron density, the bending angles have been transformed (through Abel integral) to a vertical refractivity profile, which, in turn, has been converted to an ionospheric electron density profile.In this work, the results obtained by the application of the mentioned inversion algorithm to experimental data will be presented, with particular focus on the retrieval of the ionospheric electron density profiles. References Ao, C. O., C. D. Edwards Jr., D. S. Kahan, X. Pi, S. W. Asmar, and A. J. Mannucci (2015), A first demonstration of Mars crosslink occultation measurements, Radio Sci., 50, 997–1007, doi:10.1002/2015RS005750. Fjeldbo, G., A. J. Kliore, and V. R. Eshleman (1971), The neutral atmosphere of Venus as studied with the Mariner V radio occultationexperiments, Astron. J., 76, 123–140.

Ganymede is the only moon in our Solar System known to have its own global magnetic field, which generates a miniature moon magnetosphere inside the Jovian magnetosphere. Due to this unique characteristic of Ganymede, its auroral zone is also of particular scientific interest, as it is the only known example of this specific kind of interaction. The JUICE spacecraft will orbit Ganymede for almost a year, with a high inclination orbit with multiple auroral zone crossings. JUICE will study the auroral zone of Ganymede in more detail than ever before, providing both in-situ and remote sensing observations. In this work, we use Spacecraft Plasma Interaction Software (SPIS) simulations to study the spacecraft charging of JUICE in the auroral zone. Hubble Space Telescope observations of the aurora of Ganymede show localized regions of bright spots superimposed on a continuous background emission (e.g. Feldman et al. 2000, Eviatar et al. 2001). In order to produce bright auroras, the electron population needs to be accelerated up to hundreds of eV (Eviatar et al. 2001). Preliminary simulation results, using an auroral electron population with temperature Te = 200 eV and density ne = 300 cm-3, shows frame charging (i.e. spacecraft ground) of around 10 V and differential charging of around 30 V. High frame and differential potentials can cause disturbances in both particle and electric field measurements and prevent accurate characterization of the environment. Since the auroral zone of Ganymede is of particular scientific interest, it is important to study and prepare for this kind of disturbances. References D. Feldman et al., HST/STIS ultraviolet imaging of polar aurora on Ganymede, The Astrophysical Journal, 535(2), 2000 A. Eviatar et al., Excitation of the Ganymede ultraviolet aurora, The Astrophysical Journal, 555(2), 2001

Conventional Radio Occultation measurements take place between a spacecraft in orbit around a planet (or during an occasional fly-by), and a receiving ground station on the Earth. When the straight line between the sender and receiver crosses the ionosphere and the atmosphere of the planet the signal is affected by the electrons in the ionosphere and the molecules in the neutral atmosphere. This signal change is observed as a slight frequency shift in the received radio signal with respect to the transmitted signal. The shift is very small compared to the Doppler shift in the frequency due to the differential relative velocity of the spacecraft and the Earth, but can for a stable system be isolated, and profiles of electron density in the ionosphere and profiles of atmospheric density in the neutral atmosphere can be derived. Here we are attempting to do occultation measurements between two spacecraft at Mars, Mars Express and ExoMars Trace Gas Orbiter, using the UHF link normally used for communicating between the orbiters and the landers on the surface of Mars, rather than using the X- or S-band transmitter used for the communication between a spacecraft and the Earth. This was first exercised between NASA’s Odyssey and MRO at three occasions in 2007 (Ao, et al., Radio Sci., 2015), but has not been repeated since then. Such a Mutual Occultation technique has several significant advantages over the traditional spacecraft to Earth occultation measurements. In particular, it allows a much more even distribution of the samples over the surface of the planet and in time, and the observations are independent of Earth occultation seasons. Due to the short period of the orbit of TGO many occultations take place every day. The distance between the two spacecraft is much smaller than the distance to the Earth and therefore a much higher signal to noise ratio can be achieved, resulting in very low statistical errors. The Mutual Occultation s/c to s/c link also avoids that the signal passes through the Earth’s ionosphere and atmosphere. The compensation for the Earth atmosphere and ionosphere is far from perfect and results in major contributions to the systematic errors. An additional positive effect is that the interplanetary plasma disturbance to the signal is a minimum due to the short distance between the two s/c and the large distance from the Sun. The first experiment was carried out on the 2nd of November 2020 and demonstrated the feasibility of the technique. Several more experiments took place during November-December. These initial test used a lander (Beagle2) hail signal from Mars Express/Melacom, consisting of a series of 20 seconds of modulated and unmodulated transmissions interspersed with 2 seconds without transmission. In March 2021 a software update was applied to Melacom, which allowed it to generate an uninterrupted, unmodulated carrier-only signal, since. This has improved the performance and the quality of the measurements significantly. To date four passes with this improved performance have been executed. These data are presently being analysed. This presentation will summarise recent simulations and show the latest results from these new experimental measurements. Example of Occultation Geometry: The orbits of TGO and MEX are indicated by tan respective red lines and the direct path of the radio signal is indicated by a white line. The part of the propagation through the atmosphere is indicated by a yellow box.

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 (https://www.cosmos.esa.int/web/spice/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

Mars Express has been exploring Mars since late 2003 giving us fascinating views of the red planet as well as unique science insights. The long-life of Mars Express is an asset in order to understand the long-term evolution of the atmosphere of Mars, and in particular, of the ionosphere and upper atmosphere. Currently, we have about 16 years of ionospheric observations at Mars, which cover near two cycles of solar activity. This is the first time that such a long time series observations can be investigated in other planet rather than Earth. In particular, this work focuses on Mars’ ionospheric behaviour during two consecutive solar minima (23/24 and 24/25) using the same dataset. We use the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express to investigate the total electron content behaviour of the whole atmosphere in relation to key parameters, such as the solar flux, solar illumination, dust storms and Sun-Mars distance. The topside variability of the ionosphere is also investigated through variations in the neutral scale height, and results are supported by ionospheric modelling. The main goal of this study is to investigate whether the ionosphere during the recent solar minimum 24/25 was similar (and to which degree) to the ionosphere at solar minimum 23/24, which was characterised by a pronounced reduction of both the topside and bottomside regions.

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

ESA’s Mars Express spacecraft has been exploring the red planet since January 2004. The spacecraft is able to receive and transmit signals at both S-band and X-band. Changes in the radio signals properties while propagating through the atmosphere yield information on the electron density profile. The Mars’ ionosphere is very variable in time. One source of its variability is the solar activity. Here we study the effect of solar flares and coronal mass ejections on the Mars’ ionosphere using the publicly available Mars Express radio occultation data. A new software in python was developed to process the residual frequencies data sets, and was validated using higher-level archival data. We calculate the electron density profiles before, during, and after such solar events. The variations in ionospheric parameters (total electron content, peak density and altitude) are quantified in order to understand the ionospheric changes due to solar activity. As expected, significant variations in these parameters have been identified. The results are compared with additional measurements and models. Figure 1: Electron density profiles observed by MEX on 2011 during 2 days following a solar event.

JUICE - JUpiter ICy moons Explorer - is the first large mission in the ESA Cosmic Vision 2015-2025 programme. The mission was selected in May 2012, and is now (May 2021) in the final stages of its integration and testing campaigns. Following its arrival at Jupiter in July 2031, JUICE will spend 4.2 years making detailed observations of Jupiter and three of the Galilean moons, Ganymede, Callisto and Europa. We present here the plan of activities to be carried out during the interplanetary cruise phase. 1. Trajectory during the cruise phase The mission will be launched from Kourou with an Ariane 5. The baseline launch window is between 26 August – 15 September 2022, with a backup launch slot in August 2023. The interplanetary transfer sequence relies on gravity assist manoeuvres with Venus, Earth and the Moon. The Jupiter orbit insertion will be performed in July 2031. The cruise phase officially starts after the end of the near-Earth commissioning (launch + 3 months) and ends six months before the Jupiter orbit insertion, when the nominal science mission begins. The list of planetary gravity assists is given in Table 1: Date Planet Pericenter altitude [km] 01/09/2023 02/09/2023 Moon Earth 300 180,000 23/08/2024 Earth 13,400 31/08/2025 Venus 5,100 29/09/2026 Earth 8,600 18/01/2029 Earth 4,600 2. Spacecraft and mission constraints during the cruise phase The main limitations during the cruise phase are: Priority of operations in support of navigation and spacecraft safety, especially in the preparation of gravity assist manoeuvres, in particular the world premiere of Lunar-Earth gravity assist The thermal design of the spacecraft which imposes, for heliocentric distances lower than 1.34 AU, restricted pointing capabilities and limitations in the number of instruments that can operate simultaneously. Quiet Cruise baseline: Reduced number of ground contacts (one ground station pass per week except for planetary flybys). Operational constraint to minimize the use of the mass memory during Cruise to preserve unit lifetime, limiting the possibility to operate the instruments and store their data. Budgetary restrictions that result in small operation teams during this phase. The baseline operations of the instruments are two one-week checkouts per year. It is expected that at least some of the observations will take place during the planetary flybys, with different possibilities still under study. The requests for operating the payload beyond this baseline will be carefully analyzed and agreed on a best-effort basis. The next section gives example of possible observations. 3. Potential scientific investigations to be performed during the cruise phase Instrument operations during the cruise phase are always useful: they allow calibrating instruments in known environments (e.g. solar wind, Earth’s magnetosphere), checking possible interferences between instruments, they provide scientific results (sometimes not expected and outstanding, resulting in high standard publications) and attract public attention. During the long cruise phase of the JUICE mission, a number of scientific opportunities have been identified, beyond the obvious case of the planetary flybys. They include: solar wind campaigns, neutral atoms imaging in the heliosphere, measurement of Jovian escaping relativistic electrons, measurements of interplanetary dust, test of the general relativity around solar conjunctions, observation of the cosmological radio background, and an asteroid flyby (to be further studied). 4. JUICE teams activities during the cruise phase The JUICE teams (mission operation center, science operations center, instrument teams, science working team, working groups, project scientist and mission manager) will be busy with numerous activities. A non-exhaustive list is given below: Spacecraft navigation and operations in the inner solar system, including planetary flybys and payload checkouts; Decision about an asteroid flyby (shortly after launch); Analysis of the cruise data, instrument calibration, scientific analysis, publication and archive; Refinement and agreement (three years before Jupiter orbit insertion) on the trajectory within the Jupiter system; Preparation of the science planning of the nominal mission and of the science analysis of the future data; Studying and preparing the coordination with ground-based and space observatories; Publication of a special issue with mission, instruments and science articles; Recruitment of guest investigators and of potentially additional interdisciplinary scientists; Continue the fruitful collaboration with the NASA Clipper teams. Conclusions The JUICE cruise phase, despite its long duration (9 years), will be busy with many activities, with the goal of being fully ready to start the scientific observations from early 2031 onwards. Figures Figure 1: Evolution of distances (in AU) between the Sun, the Earth, Jupiter and JUICE as a function of time. Figure 2: Overall cruise projected in the ecliptic frame Figure 3: Lunar-Earth flyby in 2023 Figure 4: Illustration of the 300 km lunar flyby in 2023. The lunar and Earth disks are dark due the Sun position.

The elongated crater population on Mars is of great interest in planetary sciences [1]. In 1982, 1988 and 2000 three datasets were established using Viking images [1,2,3]. In 2011 a new and improved set of data with 261 craters and a database including up-to-date images taken by the High Resolution Stereo Camera (HRSC) onboard ESA’s Mars Express were established [4] and continuously updated [5]. Based on the crater shape [6] and the ejecta deposit [7] the direction of impact can be estimated. Since this is not always a straightforward determination, it was decided to check whether the topographic profile could help in ambiguous cases. 1. Introduction Impact cratering is one of the fundamental processes in shaping the surface of terrestrial planets. Even though most impact structures have a circular shape, a small fraction of elongated craters formed by oblique impacts can be observed. Using the recently developed Mars Express database, the current study focuses on the impact direction of elongated craters on Mars. 2. Motivation for the study One major driver for conducting this study is the ongoing discussion of the decaying moonlets hypothesis, which might be an explanation for at least a fraction of the elongated crater population on Mars, as well as the unsolved origin of the Martian moons, Phobos and Deimos [5,9]. 3. Direction of impact As a precondition the crater orientation is analysed and a strong correlation to the crater age can be stated. For the identification of the impact direction a two-stage process is applied and this presentation shows the usefulness of the procedure. A first estimation is based on the crater shape [6] and the ejecta deposit [7]. Figure 1 shows a crater example and table 1 the summarized results for the elongated crater population on Mars. Subsequently, the estimation is reviewed by an analysis of the topographic crater profiles, which result from the pressure distribution during impact [8]. Figure 2 presents the corresponding profile for the first example crater. This review process indicates the validation of the results of the first stage. For craters with morphological characteristics not allowing a distinct identification of the impact direction in stage one this approach helps to improve the dataset dramatically (figures 3 and 4). 4. Figures Figure 1: Estimation of impact direction based on crater shape and ejecta deposit for example crater 1 Figure 2: Altitude profile of example 1 based on HRSC MOLA blended DEM (200m) confirms East to West impact direction Figure 3: Distinct identification of impact direction based on crater shape and ejecta deposit difficult for example crater 2 Figure 4: Altitude profile indicates West to East impact direction 5. Table Table 1: Direction of impact Acknowledgements B. Buchenberger sincerely thanks O. Witasse for more than a decade of great cooperation and friendship that gave him the opportunity to work on such wonderful topics like this. References [1] Schultz, P. H. and Lutz-Garihan, A. B.: Grazing Impacts on Mars: A Record of Lost Satellites, Journal of Geophysical Research, Vol. 87, p. 84-96, 1982. [2] Barlow, N. G.: Crater Size-Frequency Distributions and a Revised Martian Relative Chronology, Icarus, Vol. 75, p. 285-305, 1988. [3] Bottke, W. F., Love, S. G. et al.: Interpreting the Elliptical Crater Populations on Mars, Venus and the Moon, Icarus, Vol. 145, p. 108-121, 2000. [4] Buchenberger, B., Witasse, O. et al.: Elongated craters on Mars revisited: Test of the decaying moonlets hypothesis? EPSC-DPS Joint Meeting, p. 738, 2011. [5] Sefton-Nash, E., Faes, Z. et al.: The Orbit Planes of Impactors that Formed Elongated Martian Craters, EPSC, p.286, 2017. [6] Burchell, M.J., Robin-Williams, R. et al.: The SMART-1 lunar impact, Icarus, Vol. 207, p. 28-38, 2010. [7] Melosh, H.J.: Impact Cratering - A Geologic Process, Oxford University Press, New York, p. 101, 1996. [8] Pierazzo, E. and Melosh, H. J.: Melt Production in Oblique Impacts, Icarus, Vol. 145, p. 252-261, 2000. [9] Craddock, R.A.: Are Phobos and Deimos the result of a giant impact? Icarus, Vol. 211, Iss. 2, p. 1150, 2011.

We present the first long-term characterization of the lower ionosphere of Mars, a region previously inaccessible to orbital observations, based on an analysis of radar echo blackouts observed by MARSIS on Mars Express and SHARAD on the Mars Reconnaissance Orbiter from 2006 to 2017. A blackout occurs when the expected surface reflection is partly to fully attenuated for portions of an observation. Enhanced ionization at altitudes of 60 to 90 km, below the main ionospheric electron density peak, results in the absorption of the radar signal, leading to a radar blackout. MARSIS, operating at frequencies between 1.8 and 5 MHz suffered more blackouts than SHARAD, which has a higher carrier frequency (20 MHz). More events are seen during solar maximum while there is no apparent relationship between blackout occurrence and crustal magnetic fields. Blackouts do occur during both nightside and dayside observations, and have an interesting variation with solar zenith angle. Analysis of MAVEN Solar Energetic Particle (SEP) electron counts between 20 and 200 keV during selected events demonstrates that these electrons are responsible for such events, and we investigate the minimum SEP electron fluxes required to ionize the lower atmosphere and produce measurable attenuation. When both radars observe a radar blackout at the same time, the SEP electron fluxes are at their highest. For certain events, we find that the average spectrum responsible for a blackout is particularly enhanced at the higher energy end of the spectrum, i.e. above 70 keV . This study is, therefore, important for future communications for human exploration of Mars.

The exploration of Callisto is part of the extensive interest in the icy moons characterization. Indeed, Callisto is the Galilean moon with the best-preserved records of the Jovian system formation. Led by the National Space Science Center (NSSC), Chinese Academy of Science (CAS), the planned Gan De mission aims to send an orbiter around Callisto in order to characterize its surface and interior. Potential orbit configurations are currently under study for the Gan De mission proposal.As part of a global characterization of Callisto, its gravity field can be inferred using radio tracking data from an orbiter. Mission characteristics such as orbit type, Earth beta angle and solar elongation will have a direct influence on the recoverability of its gravity field parameters. In this study, we will analyse this influence from closed-loop simulations using the planetary extension of the Bernese GNSS Softwareai.A number of reference orbits with different orbital characteristics will be selected for the Gan De mission and, using an extended force model, will be propagated from different starting dates and different initial Earth beta angles. Realistic Doppler tracking data (2-way X-band Doppler range rate) will be simulated as measurements from ground stations, with a dedicated noise model. These observations will then be used to reconstruct the orbit along with dynamical parameters. The focus of this presentation will be on the quality of the retrieved gravity field parameters and tidal Love number k2.

We study the interaction of three solar wind structures, two stream interaction regions and one interplanetary coronal mass ejection, with Mars' plasma environment during 20-27 November 2007. This period corresponds to the solar minimum between the solar cycles 23 and 24 which was characterized by very low values of the solar wind density and dynamic pressure and low IMF magnitude. During that time the Mars-Express orbit was in the terminator plane, while the Earth, Sun, and Mars were almost aligned, so we use the ACE and STEREO probes as solar wind monitors in order to identify and characterize the structures that later hit Mars. We find that the passage of these structures caused strong variations of in the bow shock location (between 2.2 and 3.0~R$_M$), compression of the magnetospheric cavity (up to 45~\%) and an increased transterminator flow below 2~R$_M$ (by a factor of $\leq$8). This study shows that during times of low solar activity, modest space weather phenomena may cause large variations of plasma flow at Mars., Comment: Published in Journal of Geophysical Research

The Mars' ionospheric behavior during two consecutive solar minima (23/24 and 24/25) is investigated with the same dataset. In particular, we use the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express to investigate the total electron content behavior of the whole atmosphere in relation to the solar irradiance (EUV and X-ray fluxes), the solar zenith angle and the heliocentric distance. The topside variability of the electron density profiles is also investigated through variations in the peak density and neutral scale height. Moreover, the equations of the NeMars empirical model of the Martian ionosphere for low solar activity are tested for both minima. We have found that the topside ionosphere of Mars behaved similarly at both solar minima. However, when considering the bottomside, a pronounced reduction in ionization in particular cases is suggested. In addition, larger TEC values are found during the solar minimum 24/25 in the nightside sector that may indicate possible larger plasma transport than during the minimum 23/24. Finally, this study confirms that the ionospheric empirical NeMars model equations derived by Sanchez-Cano et al. (2016) for the low solar activity period during the solar minimum 23/24 are also valid and accurate for the solar minimum 24/25. The long duration of Mars Express is a critical factor for determining the long-term Martian ionospheric variability, which in turn, is essential for understanding the global evolution of the planet's atmosphere.

Carri, B., et al., [Context] Dynamical models of Solar System evolution have suggested that the so-called P- and D-type volatile-rich asteroids formed in the outer Solar System beyond Neptune's orbit and may be genetically related to the Jupiter Trojans, comets, and small Kuiper belt objects (KBOs). Indeed, the spectral properties of P- and D-type asteroids resemble that of anhydrous cometary dust. Aims. We aim to gain insights into the above classes of bodies by characterizing the internal structure of a large P- and D-type asteroid. [Methods] We report high-angular-resolution imaging observations of the P-type asteroid (87) Sylvia with the Very Large Telescope Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument. These images were used to reconstruct the 3D shape of Sylvia. Our images together with those obtained in the past with large ground-based telescopes were used to study the dynamics of its two satellites. We also modeled Sylvia's thermal evolution. [Results] The shape of Sylvia appears flattened and elongated (a/b 1.45; a/c 1.84). We derive a volume-equivalent diameter of 271 ± 5 km and a low density of 1378 ± 45 kg m-3. The two satellites orbit Sylvia on circular, equatorial orbits. The oblateness of Sylvia should imply a detectable nodal precession which contrasts with the fully-Keplerian dynamics of its two satellites. This reveals an inhomogeneous internal structure, suggesting that Sylvia is differentiated. [Conclusions] Sylvia's low density and differentiated interior can be explained by partial melting and mass redistribution through water percolation. The outer shell should be composed of material similar to interplanetary dust particles (IDPs) and the core should be similar to aqueously altered IDPs or carbonaceous chondrite meteorites such as the Tagish Lake meteorite. Numerical simulations of the thermal evolution of Sylvia show that for a body of such a size, partial melting was unavoidable due to the decay of long-lived radionuclides. In addition, we show that bodies as small as 130-150 km in diameter should have followed a similar thermal evolution, while smaller objects, such as comets and the KBO Arrokoth, must have remained pristine, which is in agreement with in situ observations of these bodies. NASA Lucy mission target (617) Patroclus (diameter ≈140 km) may, however, be differentiated., This work has been supported by the Czech Science Foundation through grants 20-08218S (J. Hanuš, J. Ďurech) and by the Charles University Research program No. UNCE/SCI/023. The work of TSR was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This work was supported by the MINECO (Spanish Ministry of Economy) through grant RTI2018-095076-B-C21 (MINECO/FEDER, UE). This material is partially based upon work supported by the National Science Foundation under Grant No. 1743015.

Galactic Cosmic Rays (GCRs) are an intrinsic part of the heliospheric radiation environment, and an inevitable challenge to long-term space exploration. Here we show solar cycle induced GCR modulation at Mars in the period 2005-2020, along with GCR radial gradients, by utilising Mars Express and Rosetta engineering parameters compared to sunspot number time series. The engineering parameter used is called EDAC (Error Detection And Correction), a cumulative counter which is triggered by charged energetic particle causing memory errors in on-board computers. EDAC data provides a new way of gaining insight into the field of particle transport in the heliosphere, allowing us to circumvent the need for dedicated instrumentation as EDAC software is present on all spacecraft.This data set can be used to capture variations of GCRs in both space and time, yielding the same qualitative information as ground-based neutron monitors. Our analysis of the Mars Express EDAC parameter reveals a strong solar cycle GCR modulation, yielding an anticorrelation coefficient of -0.5 at a time lag of ~5.5 months. By combining Mars Express with Rosetta data, we calculate a 5.3% increase in EDAC count rates per astronomical unit, attributed to a radial gradient in GCR fluxes in accordance with established literature.The potential of engineering data for scientific purposes remains mostly unexplored. The results obtained from this work demonstrates, for the first time for heliophysics purposes, the usefulness of the EDAC engineering parameter, data mining and the utility of keeping missions operational for many years, providing complimentary data to nominal science instruments.

This is an open access article under the CC BY-NC-ND license., This work uses a subset of “quiet” MaRS ionospheric dayside observations (MaRSquiet, 2004–2017) and a 1-D photochemical model (IonA-2) to investigate the potential formation processes of the excess electron densities merged with the base of the main ionosphere (Mm). 42% of the investigated MaRS observations contain identified Mm, which occur in a large variety of shapes ranging from smoothly decreasing electron densities to peak structures below the base of M1. The Mm appear over the full range of accessible solar zenith angles (50° - 90°) and are found between approximately 70 and 110 km altitude. Their base is found on average deeper in the atmosphere than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the Mm formation on energy sources that penetrate deep into the atmosphere. This is supported by a strong positive correlation with increasing solar activity when solar flares, coronal mass ejections, and enhanced short solar X-ray and Ly-α intensities are more common. No relationship is found between the Mm occurrence rate and the magnitude/inclination of the weak crustal crustal magnetic field in MaRSquiet. Investigations with the IonA-2 photochemical model for undisturbed and flare conditions show that the ionization of the local neutral atmosphere by solar X-ray radiation, The Mars Express Radio Science Experiment (MaRS) is funded by the German Space Agency (DLR) under the Grant 50QM1802. Support for Mars Express Radio Science at Stanford University is provided by NASA through a JPL Contract. Support for the Multimission Radio Science Support Team is provided by NASA/JPL. Portions of this research were performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA. We thank everyone involved in the Mars Express project at ESTEC, ESOC, ESAC, JPL, and the ESTRACK and DSN ground stations for their continuous support. K. P. and M. P. acknowledge funding for this project by the Deutsche Forschungsgemeinschaft (DFG) under Grant PA 525/11-1, PA525/11-2 and PA 525/14-1. F.G.G. is funded by the Spanish Ministerio de Ciencia, Innovacion y Universidades, the Agencia Estatal de Investigacion and EC FEDER funds under project RTI2018-100920-J-I00, and 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 Astrofisica de Andalucia (SEV-2017-0709). Solar Irradiance Platform historical irradiances are provided courtesy of W. Kent Tobiska and Space Environment Technologies. These historical irradiances have been developed with partial funding from the NASA UARS, TIMED, and SOHO missions. We acknowledge support for work conducted on the development of the Mars Climate Database from the European Space Agency (under ESTEC Contract 11369/95/NL/JG(SC)) and from CNES. CNRS (LMD group), the IAA and the UK Particle Physics and Astronomy Research Council (AOPP, Oxford Group) also provided support during the development of the Martian General Circulation Models.

The comparative study of planetary systems is a unique source of new scientific insight: following the six “key science questions” of the “Planetary Exploration, Horizon 2061” long-term foresight exercise, it can reveal to us the diversity of their objects (Question 1) and of their architectures (Question 2), help us better understand their origins (Question 3) and how they work (Question 4), find and characterize habitable worlds (Question 5), and ultimately, search for alien life (Question 6). But a huge “knowledge gap” exists which limits the applicability of this approach in the solar system itself: two of its secondary planetary systems, the ice giant systems of Uranus and Neptune, remain poorly explored. Starting from an analysis of our current limited knowledge of solar system ice giants and their systems in the light of these six key science questions, we show that a long-term plan for the space exploration of ice giants and their systems will greatly contribute to answer these questions. To do so, we identify the key measurements needed to address each of these questions, the destinations to choose (Uranus, Neptune, Triton or a subset of them), the combinations of space platform(s) and the types of flight sequences needed. We then examine the different launch windows available until 2061, using a Jupiter fly-by, to send a mission to Uranus or Neptune, and find that: (1) an optimized choice of platforms and flight sequences makes it possible to address a broad range of the key science questions with one mission at one of the planets. Combining an atmospheric entry probe with an orbiter tour starting on a high-inclination, low periapse orbit, followed by a sequence of lower inclination orbits (or the other way around) appears to be an optimal choice. (2) a combination of two missions to each of the ice giant systems, to be flown in parallel or in sequence, will address five out of the six key questions and establish the prerequisites to address the sixth one: searching for life at one of the most promising Ice Giant moons. (3) The 2032 Jupiter fly-by window, which offers a unique opportunity to implement this plan, should be considered in priority; if this window cannot be met, using the 2036 Jupiter fly-by window to send a mission to Uranus first, and then the 2045 window for a mission to Neptune, will allow one to achieve the same objectives; as a back-up option, one should consider an orbiter + probe mission to one of the planets and a close fly-by of the other planet to deliver a probe into its atmosphere, using the opportunity of a future mission on its way to Kuiper Belt Objects or the interstellar medium; (4) based on the examination of the habitability of the different moons by the first two missions, a third one can be properly designed to search for life at the most promising moon, likely Triton, or one of the active moons of Uranus. Thus, by 2061 the first two missions of this plan can be implemented and a third mission focusing on the search for life can be designed. Given that such a plan may be out of reach of a single national agency, international collaboration is the most promising way to implement it.

We investigate nine Titan impact craters using Visual and Infrared Mapping Spectrometer (VIMS) data and a radiative transfer code (RT) [e.g. 1] in addition to emissivity data, in order to constrain the spectral behavior and composition of the craters. Past studies have looked at the chemical composition of impact craters either by using qualitative comparisons between craters [e.g. 2;3] or by combining all craters into a single unit [4], rather than separating them by geographic location or degradation state. Here, we use a radiative transfer model to first estimate the atmospheric contribution to the data, then extract the surface albedos of the impact crater subunits, and finally constrain their composition by using a library of candidate Titan materials. Following the general characterization of the impact craters, we study two impact crater subunits, the ‘crater floor’, which refers to the bottom of a crater, and the ‘ejecta blanket’, which is the material thrown out of the crater during an impact event. The results show that Titan’s mid-latitude plain craters: Afekan, Soi, and Forseti, in addition to Sinlap and Menrva are enriched in an OH-bearing constituent (likely water-ice) in an organic based mixture, while the equatorial dune craters: Selk, Ksa, Guabonito, and Santorini, appear to be purely composed of organic material (mainly unknown dune dark material). This follows the pattern seen in [4], where midlatitude alluvial fans, undifferentiated plains, and labyrinths were found to consist of a tholin-like and water-ice mixture, while the equatorial undifferentiated plains, hummocky terrains, dunes, and variable plains were found to consist of a dark material and tholin-like mixture in their very top layers. These observations also agree with the evolution scenario proposed by [3], wherein the impact cratering process produces a mixture of organic material and water ice, which is later “cleaned” through fluvial erosion in the midlatitude plains; a cleaning process that does not appear to operate in the equatorial dunes, which seem to be quickly covered by a thin layer of sand sediment. This scenario agrees with other works that suggest that atmospheric deposition is similar in the low-latitudes and midlatitudes on Titan, but with more rain falling onto the higher latitudes causing additional processing of materials in those regions [e.g. 5]. In either case, it appears that active processes are working to shape the surface of Titan, and it remains a dynamic world in the present day.[1] Hirtzig, M., et al. (2013). Icarus, 226, 470–486; [2] Neish, C.D., et al. (2015), Geophys. Res. Lett. 42, 3746–3754; [3] Werynski, A., et al. (2019), Icarus, 321, 508-521; [4] Solomonidou, A., et al. (2018), J. Geophys. Res, 123, 2, 489-507; [5] Neish, A.C., et al. (2016), Icarus, 270, 114–129.