The formation history of giant planets inside and outside the Solar System remains unknown. We suggest that runaway gas accretion is initiated only at a mass of ∼100 M_⊕ and that this mass corresponds to the transition to a gas giant, a planet whose composition is dominated by hydrogen and helium. Delayed runaway accretion (by a few million years) and having it occurring at higher masses is likely a result of an intermediate stage of efficient heavy-element accretion (at a rate of ∼10⁻⁵ M_⊕ yr⁻¹) that provides sufficient energy to hinder rapid gas accretion. This may imply that Saturn has never reached the stage of runaway gas accretion and that it is a “failed giant planet”. The transition to a gas giant planet above Saturn’s mass naturally explains the differences between the bulk metallicities and internal structures of Jupiter and Saturn. The mass at which a planet transitions to a gas giant planet strongly depends on the exact formation history and birth environment of the planet, which are still not well constrained for our Solar System. In terms of giant exoplanets, the occurrence of runaway gas accretion at planetary masses greater than Saturn’s can explain the transitions in the mass-radius relations of observed exoplanets and the high metallicity of intermediate-mass exoplanets.

How to cite: Helled, R.: Is Saturn a failed gas giant?, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-31, https://doi.org/10.5194/epsc2024-31, 2024.

Studying the interiors of giant planets is important for understanding their origin. While accurate measurements of gravity field and seismology from Juno and Cassini teach us about the present-day internal structures of Jupiter and Saturn, further insights can be gained from simulating their evolution. Understanding the long-term thermal evolution of the giant planets is also critical to connect the planetary formation and the current-state structure of planets.

In Jupiter and Saturn, a phase separation between hydrogen and helium occurs, resulting in the formation of helium droplets which settle towards the deep interior. This phenomenon, also known as helium rain, can strongly affect their thermal evolution. Therefore, studying the evolution of Jupiter and Saturn, and giant planets in general, requires a good comprehension of the phase diagram of hydrogen and helium. However, the latter remains uncertain given the discrepancy between various theoretical calculations and experimental data.

I will present results of evolution models of Jupiter and Saturn with helium rain, using different phase diagrams. We find that a consistency between Jupiter's evolution and the Galileo measurement of its atmospheric helium abundance is achieved only if a shift in temperature in the existing phase diagrams is applied. We next use the shifted phase diagrams to model Saturn's evolution and find consistent solutions for both planets. We confirm that demixing in Jupiter is modest while in Saturn, the process of helium rain is significant. I will discuss the inferred structure of both planets, the inferred atmospheric helium content in Saturn and show the importance of equations of state and atmospheric models on evolution models. This analysis reveals key information on the interiors of giant planets and can also be used to constrain the phase diagram of hydrogen and helium.

How to cite: Howard, S., Müller, S., and Helled, R.: The evolution of Jupiter and Saturn with helium rain, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-181, https://doi.org/10.5194/epsc2024-181, 2024.

In the atmospheres of the Solar System planets, several kinds of atmospheric waves can be found. They are an important atmospheric phenomenon as, besides disturbing the atmosphere, they can also alter the mean structure and affect the general circulation [1]. Therefore, understanding their effects on the atmospheric dynamics is of paramount importance.

The thermospheres of the Giant Planets are hotter than what would be expected if solar heating were considered the only energy source. Historically, the few existing numerical models have had difficulties to reproduce the observed temperature structure at mid and low latitudes. It was long thought that waves propagating vertically might play an important role in determining the temperature by either heating the thermosphere [2, 3] or by weakening the intense high-latitude westward jets allowing the meridional transport of energy trapped in the polar regions [4, 5]. Recently, the detection of gravity waves in Saturn's thermosphere has been reported using data from Cassini INMS and UVIS occultation data [4, 5, 6].

The Saturn Thermosphere-Ionosphere Model (STIM) is the only 3D general circulation model published so far for Saturn's upper atmosphere [7, 8]. The model couples dynamically and chemically the thermosphere and the ionosphere. In this work, we have used the STIM model to perform high resolution numerical simulations of Saturn's thermosphere. They show the development of supersonic spiral gravity waves in the northern and southern auroral regions driven by the high latitude forcing produced by Joule heating and ion drag. We analyze the morphology and properties of these waves, their propagation and the forcing they impose on the mean flow. We also show simulations with forcing introduced at the bottom of the model to explore waves that can propagate through the thermosphere and their potential impact on the circulation.

References:

[1] Vallis. Atmospheric and Ocean Fluid Dynamics. Cambridge University Press, Cambridge, UK, 2006.

[2] Young et al. Gravity waves in Jupiter's thermosphere. Science, 276, 1997.

[3] O'Donoghue et al. Heating of Jupiter's upper atmosphere above the Great Red Spot. Nature, 536, 2016.

[4] Müller-Wodarg et al. Atmospheric Waves and Their Possible Effect on the Thermal Structure of Saturn's Thermosphere. Geophysical Research Letters, 46, 2019.

[5] Brown et al. Evidence for gravity waves in the thermosphere of Saturn and implications for global circulation. Geophysical Research Letters, 49, 2022.

[6] Brown et al. A pole-to-pole pressure-temperature map of Saturn's thermosphere from Cassini Grand Finale data. Nature Astronomy, 4, 2020.

[7] Müller-Wodarg et al. A global circulation model of Saturn's thermosphere. Icarus, 180, 2006.

[8] Müller-Wodarg et al. Magnetosphere-atmosphere coupling at Saturn: 1 – Response of thermosphere and ionosphere to steady state polar forcing. Icarus, 221, 2012.

How to cite: Iñurrigarro Rodriguez, P., Medvedev, A. S., Müller-Wodarg, I. C. F., and Moore, L.: Numerical simulations of atmospheric waves in Saturn's upper atmosphere, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-234, https://doi.org/10.5194/epsc2024-234, 2024.

Cassini observations of the micrometeoroid bombardment flux, ring mass and fractional pollution constrain the origin and history of Saturn’s rings. In the simplest model, the age of the rings can be estimated by assuming the rings are a closed system with constant bombardment at the current rate. However, the rings are not a closed system and Cassini spectroscopy is consistent with space weathering of the cosmic dust polluting the rings. The remote sensing of the rings shows a red slope, with higher pollution at the shortest wavelengths, consistent with reddening due to space weathering of atmosphereless bodies. If processes at the time of the micrometeorite impacts or subsequent chemical and physical weathering can degrade the original pollutants, this means that laboratory spectra are not appropriate to determine the total extrinsic material that has struck the rings over their lifetime. Laboratory results for irradiation of icy outer solar system analogues indicate oxidation of organics and other pollutants over time. It is now generally agreed that the radiolysis of ice by energetic ions, electrons and solar UV photons produces the oxygen, ozone and peroxide seen at many icy satellites. The porosity of ice provides sufficient space for chemical reactions and mobility. The ring particle surfaces are in addition continually gardened by particle collisions and meteoritic impacts. Because of the loss processes, the current fractional pollution provides only a lower limit on the total integrated pollution flux, and thus a lower limit for the ring age. Rosetta data on the dust composition and surface reflectivity of Comet 67P provide our starting point for the composition of the bombarding material: Two independent analyses of Cassini UVIS spectra of Saturn’s rings give fractional pollution in the outer B ring of 1.4 -3%. This provides a lower limit of 400 to 1600 million years for the most opaque parts of Saturn’s B ring, depending on whether we use the maximum or minimum values for the bombardment rate reported by Cassini CDA. This result shows Saturn’s rings may be as old as the planet Saturn itself, consistent with the estimates for the orbital evolution of ring moons Atlas and Epimetheus. The A and C rings may have formed more recently.

How to cite: Esposito, L. W., Elliott, J. P., and Bradley, E. T.: Space Weathering Provides a Lower Limit on the Age of Saturn’s Rings, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-562, https://doi.org/10.5194/epsc2024-562, 2024.

One of the most exciting and controversial aspects of Saturn’s magnificent rings is that they may actually be a recent phenomenon in the solar system – forming long after the Earth, Saturn, and its moon. This possibility has been vigorously debated for nearly 40 years, since the Voyager flybys of Saturn. Over the years, the most powerful support for this hypothesis has turned out to be the puzzle of the rings’ nearly pure water ice composition – unique in the family of planetary rings – in spite of the constant hail of rocky-carbon meteoroids from outside the Saturn system. However, three major uncertainties have left the young-ring hypothesis unproven. Two of these have already been resolved by the Cassini mission: the amount of non-icy material currently in the rings, and the total ring mass. The third main constraint is the mass flux of non-icy meteoroids falling onto the rings. 

Measuring this mass flux was always a main science goal of the Cassini mission, and could only be achieved by the Cassini Cosmic Dust Analyzer instrument (CDA). In this talk we will report about the determination of the mass flux of non-icy material coming into the Saturn system, which completes the trifecta of constraints that are required to strongly support a youthful ring system [1]. The measurements present a thorough and detailed analysis of the series of unconnected individual particle detections by CDA over Cassini’s entire 13 year mission, converting these detections into the desired mass flux. The CDA detections determine the incident particle orbits, and they come (surprisingly) not from comets as expected, but mostly from Kuiper Belt Objects. This means that most of the particles have low speeds relative to Saturn and are strongly focused gravitationally, such that the flux at the rings is even larger than previously estimated. The derived mass flux implies a ring exposure time of less than 100 to 400 million years, which is in support of recent ring formation scenarios.

How to cite: Kempf, S., Altobelli, N., Schmidt, J., Cuzzi, J., Estrada, P., and Srama, R.: How old are Saturn’s rings?, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-920, https://doi.org/10.5194/epsc2024-920, 2024.

Introduction: Enceladus’ activity is concentrated across its south polar terrain (SPT). The activity is concentrated along four fractures dubbed tiger stripes. Constraining the magnitude of the endogenic emission from this region has proved challenging, with estimates varying from 4.2 GW to 5.8 ± 1.9 GW and 15.8 ± 3.1 GW [1-3].

The reason for the discrepancy is due to how different studies disentangled the total (passive and endogenic) emission observed to isolate the endogenic component. In the 15.8 GW study estimated the magnitude of the passive emission through modelling and then subtracted from the total. The other studies instead assumed the low-temperature passive component was negligible by only considering only high-temperature (>80 K) endogenic emission across the SPT (5.8 GW), or just the tiger-stripes (4.2 GW).

So the different studies could not be contradicting each other. It’s feasible that the total endogenic emission is ~15.8 GW, but the tiger-stripe only emission is ~4.2 GW with the remaining ~11 GW coming from elsewhere. The most logical place for this extra emission is the region between the stripes, called interstitial (or funiscular) terrain. Thus, it is emission from this is the area that we focus upon in this work.

Data: Cassini Composite Infrared Spectrometer (CIRS) was highly sensitive to the peak power of their blackbody emission. CIRS was a Fourier transform spectrometer with three focal planes covering 10 to 1400 cm^-1 (7.1 to 1000 μm) [4]. Focal plane 1 (FP1) covered 10 to 600 cm^-1 (16.7 to 1000 μm) with a spatial reolution of 3.9 mrad, enabling it to be sensitive to thermal radiation above 40 K. This made FP1 particularly sensitive to the typical solar heated surface temperatures of the icy satellites and thus the focus of this work  . The specific CIRS FP1 data to be analysed is given in Table 1. An example of the CIRS coverage for a single scan is shown in Figure 2.

Table 1 – Details of the CIRS FP1 observations to be analysed. All encounters have a single FP1 scan except for Orbit 136 (6 scans) and Orbit 228 (4 scans).

Figure 1 – Footprints (shown by black elipses) of CIRS FP1 over Enceladus’ tiger stripe fractures taken during Orbit 136, first scan. The names of the tiger stripes are given. The solid line indicates the location of the center of the FP1 field of view, where the detector is most sensitive.

Method: Seasonal thermophysical models [5] are run over a range of expected thermophysical properties. The model accounts for Saturnshine and eclipses. The surface temperatures predicted by these models is then determined for each observation. Accounting for the Gaussian response of FP1 the surface temperatures are translated into a predicted radiance for each CIRS footprint and each set of thermophysical properties. These are then compared to the CIRS observed radiances to determine how well they agree.

Models that can fit the CIRS data close to, but not inside the SPT, imply they have thermophysical parameters consistent with those observed close to Enceladus’ SPT and thus provides a reasonable estimate of their value.

For these “good fit” models the difference between the observed and predicted radiances inside of the active regions can then be determined, to provide a constraint on the endogenic emission observed there.

Results: The analysis is ongoing, but here we present new results of a single scan taken during Orbit 136. The ground coverage of this scan was already shown in Figure 1. Model temperatures were obtained for albedos ranging from 0.7 to 0.9 in 0.25 increments, and between 5 and 100 MKS in 5 MKS increments below 50 MKS and 10 MKS increments above it. A subset of the model results are compared to the CIRS-derived temperatures in Figure 2.

The results show that the location of the tiger stripes is obvious in the FP1 data. The location of each stripe (indicated by vertical dotted line) coincides with a distinct increase in surface temperature ~10 K above the background. Between the fractures the temperature decreases to close to non-SPT temperatures. All model parameter sets are able to predict the CIRS observed temperatures at the start of the scan (outside the SPT). So none can be ruled out based on this study alone. However, it is obvious that the discrepancy between the predicted temperatures and those observed across the tiger stripes is highly model dependent.

Figure 2 – Model temperatures (red) compared with CIRS-derived surface temperatures (black). The distance given is from the start point (shown in Figure 1) following the FP1 ground track (shown by the solid line cutting through the ellipses in Figure 1). The dotted vertical lines indicate where an observation cuts across a fracture, from left to right these are: Damascus, Baghdad and Cairo. Note there is no model fit to the first six observations as they cover latitudes currently not modelled.

The difference between the model and observed temperatures is endogenic emission, so each model will predict a different endogenic emission. The range in endogenic emissions these models predict for this swath is between 0.02 and 0.45 MW km^-2, with a mean value of 0.13 MW km^-2. Assuming this mean value, and an SPT area of 13,500 km² (the area polewards of 60  S) and such emission levels are constant across the active region then this implies the SPT could have a background endogenic emission of ~1.7 GW. We note this estimate is very preliminary and additional analysis will better constrain the thermophysical property regime and the spatial variation of the emission.

Acknowledgments: This work is funded through NASA CDAP 80NSSC19K0885.

References: [1] Spencer, J.R. et al., In Enceladus and the Icy Moons of Saturn, University of Arizona, 2018. [2] Spencer, J.R. et al., Science 311, 1401-1405, 2006. [3] Howett, C.J.A. et al., Journal of Geophysical Research 116, E03003, 2011. [4] Flasar, F.M.et al., Space Science Reviews 115, 169-297, 2004, [5] Spencer et al., Icarus 78, 337-354, 1989.

How to cite: Howett, C., Nimmo, F., Spencer, J., and Miles, G.: Constraining Enceladus' Endogenic Emission, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-981, https://doi.org/10.5194/epsc2024-981, 2024.

Introduction

Enceladus exhibits several remarkable characteristics, including a unique cryovolcanic activity [1] accompanied by anomalous heat flux [2], large ice shell thickness variations [e.g., 3], and a complex geological history recorded on the surface [4]. These features result from a complex interplay of internal and external forces operating over various temporal scales. Here, our main objective is to provide a comprehensive synthesis of various processes affecting the stress state of the ice shell from tidal to geological time scales. We study the individual contributions and assess their relative importance. We focus on the eccentricity-driven diurnal stresses enhanced by frictional faults [5-6]  and the effect of viscoelasticity. On longer time scales, we assess the impact of non-synchronous rotation [7]. We also investigate the possible role of the long-term viscous flow [3] driven by gravitational instabilities due to the variable ice shell thickness.

Model set-up

To achieve the objectives, we model the elastic or viscoelastic response of the ice shell with variable ice shell thickness with frictional faults to eccentricity tides and non-synchronous rotation. On long-time scales, we investigate the viscous flow resulting from the ice shell thickness variations [3]. In all cases, we employ FE method [8] in order to include the 3D character of the ice shell thickness variations and the presence of faults.

Eccentricity tides

To assess the stress associated with eccentricity tides, we account for variations in ice shell thickness and the presence of frictional faults. Considering the short time scale of 1.37 days and assuming a conductive temperature profile, we model the ice shell's rheology as elastic. The mechanical properties of the fault zone are approximated using a Mohr-Coulomb-type yield criterion [5]. During one orbital period, stress reaches up to 100 kPa in the vicinity of faults and for a low friction coefficient (Fig. 1). The stress regime changes from compressional to extensional and vice versa with depth mainly in the direction parallel to the faults due to the bending stresses. Due to nonlinear friction law, the response is coupled over various periods, effectively contributing to stresses on other time scales. In particular, the friction leads to the emergence of a background static stress in the model. The amplitude of the background stress increases with the friction coefficient and can become comparable to the diurnal component for high friction coefficients.

Non-synchronous rotation

The stress associated with possible non-synchronous rotation (NSR) of Enceladus is examined, taking into account variations in the ice shell thickness. This analysis considers a time scale ranging from 0.01 to 1 million years (Myr) for the period of NSR, as indicated by Patthoff et al. [7]. In our study, we adopt a viscoelastic (Maxwell) rheology, where the viscosity follows an Arrhenius dependence on temperature. A conductive temperature profile is assumed to characterize the thermal dependence of the ice rheology. Our simulations predict that the stress induced by NSR can reach approximately 4 MPa, dominating in stress magnitude among the investigated mechanisms. Furthermore, due to viscoelastic relaxation on NSR periods, a low-stress layer develops near the bottom boundary in the region of decreased viscosity (Fig. 2). However, this layer's impact on surface stress is found to be limited across all studied cases (bottom viscosity ≥10¹⁴ Pas). These findings highlight the importance of considering non-synchronous rotation, if present, and its associated viscoelastic effects when evaluating stress dynamics in Enceladus' icy shell.

Viscous flow

We quantify the flow and stress triggered by gravitational instabilities arising from uneven ice shell thickness [3]. On a geological time scale, we model the ice shell as a viscous fluid using the Boussinesq approximation while also incorporating the gravitational signal from the boundaries. We observe significant changes in the flow characteristics, transitioning from radial high-velocity flow (low viscosity contrast) to low-velocity tangential flow concentrated within a thin layer of low viscosity (high viscosity contrast). The maximum surface stress associated with the viscous flow is approximately 0.5 MPa (Fig. 3). Our results also emphasize the dependence of surface stress on the local ice shell thickness, with compressive stress associated with locally thin ice shells.

Summary

Our findings suggest that non-synchronous rotation induces the most significant stress, up to approximately 4 MPa, while surface stress from viscous flow reaches around 0.5 MPa, with compressive regimes in areas of the thinnest ice shell. Eccentricity tides induce comparatively lower stress, peaking at around 100 kPa in the South Polar Terrain. Frictional faults contribute to background stress, influenced by rheological properties, with viscoelastic effects proving negligible. This comprehensive assessment sheds light on the complex dynamics shaping Enceladus' icy surface.

Acknowledgements

This study received funding from the Czech Science Foundation under project number 22-20388S. The computations were conducted at the IT4Innovations National Supercomputing Center (e-INFRA CZ, ID:90254).

References

[1] Porco et al. (2006). Science 311, pp. 1393–1401. doi:10.1126/science.1123013

[2] Spencer et al. (2018) Enceladus and the Icy Moons of Saturn, pp. 163–174. doi:10.2458/azu_uapress_9780816537075-ch008 

[3] Čadek et al. (2019). Icarus 319, pp. 476–484. doi:10.1016/j.icarus.2018.10.003 

[4] Crow-Willard and Pappalardo (2015) JGR-Planets 120, pp. 928–950. doi:10.1002/2015JE004818 

[5] Pleiner Sládková et al. (2021), GRL 48, e2021GL094849. doi:10.1029/2021GL094849 

[6] Berne et al. (2023). JGR-Planets 128.6, 2022JE007712. doi:10.1029/2022JE007712 

[7] Patthoff et al. (2019). Icarus 321, pp. 445–457. doi:10.1016/j.icarus.2018.11.028 

[8] Alnaes et al. (2015). Archive of Numerical Software 3.100, pp. 9–23. doi:10.11588/ans.2015.100.20553

How to cite: Behounkova, M., Soucek, O., Choblet, G., Tobie, G., and Kihoulou, M.: Deformation of Enceladus’ Ice Shell: From Tidal Forces to Viscous Flow, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-102, https://doi.org/10.5194/epsc2024-102, 2024.

Introduction and Motivation

The Cassini probe (2004-2017) had onboard a RADAR operating both in an active and passive (or radiometry) mode at 2.2-cm wavelength. In its active mode, the RADAR recorded the backscatter from the surface (through the normalized radar backscatter cross section σ⁰) while in its passive mode it measured brightness temperatures (T_b). Both resolved and unresolved observations of Enceladus have concluded on the extremely radar-brightness of Enceladus, the largest in the Solar system [1,2]. Such radar-brightness can be partially explained by the presence of ultra-clean water ice particles at Enceladus’s surface, in particular in the SPT (South Polar Terrain), which offers a favorable medium for scattering. Nevertheless, so far none of the purely random wave scattering models have succeeded to reproduce the measured σ⁰ or radar albedos. Therefore, it remains an outstanding question to identify the scattering structures responsible for such high radar returns [4, 5]. Furthermore, during Cassini’s unique close flyby of Enceladus’ SPT (E16 flyby) swathing an area of few tens of kilometers, the Cassini RADAR used as a radiometer revealed thermal anomalies that had not been detected in the infrared [3].  However, the amplitude of the internal heat flux remains to be constrained.

To understand better both the scattering and thermal anomalies of Enceladus’ SPT, we developed a model able to jointly simulate Cassini active and passive observations. Comparison to the E16 data provide new insights into the compositional, thermal and structural properties of the subsurface of Enceladus’ SPT.

Method

To predict the backscatter (σ⁰) and thermal emission in the microwave domain we combine two models: (1) a thermal model providing depth profiles of the physical temperature beneath the surface at the E16 flyby epoch, and (2) a radiative transfer model to simulate both active and passive observations.

Thermal Model – We adapted a multilayer thermal model called Multi-layered Implicit Heat Transfer Solver to the case of Enceladus. This model accounts for Solar flux at the surface and a possible endogenic heat flux from the bottom [4]. The subsurface of Enceladus is modeled as a bi-layer medium with an icy porous regolith overlying a denser water ice substrate. The main parameters of the model are the porosities of the top and bottom layers ϕ₁ and ϕ₂ (which primarily control the effective thermal properties), the thickness d of the top layer and the endogenic heat flux (F_endo). Based on plume deposition rate modeling [5] a value of up to 90% was assumed for ϕ₁. The thickness d is unknown, but [5] suggest it could be up to 700 meters and at least few decimeters based on [6]. We thus vary d from 10 cm to 500 meters. Lastly, F_endo is assumed to be in the range 0 - 0.5 W/m² based on both infrared and microwave observations [7,3].Fig. 1 displays simulated temperature profiles obtained for ϕ₁ = 60%, ϕ₂= 10%, d = 10 m at the SPT for different values of F_endo. As F_endo increases, temperatures at depth increase up to values larger than 100 K. Further, the temperature profiles clearly show a discontinuity at the transition from the top to the bottom layer, the top layer acting like an insulating layer (especially if very porous). Additional analyses with different parameter combinations will be presented. The thermal model outputs are used as inputs for the radiative transfer model.

Fig. 1: Temperature profiles for a bi-layer subsurface model at latitude=-60°, longitude=150° at the epoch of the E16 flyby. And δ_top/bottom^{season/diurnal} are the diurnal and seasonal skin depths for both layers.

RT Model - We used the Snow Microwave Radiative Transfer model, a multi-layer RT model initially designed for snow or sea-ice [8]. The assumed effective permittivity of the water ice subsurface depends on its porosity and includes possible contaminants fraction, here assumed as organic dust. The parameters of the RT model are thus the dust fraction and the water ice grain radius size (r₁, r₂) in the top and bottom layers. It is consistent with the assumptions on ϕ₁, ϕ₂ and d used for the thermal model. We vary r  from 50 microns to 1 mm [9].

Preliminary Results

Fig. 2: SMRT outputs for different cases noted in subplot legends and titles. Radar backscatter (σ_VV⁰ ) and brightness temperature (T_b) is for vertical polarization.

Fig. 2 presents outputs from the SMRT model in the case of a monolayer water ice subsurface with a constant porosity of 50% and the geometry of observation of the E16 flyby. Each simulation points corresponds to different locations at the SPT. We investigated the effect of r, F_endo and dust fraction on both σ⁰ and T_b. As expected, these quantities are anti-correlated. In particular, as the r  increases volume scattering increases leading to larger σ⁰and smaller T_b. Decreasing the dust fraction has a similar effect as dusts are more absorbent than pure water ice and thus not favorable to volume scattering. Adding an endogenic heat flux has primarily an effect on T_b.

Fig. 3 displays the comparison between the E16 active and passive Cassini RADAR observations and outputs from the model used in monolayer configurations. While the T_b range of values is well reproduced, the model predicts σ⁰ much smaller than the ones measured. In the presentation we will explore more assumptions.

Figure 3: Comparison of the data and model outputs obtained for different combinations of parameters (1. dust = 5%, r = 0.1 mm, F_endo=0, Θ=50^◦, 2. dust = 5%, r = 1 mm, F_endo=0, Θ=50^◦, 3. dust = 5%, r = 1 mm, F_endo=0.05W/m², Θ=50^◦, 4. dust = 0%, r = 0.2 mm, F_endo=0.05W/m², Θ=50^◦).

References

[1] Le Gall et al., 2019, Geophysical Research Letters 46, 11747–11755

[2] Le Gall et al., 2023, Icarus 394, 115446.

[3] Le Gall et al., 2017, . Nature Astronomy 1, 0063.

[4] Mergny et al., 2023, Article Submitted.

[5] Martin et al., 2022, Icarus 392, 115369.

[6] Black et al., 2001, Icarus 151, 167–180.

[7] Howett et al., 2011, Geophys. Res. 116, E03003

[8] Picard et al., 2018, Geoscientific Model Development 11, 2763–2788.

[9] Jaumann et al., 2007, Icarus 193, 407.

How to cite: Raza, M. S., Le Gall, A., Bonnefoy, L., Schmidt, F., Leyrat, C., Mergny, C., and Picard, G.: Insigths into scattering and thermal anomalies at Enceladus’ SPT from the active and passive Cassini radar observations., Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1032, https://doi.org/10.5194/epsc2024-1032, 2024.