Introduction

After the detection of a remarkable jet activity above Enceladus' south-polar region [1], Cassini utilized flybys for a unique sampling opportunity, confirming the presence of a subsurface water ocean and pointing at the enormous astrobiological potential of the moon [2]. Understanding the plume activity and its relationship to internal processes has since become a task of primary scientific interest. The observed brightness variations in the plume [3] indicate their tidal origin, but the existence and timing of two activity peaks during the tidal period appear to contradict stress analysis predictions. Numerical models of deformation at the faults or localized hydraulic and thermodynamic processes within the fault system have so far failed to provide a comprehensive explanation of the temporal pattern in the observed plume activity. 

Data & Model

As input, we employ the plume brightness dataset representing the ice particle content published by Ingersoll and coworkers [3], who identified several tens of groups based on the days of data acquisition. We process the data by removing the long-period variability using a template constructed from a selected subset of measurements with maximal coverage over the tidal period (Fig. 2, a). 

In this study, we provide a synthesis and extension of some of these previous approaches and combine a 3D global model of tidal deformation of the fractured ice shell [4] with a 1D local model of transport processes within south-polar faults that involves dynamic motion of the water column induced by tides and shell deformation (inspired by [5]), and vapour transport within the fissures (inspired by [6]), combining two independent vapour transport mechanisms: slip-controlled jet flow and normal-stress-controlled ambient flow (Fig. 1). 

Figure 1: Our model for diurnal fluctuations in activity comprises both global and local components. The 3D global model (depicted on the left) delivers the tidal deformation of an ice shell with fractures, incorporating realistic variations in thickness [7] and fault surface geometry based on [8]. Kinematic and dynamic quantities are averaged along faults and across the shell's depth, serving as input for the local model. The local 1D model (shown in panel b) integrates models for liquid water hydraulic motion (I, depicted in white), vapour and solid grain transport above the water table (II, illustrated in blue), and a near-surface aperture model (III, depicted in brown) comprising two mechanisms: slip-controlled mechanism corresponding to macroscopic jet flow (panel c) and a normal-stress-controlled mechanism corresponding to ambient (more diffuse) vapour flow (panel d). The main controlling parameters in the model are also indicated: the effective friction at faults (µ_f), initial slip at faults (u_0slip), the effective Young’s modulus of the fault zones (E_eff), and the activity scaling factors for the two mechanisms (A_0u and A_0σ).

Results

Our physical model involves 5 parameters that are identified in a Bayesian Monte-Carlo Markov chain fitting procedure to the activity data: effective friction coefficient and initial slip at the faults, effective Young modulus of the fault zone material, and two amplitudes of the vapour transport mechanisms (slip controlled and normal-stress controlled one). The model successfully predicts the main features of the observed plume’s temporal variability: the two-peak activity curve with a main maximum located at MA≈200˚ and a less pronounced local maximum at MA≈50˚. The model allows us to explain the relative amplitudes of the two peaks and captures the activity ramp between the two maxima (Fig. 3). 

Figure 2:  panel a: The diurnal variations of Enceladus’ plume activity, with filtering applied to remove longer-term trends. The symbols denote data sets corresponding to measurements acquired within a single observational day, with colour-coded representation indicating the respective years of data collection. The main activity curve characteristics are the dual peaks (at approximately MA≈40˚ and MA≈200˚) alongside a discernible ramp (occurring around MA≈80−140˚). In the top panel, we present, for comparative analysis, the temporal evolution of normal stress (depicted in gold) and horizontal slip (illustrated in red), computed by a 3d model of tidal deformation of Enceladus’ fault with frictional faults [4]. The temporal alignment of peaks and troughs in horizontal slip with the occurrence of early and mid-period plume peaks is apparent, whereas maximal tensile stress coincides with the observed ramp in plume activity positioned between the activity peaks.

panel b: The activity model's prediction (illustrated by the black solid line) compared to the rescaled data depicted in Figure 1 (shown here as grey symbols) provides insight into the intricate "plumbing system" of Enceladus. The activity signal comprises both primary slip-controlled (depicted by the red dashed line) and secondary normal-stress-controlled components (illustrated by the gold dash-dotted line). The former accounts for the two peaks occurring around 40-50 MA and 200 MA, while the latter explains the ramp observed around 80-140◦ and the subsequent increase in activity beyond 300 MA. 

In addition, the model provides a possible explanation for the differences between the vapour and solid emission rates during the diurnal cycle and the observed fractionation of the various icy particle families. Our model prediction could be tested by future JWST observations targeted when Enceladus is at different positions on its orbit and could be used to determine the optimal strategy for plume material sampling by future space missions.

Acknowledgements

This research was supported by the Czech Science Foundation (project no. 22-20388S).

References

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

[2] Waite et al. (2009), Nature 460, 487-490, doi:10.1038/nature08153

[3] Ingersoll et al. (2020), Icarus 344, 113345, doi:10.1016/j.icarus.2019.06.006

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

[5] Kite & Rubin (2016), PNAS 113, 3972-3975, doi: 10.1073/pnas.1520507113

[6] Nakajima & Ingersoll (2016), Icarus 272, 309-318, doi: 10.1016/j.icarus.2016.02.027

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

[8] Porco et al. (2014), AJ 148, 45, doi: 10.1088/0004-6256/631148/3/45

How to cite: Souček, O., Běhounková, M., Lanzendörfer, M., Tobie, G., and Choblet, G.: Variations in Enceladus’ plume activity explained through the dynamics of water-filled faults on Enceladus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-167, https://doi.org/10.5194/epsc2024-167, 2024.

Introduction

Ocean worlds of the Solar System have all the prerequisite ingredients for the emergence of life as we know it: organic compounds, liquid water and energy sources. Biosignatures and prebiotic compounds are organic molecules particularly searched for, such as amino acids, proteins, peptides or fatty acids. Several icy moons orbiting the giant planets are part of this ocean worlds’ family. Among them, the two saturnian moon Titan and Enceladus are of a primary interest and particularly targeted by the space agencies. Titan is the largest moon of Saturn and a unique environment in the Solar System. The dense atmosphere of Titan has been extensively studied by the Cassini-Huygens space mission and has revealed a world dominated by organic aerosols from the high atmosphere to the surface. Mass spectrometers on board the Cassini orbiter, INMS (Ion and Neutral Mass Spectrometer) and CAPS (Cassini Plasma Spectrometer), have detected positive ions and neutrals up to 100 u and heavy positive and negative ions of tens and even thousands of mass units [1]. Enceladus is a very small moon (with a diameter around 500 km) orbiting Saturn, inside the E-ring. Plumes have been detected at its south pole. The matter ejected by these plumes is composed by water ice and heavy organic compounds [2]. Both moons house a subsurface liquid water ocean and identifications of heavy organic compounds have been suggested [1-3]. On Titan, some liquid water bodies remain at the surface about a thousand years (in crater melt pools). New mission concepts and instruments are currently in development or in preparation for the future exploration of these ocean worlds.

A High Resolution Mass Spectrometer based on the Orbitrap^TM technology

Future astrobiology space mission payloads should include High Resolution Mass Spectrometer (HRMS) instrument to provide unequivocal identifications of most biosignatures and prebiotic molecules. A new generation of high-resolution mass analyzer is currently developed by a consortium of six laboratories, led by the LPC2E (Orléans, France) and funded by CNES. This mass analyzer called CosmOrbitrap [4] is based on the Orbitrap^TM technology, developed by Alexander Makarov [5] and commercialized by Thermo Fischer scientific since 2000s. Work was undertaken in collaboration with the University of Maryland and the Goddard Space Flight Center (GSFC) to develop a laser CosmOrbitrap-based mass spectrometer for future spaceflight applications. Among them, the CORALS (Characterization of Ocean Residues and Life Signatures) spaceflight prototype instrument recently achieved Technical Readiness Level (TRL) of 5+ [6]. This instrumentation challenges other MS and HRMS developments and promises unprecedented analytical performances in space. They have already been demonstrated in the lab on a large and various organic samples range: pure amino acids, short peptide biosignatures similar to those observed for instance in extremophiles on Earth, mixtures of organics with water or salts and complex organic matter analog to those found in the Titan atmosphere [7-11]. The laboratory test bench based on the same principle is coupling the mass analyzer CosmOrbitrap with a commercial Nd-YAG laser ionization system operating at 266 nm and has been fully described in [4,8]. This work aims at demonstrating the potential of laser CosmOrbitrap-based instruments for the future exploration of ocean worlds by the analysis of analogs and representative samples of Titan and Enceladus.

Representative samples of ocean worlds

In this work, three kinds of sample have been analyzed: (1) analogs of Titan’ aerosols produced in laboratory called tholins, (2) commercial tripeptides and (3) natural analogs representative of the Enceladus’ ocean.

Various laboratory experiments allow the synthesis of analogs of the so-called “tholins”. One of them, the PAMPRE experiment [12] uses a radio frequency reactive low-pressure plasma to produce solid particles and solid films onto metallic surfaces placed inside the PAMPRE reactor, mimicking the coupled ion-neutral chemistry occurring in Titan ionosphere [1,13]. Tholins studied are produced with a gas mixture of 5% of methane (CH₄) and 95% of nitrogen (N₂). Commercial tripeptides are combinations of leucine-glycine-glycine (LGG) and phenylalanine-glycine-glycine (PGG) dropped onto titanium and nickel plates. Natural analogs are coming from the hypersaline Mono lake (California, USA) used to be considered as an analog environment of the salted and subsurface ocean of Enceladus [14]. A comparative study is undertaken with results obtained on the same Mono lake samples with another technique coupling a separative method (capillary electrophoresis, CE) with a mass spectrometer [15].

Laser CosmOrbitrap mass spectrometer analyses

Latest results obtained with the laser CosmOrbitrap test bench using these various samples will be presented. Compound identifications, analytical performances and other techniques comparisons (like CE-MS) will be part of these results. They participate to a better understanding of these unique extraterrestrial environments but they also demonstrate the need of this specific, highly capable, instrumentation for the future space missions dedicated to the ocean worlds exploration.

Acknowledgments

We gratefully acknowledge the CosmOrbitrap consortium (LPC2E, LATMOS, LISA, IPAG, IJC Lab, J. Heyrovsky institute of Physical Chemistry), Alexander Makarov (Thermo Fisher Scientific) and the CNES for its technical and financial support.

We acknowledge the CORALS and AROMA teams, the NASA GSFC and the University of Maryland for a very fruitful collaboration.

We also acknowledge the LATMOS and PIIM teams for the production of the Titan’ aerosols analogs and Maria Mora from JPL for providing us some Mono Lake samples. 

References

[1] Waite et al. 2007, Science, 316

[2] Postberg et al. 2018, Nature, 558

[3] Waite et al. 2009, Nature, 460

[4] Briois et al. 2016, Planetary and Space Science, 131

[5] Makarov 2000, Analytical Chemistry, 72

[6] Willhite et al. 2021, IEEE Aerospace conference

[7] Arevalo et al. 2018, Rapid Communications in Mass Spectrometry, 32

[8] Selliez et al. 2019, Planetary and Space Science, 170

[9] Selliez et al. 2020, Rapid Communications in Mass Spectrometry, 34

[10] Selliez et al. 2023, Planetary and Space Science, 225

[11] Ni et al., 2023, Astrobiology, 23

[12] Szopa et al. 2006, Planetary and Space Science 54

[13] Dubois et al. 2020, Icarus 338

[14] Preston and Dartnell 2014, International Journal of Astrobiology, 13

[15] Mora et al. 2022, Astrobiology, 22

How to cite: Selliez, L., De Jesus, R., Lavoisé, J., Thirkell, L., Arevalo, R., Lebreton, J.-P., Gaubicher, B., Colin, F., and Briois, C.: Chemical analyses provided by High Resolution Mass Spectrometry based on CosmOrbitrap for future space missions to ocean worlds, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-842, https://doi.org/10.5194/epsc2024-842, 2024.

The Ceres Autonomous Lander Into Crater Occator (CALICO) was proposed in response to ESA’s M7-call for future space missions. CALICO is a mission to study the surface and interior of dwarf planet Ceres, the closest ocean world to Earth. Ceres’ accessible location in the main belt makes it a compelling target for exploration since it was classified as an ocean world in the wake of NASA’s Dawn mission, which provided a wealth of data. DAWN also left open many questions, such as whether the interior of Ceres ever had the potential for habitability.

CALICO’s space segment consists of two components: the CALICO surface element is to land in Occator crater to analyse the salt- and organic-rich deposits of Vinalia Faculae, where building blocks of life might be accessible on the surface. The CALICO orbital element will characterize Ceres’ magnetic and particle environment to establish the presence of subsurface volatile reservoirs.

We present an overview of CALICO’s science objectives, instrument payload and a possible mission scenario.  Although not selected to go forward into Phase A, CALICO - or any CALICO-like mission - has great potential to answer such fundamental questions about our place in the Solar System. The CALICO consortium will continue its efforts to realize a mission to Ceres.

How to cite: Hagermann, A., Schroeder, S., Buchwald, R., Bründl, T.-M., Küppers, M., Benkhoff, J., Otto, K., Grott, M., Pilorget, C., Olsson-Francis, K., Sheridan, S., Morgan, G., Schröder, C., Barabash, S., Fatemi, S., Reiss, P., Palomba, E., Longobardo, A., Van Hoolst, T., and Kindracki, J. and the The CALICO Consortium: CALICO - a Mission Proposal to Explore Ceres In Situ, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-19, https://doi.org/10.5194/epsc2024-19, 2024.

We analyse data recorded by the Cosmic Dust Analyzer (CDA) on board the Cassini spacecraft that were obtained during traversals of the Enceladus dust plume. The focus of our work are profiles of relative abundances of grains of different compositional types derived from mass spectra recorded with the Dust Analyzer (DA) subsystem during the Cassini flybys E5 and E17. The profile from E5, corresponding to a steep and fast (17.7 km/s) traversal of the plume, was analyzed in previous work (Postberg et al, 2011). Here we present and include into the analysis a second compositional profile obtained at a very different geometry during flyby E17, with a nearly horizontal traversal of the South Polar Terrain (SPT) at a significantly lower relative velocity (7.5 km/s). Additionally, we employ in our analysis rates of dust detections registered in the plume by the High Rate Detector (HRD) subsystem of CDA at two different Enceladus flybys (E7 and E21). We derive the ranges of grain sizes that were sampled by the two CDA subsystems at these flybys and use the data sets to constrain the parameters of a new dust plume model.  That model we construct from a recently developed mathematical description of dust ejection (Ershova and Schmidt, 2021) using the software package DUDI, publicly available at https://github.com/Veyza/dudi. Further constraints we use for our model are published velocities of gas ejection and the positions of gas and dust jets on the SPT.  From our model we derive production rates of dust mass for the different compositional types of grains detected by CDA, amounting to a total rate equal to or larger than about 28 kg/s.  The contribution of salt-rich dust to the plume was previously believed to be dominant in mass, based on the analysis of E5 flyby data alone (Postberg et al, 2011). However, including both compositional profiles (E5 and E17) in our analysis, we find that the salt-rich dust contribution is only about one percent by mass or less. This finding follows in part from an improved understanding of the masses of grains of various compositional types that implies a generally smaller size for salt rich grains than previously thought. Furthermore, the E17 compositional profile exhibits a dominance of organic enriched grains over the SPT, a region of the plume that was poorly constrained, if at all, by the E5 data. Our new dust plume model can be used to predict numbers and masses for grains of various compositional types that a detector on a future mission will collect during a plume traversal.

How to cite: Schmidt, J., Ershova, A., Postberg, F., Khawaja, N., Nölle, L., Srama, R., Kempf, S., and Southworth, B.: The Enceladus Dust Plume from the Cassini Cosmic Dust Analyzer, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-510, https://doi.org/10.5194/epsc2024-510, 2024.

How to cite: Hsu, H.-W. and Dong, Y.: Plume dust mass density derived from dust impact plasma measured by in situ Cassini instruments at Enceladus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-692, https://doi.org/10.5194/epsc2024-692, 2024.

The search for habitable environments in the outer solar system is at the forefront of contemporary space exploration. The presence of subsurface liquid water, energy sources, and organic molecules make some icy moons with subsurface oceans potential sites to search for extraterrestrial life. Among these bodies are the Jovian moon Europa and the Saturnian moons Enceladus and Titan. The recent detection of phosphorus (Postberg et al. 2023) and HCN (Peter et al. 2024) in the ocean of Enceladus has further enhanced its astrobiological potential.

Enceladus ejects subsurface material into space in the form of ice grains and vapours from the moon’s south polar region. Most of the ice grains fall back onto the surface, with only a fraction of these grains escaping the moon’s Hill Sphere and forming part of Saturn’s E ring. Cassini’s on-board mass spectrometers — the Cosmic Dust Analyzer (CDA) and Ion and Neutral Mass Spectrometer (INMS) - sampled gas and ice grains both from the plume and in the E ring. INMS detected O- and N-bearing organics alongside hydrocarbon species in the gas phase directly in the plume (Waite et al. 2009). On the other hand, CDA was also able to detect different classes of organic species in the ice grains sampled in Saturn’s E ring (Khawaja et al. 2019). The detections of sodium salts, nanophase SiO2 particles, and molecular hydrogen confirm the presence of water-rock interactions and hydrothermal activity in and around the rocky core of Enceladus (Postberg et al. 2009, Hsu et al. 2015, Waite et al. 2017). CDA revealed a diverse inventory of organic compounds that led to the classification of organic species in E ring ice grains: (i) complex, hydrophobic, solid, macromolecular organic compounds with molecular masses > 200 u (Postberg et al. 2018) and (ii) the more abundant recondensed volatile organic compounds in ice grains, which produce spectral features due to low mass (< 100 u) nitrogen-, oxygen-, or single aromatic ring-bearing compounds (Khawaja et al. 2019). These organic compounds are evidence of organic chemistry in the subsurface ocean.

Thus far, organic material has only been subject to detailed investigation by CDA in E ring ice grains. Here, for the first time, we analyse organic material in freshly ejected Enceladus plume ice grains. For this purpose, Cassini’s flybys of the Enceladus plume provided a unique opportunity for CDA to collect freshly ejected subsurface oceanic material, particularly organic compounds, as opposed to settled E ring grains. Flyby data were hitherto only used to classify ice grains by general composition (i.e. organic-bearing) without an in-depth compositional analysis (Postberg et al. 2011). We analyse CDA time-of-flight mass spectral data of freshly ejected ice grains sampled at a velocity ~ 17.7 km/s during Cassini’s E5 traversal of Enceladus in 2008. These high speeds support previously unexplored fragmentation pathways of organics, opening up new diagnostic possibilities for identifying such organic components in plume ice grains. For this work, we used electron ionisation (EI) mass spectra extracted from open-source databases in a complementary fashion (NIST & MassBank; Khawaja et al. 2022) to compare the fragment ions of certain organic compounds with those obtained at such high impact velocities.

Our results confirm the presence of aryl and oxygen moieties in ice grains that were previously sampled in the E ring, providing fresh insights into the stability of these compounds at Enceladean hydrothermal sites. In addition, mass spectra of these freshly-ejected organic-bearing grains also exhibit certain spectral features which were not observed at lower impact speeds in the E ring. For the first time, we find ether/ethyl and ester/alkene group moieties in these plume ice grains that provide a basis for alterative pathways for organic synthesis in hydrothermal systems on Enceladus, which carries significant implications for the habitability of the Enceladus ocean.

References

Hsu et al. (2015), Ongoing hydrothermal acGviGes within Enceladus. Nature, 519, pp. 207–210.

Khawaja et al. (2022), Complementary mass spectral analysis of isomeric O-bearing organic compounds and fragmentaGon differences through analogue techniques for spaceborne mass spectrometers, Planet. Sci. J. 3 254.

Khawaja et al. (2019), Low-mass nitrogen-, oxygen bearing, and aromaGc compounds in Enceladean ice grains. MNRAS, 489, pp. 5231–5243.

Peter, J.S., Nordheim, T.A. & Hand, K.P. (2024), Detection of HCN and diverse redox chemistry in the plume of Enceladus. Nat Astron 8, 164–173.

Postberg et al. (2023), Detection of phosphates originating from Enceladus’ocean. Nature, 618, 489-493.

Postberg & Khawaja et al. (2018), Macromolecular organic compounds from the depths of Enceladus. Nature, 558 (7711), pp. 564-568.

Postberg et al. (2011), A salt-water reservoir as the source of a composiGonally stratified plume on Enceladus. Nature, 474, pp. 620–622.

Postberg et al. (2009), Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature, 459 (7250), pp. 1098-1101.

Waite et al. (2017), Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes. Science, 356 (6334), pp. 155–159.

Waite et al. (2009), Liquid water on Enceladus from observaGons of ammonia and 40Ar in the plume. Nature, 460, pp. 487–490.

How to cite: Khawaja, N., Postberg, F., O’Sullivan, T. R., Napoleoni, M., Hillier, J., Simolka, J., Klenner, F., and Srama, R.: Cassini’s New Look at Organic Material in Enceladus’ Plume Ice Grains with CDA: Implication for the Habitability of Ocean Worlds, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1055, https://doi.org/10.5194/epsc2024-1055, 2024.

Saturn's moon, Enceladus is considered a priority target for future planetary missions due to its high astrobiological potential [1]. Water jets presumably originating from a subsurface ocean have been observed at the south pole of Enceladus by NASA’s Cassini mission [2], and their analysis provides a direct window into the ocean composition [3] that, in turn, can help to understand the nature and amount of impurities that may exist within the ice shell.

Enceladus’ jet activity generates a highly porous material that affects the thermal state of the ice shell. The thickness of that layer and its distribution are poorly constrained, but local thicknesses of up to 700 m have been reported from the analysis of pit chains on the surface of Enceladus [4]. Such a thick porous layer can strongly attenuate the signal of radar sounders that have been proposed to investigate the Enceladus’ subsurface [5].

Here, we use numerical simulations to determine the effects of a porous layer on the two-way radar attenuation. We generate a variety of steady-state one-dimensional thermal models based on proposed parameters for Enceladus’ ice shell thickness (5 - 35 km, [6]), porous layer thickness (0 - 700 m [4]) and its thermal conductivity (0.1 - 0.001 W/mK [7,8]). We define a penetration depth, which is the depth at which the two-way attenuation reaches a 100 dB, a typical value for radar sounders on planetary missions [9]. We use two material models ("high" and "low" loss) to identify the impact of chemical impurities on attenuation [9]. While the “low” loss scenario considers an ice shell composed of pure water ice, the “high” loss case is characterized by a homogeneous mixture of water ice and chlorides in concentrations extrapolated from the particle composition of Enceladus’ plume [5].

For a conductive ice shell, the temperature itself depends on the thickness of the ice shell, and to a first order on the presence of a porous layer. Due to its insulating effect caused by its low thermal conductivity, the porous layer can significantly impact the thermal profile within the ice shell, leading to a high thermal gradient near the surface. We can clearly observe in Figure 1 that the attenuation dramatically increases with increasing porous layer thickness. This can be again attributed to the insulating effect of the porous layer, an effect that becomes more pronounced with increasing porous layer thickness.

In addition to systematically testing parameter combinations, we use two ice shell thickness maps [6] together with local thermal profiles to provide a global spatial distribution of potential penetration depths that could be achieved by radar measurements. The attenuation was calculated by treating each data point as a single temperature profile using the corresponding ice shell thickness at the respective location. We define a penetration depth, which is the depth at which the two-way attenuation reaches 100 dB. The relative penetration depth is the penetration depth divided by the total ice shell thickness of the respective scenario.

We constructed two distribution models for the porous layer under the assumption that the thickness of the porous layer varies only with latitude and: 1) exponentially decreases from a 700-m-thick layer at the south pole to 10-m-thick layer at the north pole of Enceladus, which we denote as the "exponential-global distribution" (see figure 2d) while 2) the porous layer is absent within the 5° of the south pole and then exponentially decreases from 255 m to 0 m at equator (see figure 2g), which we will call "exponential-hemispheric distribution". In all cases the conductivity of the porous layer is 0.025 Wm^-1K^-1.

In the low-loss attenuation maps (Figure 2b, e, and h), a majority of the surface has more than 90% relative penetration depth, while in all three high-loss scenarios, we observe a small band of 100% relative penetration depth roughly 5° in latitude away from the south pole. The thin ice shell at the south pole allows for the signal to reach the ice-ocean interface even in the presence of a thick porous layer.

In Figure 3, we calculate the two-way attenuation at the eutectic interfaces of ammonium chloride and ammonia [5] for the exponential-hemispheric distribution of the porous layer and assuming a thermal conductivity of the porous layer of 0.025 W/(mK).

Given the low eutectic temperature of NH₃ (175.45 K), the two-way attenuation for the high-loss case (Figure 3b) is only ~5 dB on average. For the eutectic interface of NH₄Cl (257.79 K), the two-way attenuation shows an average value of about 400 dB (Figure 3d).

In conclusion, our results show that the presence of a porous layer has a first-order effect on the two-way radar attenuation. For regions covered by porous layers with thicknesses larger than 250 m and a thermal conductivity lower than 0.025 W/(mK) the two-way radar attenuation reaches a threshold value of 100 dB before reaching the ice-ocean interface in the low loss scenarios. In the high loss cases, for similar regolith layer thicknesses and thermal conductivity, the two-way attenuation remains below 100 dB for at most 48% of the ice shell. Depending on the local ice shell thickness and properties of the snow deposits, as little as a few percent of the ice shell can be penetrated before the 100 dB limit is reached. We note, however, that the presence of a porous layer leads to high subsurface temperatures and promotes the formation of brines at shallow depth that can be detected by future radar measurements.

References:

[1] Choblet et al., 2021. [2] Hansen et al., 2006. [3] Postberg et al., 2008. [4] Martin et al., 2023. [5] Soucek et al., 2023. [6] Hemingway & Mittal, 2019. [7] Seiferlin et al., 1996. [8] Ferrari et al., 2021. [9] Kalousova et al., 2017.

How to cite: Byrne, W., Plesa, A.-C., Rückriemen-Bez, T., Benedikter, A., and Hussmann, H.: The consequences of an icy porous layer on the thermal state and radar attenuation of Enceladus' ice shell, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-694, https://doi.org/10.5194/epsc2024-694, 2024.

With new missions (Juice, Europa Clipper) to icy worlds in the outer solar system in the near future, the structure, composition and evolution of these bodies is highly relevant.  Enceladus is Saturn’s brightest moon, with an icy shell predominately made of water ice and a subsurface salty ocean.  This is principally maintained by energy dissipation due to its orbit around Saturn.  It is cryo-volcanically active in the South Polar Terrain (SPT), a region observed to be significantly warmer than the rest of the surface - acutely so in the ‘Tiger Stripes’ at the south pole from whence sub-surface material is ejected onto the surface and into space.  We care about measuring emitted power because the energy budget of Enceladus directly pertains to theories of the evolution of Saturn’s moons and the longevity of the subsurface ocean.   Current estimates of the endogenic heat at Enceladus’ active SPT region are between 4 and 19 GW or 100-500 mWm^-2 [1,2]. 

Lower intensity (background) endogenic heat conducted through the ice shell is more challenging to distinguish from passive, re-radiated emission.  Yet, because it occurs over the entire surface the total heat loss is expected to be significant for Enceladus’ total energy budget (modelled to be 25-150 GW or 2.3 - 187 mWm^-2 [3,4]).  We consider the theoretical and functional limits to the detection of potential endogenic heat on Enceladus outside the SPT using observations from Cassini’s Composition InfraRed Spectrometer (CIRS) and a surface thermal model. 

The range of thermal inertia and bolometric albedos of Enceladus were derived from a selection of CIRS observations [5].  These values are used to model expected surface temperature. The uncertainty in thermal properties can be used to assign an upper limit to temperature (or power) differences between observed and model temperatures that could be explained by these uncertainties.  Scenes warmer than this threshold value could indicate endogenic thermal emission.  We model this minimum detection limit that varies with time of day, latitude, longitude and consequently temperature – because emitting power for a given temperature change is itself strongly temperature dependent. 

Figure 1 shows the maximum excess power (observation minus model) that can be due to model uncertainty for a full range of solar latitudes (half of a Saturn year) at all latitudes for passive emission.  It is shown here as a daily average at 0° longitude but changes with diurnally varying surface temperature and longitude.  This work shows that if averaged conductive heat flow is less than the values given in this figure, it would not be detectable by remote sensing due to current uncertainty in thermal properties.  It also highlights that the optimum conditions for detecting endogenic heat are at night or in polar winter – in the coldest scenes with no insolation.  These are conditions where observation error increases due to the noise quotient.  At the coldest temperatures, observations can be averaged to minimize observation uncertainty.

We will present findings to show that even with the current range of uncertainty in thermal inertia and albedo it may be possible to detect excess endogenic power as low as 20 mWm^-2 in the coldest scenes, or 40 mWm^-2 at mid-latitudes provided there are suitable observations.  When we compare these limits with observed measurement minus model differences from CIRS, this can also infer a cap to the possible conductive heat flow.  We expect that the upper limit of the model estimate given by [4] is unlikely.  If an excess power of 187 mWm^-2 were present it would be readily apparent from CIRS observations when confronted with modelled temperatures.

Quantifying Enceladus’ endogenic heat budget and imposing observation-based constraints to models of conductive heat flow would greatly enhance our understanding of the evolution of Enceladus (and by consequence all of Saturn’s moons).  While this approach is ultimately limited by model uncertainties in the thermal properties of Enceladus’ surface, this is an observation limitation that can be improved.  The thermal inertia and albedo derivation from CIRS data by [5] is in the process of being extended to use data from the full mission.  This would be greatly enhanced by more detailed thermal observations at many local times and over as much as the Saturn year as possible – particularly in the polar regions.  Since it is very challenging to observe these locations from Earth, orbiting instruments with high inclination would be welcomed to more accurately characterise the thermal properties of these icy moons and thus understand their evolution. 

[1]      C. J. A. Howett, J. R. Spencer, J. Pearl, and M. Segura, “High heat flow from Enceladus’ south polar region measured using 10-600 cm-1 Cassini/CIRS data,” Journal of Geophysical Research E: Planets, vol. 116, no. 3, 2011, doi: 10.1029/2010JE003718.

[2]      J. Spencer et al., “Plume Origins and Plumbing (Ocean to Surface),” in Enceladus and the Icy Moons of Saturn, P. M. Schenk, R. N. Clark, C. J. A. Howett, A. J. Verbiscer, and J. H. Waite, Eds., The University of Arizona Press, 2018, pp. 163–174. doi: 10.2458/azu_uapress_9780816537075-ch008.

[3]      V. Lainey et al., “Resonance locking in giant planets indicated by the rapid orbital expansion of Titan,” Nat Astron, vol. 4, no. 11, pp. 1053–1058, Nov. 2020, doi: 10.1038/s41550-020-1120-5.

[4]      D. J. Hemingway and T. Mittal, “Enceladus’s ice shell structure as a window on internal heat production,” Icarus, vol. 332, pp. 111–131, Nov. 2019, doi: 10.1016/j.icarus.2019.03.011.

[5]      C. J. A. Howett, J. R. Spencer, J. Pearl, and M. Segura, “Thermal inertia and bolometric Bond albedo values for Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus as derived from Cassini/CIRS measurements,” Icarus, vol. 206, no. 2, pp. 573–593, Apr. 2010, doi: 10.1016/j.icarus.2009.07.016.

How to cite: Miles, G., Howett, C., and Spencer, J.: Establishing limits to endogenic heat detection on Enceladus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1044, https://doi.org/10.5194/epsc2024-1044, 2024.