Water ice has a microstructure shaped by a complex interplay of coupled multi-physics processes. Among them, ice sintering—also referred to as metamorphism or annealing—transports material from ice grains into their neck region, resulting in changes in the mechanical and thermal properties of the ice. Understanding sintering is essential to investigate the properties and microstructure of ice. While the sintering process of snow on Earth has been extensively studied, there is a scarce amount of information regarding the alteration of ice in planetary surface environments characterized by low temperatures and pressures.

Here we present a multiphysics simulation model designed to study the evolution of planetary ice microstructure.  Coupled to a heat transfer solver, we have built a new model for the sintering of ice grain  with mathematical refinement to the diffusion process. As changes in ice microstructure affect the thermal properties we have expressed the heat conductivity with a formulation that consider microstructure and porosity which enables a two coupling between sintering and heat transfers.

Our simulations of Europa's icy surface spanned a million years, allowing us to thoroughly explore the evolution of ice microstructure. Results show that the hottest regions experience significant sintering, even if high temperatures are only reached during a brief portion of the day. This process takes place on timescales shorter than Europa's ice crust age, suggests that these regions should currently have surface ice composed of interconnected grains. Accurately simulating these highly coupled processes, plays a crucial role in accurately determining the microstructure and quantitative composition of Europa's surface, a key objective for upcoming missions such as JUICE and Europa Clipper.

How to cite: Mergny, C. and Schmidt, F.: Icy Moon Surfaces Microstructure through Multiphysics Simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9751, https://doi.org/10.5194/egusphere-egu24-9751, 2024.

Europa’s hydro- and cryosphere is of primary interest in the quest for habitable environments in the solar system (e.g., [1]). The ice shell, which connects the potential subsurface ocean to the surface, may itself provide niches for life if liquid brine pockets can form and exist for extended periods of time. It is thus crucial to understand the thermal and dynamic state of the ice shell in order to characterize the existence and transport of liquid brines within the ice shell.

Recent work by [2] and [3] investigated the effects of temperature dependent thermal conductivity (k) as well as heat capacity (cp) and a complex composite rheology on convection in the ice shell. In this work, we build upon these previous efforts by combining the influence of both - varying thermodynamic parameters and complex rheology - in geodynamic simulations performed with the convection code GAIA [4]. Instead of a temperature-dependent heat capacity, we investigate the effect of a temperature- and depth-dependent thermal expansivity (α), which is a crucial term in determining the buoyancy induced by temperature differences.

We study the dynamic state (Nu-Ra scaling), the mechanical state (elastic thickness, brittle-to-ductile transition, deformation maps), and the thermal state (bottom and top boundary heat flux, occurrence of brines) of the ice shell for various setups (using both constant and variable α and k) and input parameters (ice shell thickness and grain size). For selected models, i.e. distinct thermal and dynamic states, we calculate the local two-way attenuation based on [5], [6]. The resulting two-way attenuation patterns will offer initial insights into the radar's ability to penetrate to the ice-ocean interface. If attenuation proves excessive due to the presence of hot thermal plumes, making the sampling of the ice-ocean interface unlikely, the patterns can still provide valuable insights into the dynamic state of Europa's ice shell. This includes parameters such as the thickness of the conductive layer (the so-called stagnant lid) that forms in the top part of the ice shell or the wavelength of convective structures deeper in the ice shell.

References:

[1] Coustenis & Encrenaz et al., 2013. [2] Carnahan et al. 2021. [3] Harel et al. 2020. [4] Hüttig et al., 2013. [5] Kalousova et al., 2017. [6] Soucek et al., 2023.

How to cite: Rückriemen-Bez, T., Plesa, A.-C., Byrne, W., Hussmann, H., and Benedikter, A.: The thermal and dynamic state of Europa’s ice shell: Revealed by global-scale convection models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18532, https://doi.org/10.5194/egusphere-egu24-18532, 2024.

Europa’s surface is one of the youngest in the solar system. The Jovian moon is believed to hide a global liquid water ocean under its icy crust [1] and is exposed to intense space weathering due to the continuous bombardment by electrons and ions from Jupiter’s magnetosphere [2]. To understand the processes governing the evolution of the surface it is necessary to finely characterize the microphysics of the ice (composition via endmember volume abundance, grain size and surface roughness). However, the majority of the previous studies [3,4] do not allow to constrain precisely these parameters.

Here we report the use of a radiative transfer model [5] in a Bayesian MCMC inference framework [6,7] to retrieve microphysical properties of Europa's surface using the Galileo Near-Infrared Mapping Spectrometer (NIMS) hyperspectral data [8]. We present the analysis of a calibrated spectrum of a dark lineament from the trailing Anti-jovian hemisphere. The estimated signal-to-noise ratio (SNR) is between 5 and 50, we mainly focus on the 1.0-2.5 µm region for which the SNR is higher with an uncertainty on the absolute calibration up to 10% [8].

A first work has allowed us to test all combinations of 3, 4 and 5 endmembers from a list of 15 relevant compounds [9]. We were able to test over 5,000 combinations and show that some compounds appear necessary to reproduce the observation, such as water ice and sulfuric acid octahydrate, in agreement with previous studies [3,4,10]. However, adding either hydrated sulfates or chlorine salts produces results substantially similar [9]. Here we present a follow-up study in which we focus on the few acceptable combinations identified by our Bayesian inversions and we analyze the results in terms of grain size and surface roughness. We show that the grain size of the mandatory endmembers is well constrained and similar from one combination to another [11]. The macroscopic roughness is however poorly constrained [11], as expected. Thanks to numerical optimizations we are able to invert independently every spectel of a NIMS hyperspectral cube with the bayesian MCMC algorithm. From this result, we present maps of microphysical properties on an entire hyperspectral image of a dark lineament. 

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

How to cite: Cruz Mermy, G., Schmidt, F., Andrieu, F., Cornet, T., and Belgacem, I.: Microphysics of Europa’s surface with Galileo/NIMS data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18178, https://doi.org/10.5194/egusphere-egu24-18178, 2024.

The search for habitable worlds within our solar system is guided by liquid water. Evidence for global, salty oceans hidden beneath the icy shells of moons in the Jovian system has motivated two upcoming missions: ESA’s Jupiter Icy Moons Explorer (Juice), launched April 2023, and NASA’s Europa Clipper, launching October 2024. Both spacecraft are equipped with ice-penetrating radar instruments, the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON), that will transmit radio waves into the subsurface and record energy reflected from interfaces defined by contrasts in dielectric properties, such as the ice-ocean interface.

The ocean is presumed to be the most extensive liquid water reservoir beneath the surface. However, various ice-water interfaces could exist throughout the ice shell. Dynamic processes such as impacts, convection, tidal heating, strike-slip faulting, and basal fracturing have been hypothesized to influence melt generation or inject ocean water in the ice shell interior. Even in the absence of these dynamic processes, impurities within the ice allow liquid water to be thermodynamically stable as brine at temperatures below the freezing point. In ice shells with non-zero bulk salinity, transitions from solid ice to ice-brine mixtures, or eutectic interfaces, invariably precede the ice-ocean interface. Understanding the detectability and radiometric character of eutectic interfaces is therefore a critical step towards interpreting the data collected by these ice-penetrating radar instruments.

In this work, we review measurements and models of the dielectric properties of sea ice and marine ice on Earth. We use these measurements and models as a foundation to propose a path forward for modeling the dielectric properties of eutectic interfaces within an ice shell. We assess how the ice shell's bulk salinity and the thickness of the thermally conductive layer impact the detectability and radiometric characteristics of eutectic interfaces. Our discussion includes how future laboratory measurements of existing terrestrial ice samples coupled to measurements of proxy samples consistent with off-world ocean sources can inform and refine our proposed framework.

How to cite: Wolfenbarger, N., Schroeder, D., and Blankenship, D.: From Sea Ice to Icy Shells: Modeling the Dielectric Properties of Ice-Brine Mixtures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13407, https://doi.org/10.5194/egusphere-egu24-13407, 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 700m 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]). 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. 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].

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 porous 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 effects of an icy porous layer on the two-way radar attenuation on Enceladus, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6159, https://doi.org/10.5194/egusphere-egu24-6159, 2024.

Titan is an object of fascination for scientists researching the solar system, as a ‘terrestrial-like’ world with active meteorology and fluvial and lacustrine formations based on methane chemistry and condensation. The Cassini-Huygens mission explored Titan extensively from 2004 to 2017, but since that time further observation of its slow seasonal cycle has been possible only via telescopes positioned on or close to the Earth. Titan’s unique characteristics led to a concerted post-Cassini observational campaign, with many of the most powerful telescopes available to astronomy. In this work we report on observations from 2022 & 2023 with three instruments on the James Webb Space Telescope (JWST), NIRCam, NIRSpec and MIRI, also in coordination with imaging from Keck II. In November 2022 and July 2023, Titan was the subject of multi-spectral filter imaging with JWST NIRCam and Keck II NIRC2, revealing tropospheric clouds at mid-northern latitudes, in line with climate modeling predictions for this season (late northern summer). In filters sensitive to the upper troposphere, we observed clouds growing and apparently ascending in altitude during a Titan day. JWST NIRSpec spectroscopy yielded for the first time a high resolution (R=2700) spectrum of Titan across the entire near-infrared (1-5 microns) unobscured by telluric absorption. This, among other things, enabled measuring the detailed structure of the CO 4.7 micron non-LTE emission, including the fundamental, the first two overtone bands and two isotopic bands. It is also the first time that CO₂ emission has been resolved in the NIR and the first time it has been seen on Titan’s dayside.  Finally, very sensitive spectroscopy with JWST MIRI in the mid infrared (5-28 microns) confirmed the many stratospheric gases seen by Cassini CIRS, but also added a new detection of methyl (CH3) in the middle atmosphere, a product of methane photochemistry that was expected but not previously seen. We modeled parts of the spectra to find a global mean temperature profile and profiles of minor gases. Soon we hope to extract yet more results from the NIRSpec and MIRI spectra as our understanding of the calibration and modeling progresses. In this presentation we summarize our results to date and describe planned future observations of Titan with JWST and Keck cycles.

How to cite: Nixon, C., Bézard, B., Cornet, T., Coy, B., de Pater, I., Es-Sayeh, M., Hammel, H., Lellouch, E., Lora, J., Lombardo, N., López-Puertas, M., Rannou, P., Rodriguez, S., Teanby, N., and Turtle, E. and the Titan JWST and Keck Observation Team: Titan in Late Northern Summer from JWST and Keck Observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2392, https://doi.org/10.5194/egusphere-egu24-2392, 2024.

The atmosphere of Titan is known to be a laboratory of complex organic chemistry. (Coustenis, 2021) From the Voyager missions, and later the Cassini-Huygens mission, several hydrocarbons and nitriles have been detected and their seasonal variations have been monitored during a period of one Titan season (30 years). Other minor species have been detected from the ground mainly in the millimeter range or using space-borne observatories like ISO. These results have been included in photochemical models that have also predicted the presence of other minor species, among which some have infrared transitions in the 5-25-µm spectral range where propane (C₃H₈) and allene (CH₂CCH₂) have already been detected. We have started an observing program using the TEXES thermal infrared imaging spectrometer at the Infrared Telescope Facility (Mauna Kea Observatory) to monitor the infrared signatures of hydrogen cyanide (HCN) and cyanoacetylene (HC₃N), along with acetylene (C₂H₂ and C₂HD). In addition, we have been searching for cyanopropyne (C₄H₃N) and isobutyronitrile (C₄H₇N) in the 20-micron region. High-resolution spectra of Titan with TEXES were recorded before where Lombardo et al. (2019) measured HNC (hydrogen isocyanide) in Titan’s lower stratosphere (1 ppb around 100 km), which is the first time HNC has been measured at these altitudes.  In September 2022 we obtained spectra of Titan in the following spectral ranges: (1) 498-500 cm^-1 (C₂HD, HC₃N, search for C₄H₃N); (2) 537-540 cm^-1 (C₂HD, search for C₄H₇N); (3) 744-749 cm^-1 (C₂H₂, HCN); (4) 1244-1250 cm^-1 (CH4). Observations are presently being processed. In 2023, laboratory spectra of cyanopropyne and isobutyronitrile have been recorded at Sorbonne Université in the 495-505 cm^-1 and 510-570 cm^-1 spectral ranges, respectively, with a spectral resolution of 0.01 cm^-1 and 0.056 cm^-1 (Coustenis et al., 2023). Cross sections have been derived for these two molecules and upper limits will be derived for these two molecules in the atmosphere of Titan. TEXES data will also be used for a study of the variations of HCN and HC₃N since the end of the Cassini mission, and for a retrieval of D/H from C₂HD/C₂H₂.

References

• Coustenis, A., 2021. “The Atmosphere of Titan”. In Read, P. (Ed.), Oxford Research Encyclopedia of Planetary Science. Oxford University Press. doi:https://doi.org/10.1093/acrefore/9780190647926.013.120
• Lombardo, N.A., Nixon, C.A., Greathouse, T.K., Bézard, B., Jolly, A., Vinatier, S., Teanby, N.A.A, Richter, M.J., Irwin, P.J.G., Coustenis, A., Flasar, F.M., 2019. Detection of propadiene on Titan. Astroph. J. Lett. 881, Issue 2, article id. L33, 6 pp.
• Coustenis, A., Nixon, C. A., Encrenaz, Th., Lavvas, P., 2023. Titan’s chemical composition from Cassini and ground-based measurements. IUGG 2023, Berlin, Germany, 11-20 July.

How to cite: Coustenis, A., Encrenaz, T., Greathouse, T. K., Jacquemart, D., Giles, R., Nixon, C. A., Lavvas, P., Lombardo, N., Vinatier, S., Bezard, B., Lahouari, K., Soulard, P., Tremblay, B., Jolly, A., and Steffens, B.: Ground-based monitoring of atmospheric species on Titan and a search for new nitriles with IRTF/TEXES, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5965, https://doi.org/10.5194/egusphere-egu24-5965, 2024.

In the search for life beyond Earth, the icy moons Europa and Enceladus have been brought forward as the most promising targets within our Solar System. Recently, the Enceladus Orbilander mission has gained significant interest as it has been selected as a NASA flagship mission¹. This emphasises the need for reliable in-situ instrumentation capable of biosignature detection and identification.

In-situ instrumentation must not only meet flight-capability requirements, but the detection capabilities should extend beyond single molecules or compound groups. Various groups of compounds are listed to be of astrobiological interest, such as amino acids, lipids, and nucleobases^1–3. Ideally, instruments should be capable of simultaneously detecting several different compound groups, in varying abundances from major components down to trace level. Therefore, to successfully detect both trace abundances and highly abundant compounds, a high sensitivity and wide dynamic range coverage are essential as well.

This contribution will provide a comprehensive overview of the ORIGIN (ORganics Information Gathering INstrument) space-prototype, a Laser Desorption Ionisation Mass Spectrometer (LDI-MS), designed for the in-situ detection of molecular biosignatures. ORIGIN's light-weight and robust design, includes a nanosecond pulsed laser system (λ=266 nm, 20 Hz, τ=3 ns) and a miniature reflectron-type Time-Of-Flight mass analyser (RTOF) (160 mm x Ø 60 mm)⁴. The instrument is designed to address the challenges of flight-capability, sensitivity, and dynamic range coverage, which are all essential for reliable biosignature detection on exploration missions.

ORIGIN's analytical capabilities have been demonstrated for amino acids and lipids, and have recently been extended to nucleobases^4-6. We will discuss results of the recent experiments to give an overview of ORIGIN’s detection capabilities including sensitivity and dynamic range, which are crucial for future space exploration missions. The determined limit of detection for three lipids (∼7×10−13 mol μL−1) aligns with the specified requirements in the Enceladus Orbilander mission concept (1×10−12 mol μL−1)^3,6. The application of ORIGIN towards the detection of biosignatures on icy moons and the envisioned concept of ice sample handling will also be discussed.

1. National Academies of Sciences, Engineering, and Medicine. Origins, Worlds, Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. 26522 (The National Academies Press, 2022). doi:10.17226/26522.
2. Hand, K. P. et al. Report of the Europa Lander Science Definition Team. (Jet Propulsion Laboratory, 2017).
3. MacKenzie, S. et al. Enceladus Orbilander: A Flagship Mission Concept for the Planetary Decadal Survey. vol. 2020 (John Hopkins Applied Physics Laboratory, 2020).
4. Ligterink, N. F. W. et al. ORIGIN: a novel and compact Laser Desorption – Mass Spectrometry system for sensitive in situ detection of amino acids on extraterrestrial surfaces. Sci. Rep. 10, 9641 (2020).
5. Boeren, N. J. et al. Detecting Lipids on Planetary Surfaces with Laser Desorption Ionization Mass Spectrometry. Planet. Sci. J. 3, 241 (2022).
6. Boeren N.J. et al. Laser Desorption Ionization Mass Spectrometry of Nucleobases for Future Space Exploration Missions, Planet. Sci. J., to be submitted.

How to cite: Boeren, N. J., Keresztes Schmidt, P., Tulej, M., Wurz, P., and Riedo, A.: Towards biosignature detection on Icy Moons with ORIGIN, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16084, https://doi.org/10.5194/egusphere-egu24-16084, 2024.