arXiv subjects

Cyril Mergny

Publications and source records attributed to Cyril Mergny.

6 recordsLinked to original sources

arXiv2026

The Physical Nature of Regolith on Icy Moons

Estimating surface properties such as porosity and grain sizes is key for planning lander missions and landing site selection on icy moons. However, spaceborne instruments do not measure the regolith properties directly: instead, they record proxy measurements such as thermal flux, which are then interpreted through modeling to estimate thermal inertia, porosity, grain size, etc. A striking conclusion from all thermal measurements that probed the uppermost surface (first millimeters) of icy moons is they all show an exceptionally low thermal inertia, ranging from 9 to 20 J.m-2.K-1.s-0.5. This value is orders of magnitude lower than that of bulk hexagonal water ice (2000 J.m-2.K-1.s-0.5) at these temperatures. We demonstrate that a regolith thermally dominated by hexagonal water ice may only achieve such thermal inertia through a combination of extremely high porosity (>80%), small grain radii (<1 mm), and an unconsolidated regolith (minimal contact area between grains), consistent with previous photometry and spectroscopy studies. For the Galilean moons, deeper thermal observations (>1 cm) have revealed higher thermal inertia (>~50 J.m-2.K-1.s-0.5), indicating that the regolith compacts over centimeter scales. Since gravity has no effect on compaction on such scale, we propose three formation scenarios to account for vertical layering: deposition cover, degradation by impactors, and temperature gradient metamorphism. We discuss how monodisperse grains can reach such extreme porosities and provide examples of experimental analogs that could best represent the regolith. We propose that high porosity regolith are favored on icy moons due to the adhesive nature of water ice and their low-gravity environment.

astro-ph.EP↗

arXiv2025

The Blinking Crystallinity of Europa: A Competition between Irradiation and Thermal Alteration

The surface of Europa experiences a competition between thermally-induced crystallization and radiation-induced amorphization processes, leading to changes of its crystalline structure. The non-linear crystallization and temperature-dependent amorphization rate, incorporating ions, electrons and UV doses, are integrated into our multiphysics surface model (MSM) LunaIcy, enabling simulations of these coupled processes on icy moons. Thirty simulations spanning 100 000 years, covering the full ranges of albedo and latitude values on Europa, explore the competition between crystallization and irradiation. This is the first modeling of depth-dependent crystallinity profiles on icy moons. The results of our simulations are coherent with existing spectroscopic studies of Europa, both methods showing a primarily amorphous phase at the surface, followed by a crystalline phase after the first millimeter depth. Our method provides quantitative insights into how various parameters found on Europa can influence the subsurface crystallinity profiles. Interpolating upon our simulations, we have generated a crystallinity map of Europa showing, within the top millimeter, highly crystalline ice near the equator, amorphous ice at the poles, and a mix of the two at mid-latitudes. Regions/depths with balanced competition between crystallization and amorphization rates are of high interest due to their periodic fluctuations in crystalline fraction. Our interpolated map reveals periodic variations, with seasonal amplitudes reaching up to 35% of crystalline fraction. These variations could be detected through spectroscopy, and we propose a plan to observe them in forthcoming missions.

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arXiv2024

LunaIcy: Exploring Europa's Icy Surface Microstructure through Multiphysics Simulations

A multiphysics simulation model incorporating a sintering model coupled with the MultIHeaTS thermal solver was developed to study the evolution of icy moons' microstructure. The sintering process is highly dependent on temperature, and this study represents the first attempt in planetary science to examine the coupled interaction between heat transfer and sintering. Our approach to ice sintering is based upon the literature while offering a refined description of the matter exchange between grains, bonds, and the pore space. By running the numerical framework, we simulate the evolution of ice microstructure on Galilean satellites, specifically tracking the changes in the ice grain and bond radii over time. LunaIcy, our multiphysics model, was applied to study the evolution of Europa's ice microstructure over 1 million yr along its orbit, with a parameter exploration to investigate the diverse configurations of the icy surface. Our results indicate that effective sintering can take place in regions where daily temperatures briefly surpass 115 K, even during short intervals of the day. Such sintering could not have been detected without the diurnal thermal coupling of LunaIcy due to the cold daily mean temperature. In these regions, sintering occurs within timescales shorter than Europa's ice crust age, suggesting that, in present times, their surface is made of an interconnected ice structure.

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arXiv2024

MultIHeaTS: A Fast and Stable Thermal Solver for Multilayered Planetary Surfaces

A fully implicit scheme is proposed for solving the heat equation in 1D heterogeneous media, available as a computationally efficient open-source Python code. The algorithm uses finite differences on an irregular grid and is unconditionally stable due to the implicit formulation. The thermal solver is validated against a stiff analytical solution, demonstrating its robustness in handling stiff initial conditions. Its general applicability for heterogeneous cases is demonstrated through its use in a planetary surface scenario with nonlinear boundary conditions induced by blackbody thermal emission. MultIHeaTS's advantageous stability allows for computation times up to 100 times faster than Spencer's explicit solver, making it ideal for simulating processes on large timescales. This solver is used to compare the thermal signatures of homogeneous and bilayer profiles on Europa. Results show that homogeneous materials cannot reproduce the thermal signature observed in bilayer profiles, emphasizing the need for multilayer solvers. In order to optimize the scientific return of a space mission, we propose a strategy made of three local time observations that is enough to identify bilayer media, for instance, for the next missions to the Jovian system. A second application of the solver is the estimation of the temperature profile of Europa's near surface (first 10s m) throughout a 1 million yr simulation with varying orbital parameters. The probability distribution of temperature through depth is obtained. Among its various applications, MultIHeaTS serves as the core thermal solver in a multiphysics simulation model detailed in the companion article by C. Mergny & F. Schmidt.

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arXiv2024

Gravity-Induced Ice Compaction and Subsurface Porosity on Icy Moons

Our understanding of the surface porosity of icy moons and its evolution with depth remains limited, including the precise scale at which ice compaction occurs under self-weight pressure. This parameter is of crucial interest for the correct interpretation of current remote sensing data (spectroscopy in the visible, infrared to passive microwave) but also for planetary exploration when designing a lander, a rover or a cryobot. In situ exploration of the ice crust would require knowledge about subsurface porosity. This study employs a compaction model solely driven by overburden pressure based on prior research. The formulation for density as a function of depth, incorporates an essential parameter: the ice compaction coefficient. To determine this coefficient, we fit our depth-dependent density model to existing data obtained from Earth-based measurements of ice cores in Antarctica and North Greenland. Our results yield a typical lengthscale for ice compaction on Earth of approximately 20.1 $\pm$ 0.6 m , consistent with the existing literature. We apply the model to Europa, which due to its lower gravity, has a typical ice compaction scale of 150 $\pm$ 4 m. We compare it with the depths scanned by current spaceborne data and find that porosity can be considered constant when accounting only for gravity-induced compaction.

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arXiv2019

A generic frequency dependence for the atmospheric tidal torque of terrestrial planets

Thermal atmospheric tides have a strong impact on the rotation of terrestrial planets. They can lock these planets into an asynchronous rotation state of equilibrium. We aim at characterizing the dependence of the tidal torque resulting from the semidiurnal thermal tide on the tidal frequency, the planet orbital radius, and the atmospheric surface pressure. The tidal torque is computed from full 3D simulations of the atmospheric climate and mean flows using a generic version of the LMDZ general circulation model (GCM) in the case of a nitrogen-dominated atmosphere. Numerical results are discussed with the help of an updated linear analytical framework. Power scaling laws governing the evolution of the torque with the planet orbital radius and surface pressure are derived. The tidal torque exhibits i) a thermal peak in the vicinity of synchronization, ii) a resonant peak associated with the excitation of the Lamb mode in the high frequency range, and iii) well defined frequency slopes outside these resonances. These features are well explained by our linear theory. Whatever the star-planet distance and surface pressure, the torque frequency spectrum -- when rescaled with the relevant power laws -- always presents the same behaviour. This allows us to provide a single and easily usable empirical formula describing the atmospheric tidal torque over the whole parameter space. With such a formula, the effect of the atmospheric tidal torque can be implemented in evolutionary models of the rotational dynamics of a planet in a computationally efficient, and yet relatively accurate way.

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