Philip L. Varghese

The morphology of the surface ice on Europa remains unknown at the meter scale. Hobley et al. (2018, https://doi.org/10.1038/s41561‐018‐0235‐0 ) suggested the possible presence of penitentes—bladed ice structures that form on Earth—on the surface of Europa, based on analogy to Earth. Characterized as instability features, penitentes grow due to differential sublimation erosion and ablation of a snowfield. Previous studies have provided contrasting results when examining the stability of the terrain on Europa from isolated radiative heat transfer and molecular transport mechanisms. We analyze these mechanisms simultaneously, leveraging Physics‐Informed Neural Networks to identify physical conditions and suitable terrains that could potentially host centimeter‐to meter‐scale ridges and penitentes on Europa. We find that for the specific condition of a pre‐defined sinusoidal ice morphology, values of ice thermal inertia near the lower limit of physically plausible values for low density, pure water ice may lead to the growth of features such as penitentes. Conversely, higher values of thermal inertia may lead to erosion of morphologies and convergence toward a flat surface. Our results can help inform the interpretation of NASA Europa Clipper and ESA Jupiter Icy Moons Explorer (JUICE) observations, and help guide landing site selection for future missions to the surface of Europa.

Conical cold gas thrusters are characterized for a contactless method to detumble space debris using plume impingement. Vacuum chamber experiments and matching simulations using Computational Fluid Dynamics (for continuum flow within the nozzle) and Direct Simulation Monte Carlo (for rarefied flow inside the vacuum chamber) are performed to infer scaling laws of imparted force on a [ ] flat plate target as a function of plenum pressure, and the target’s axial and vertical displacement. The increase in vacuum chamber back pressure over time results in plume narrowing and an associated rise in pressure force experienced by the target. The simulations capture the rise and fall of the pressure force on the target in comparison with the experimental results, with the peak force discrepancy recorded to be less than 15% in the majority of cases, validating the reliability of the simulation tools for predicting plume impingement forces at related conditions and larger offset distances in a spacelike environment. The validated simulation tool highlights the challenges and constraints associated with the ground-based testing of orbital plume impingement.

Mercury's polar cold traps host water ice deposits that are likely populated with impact‐delivered water via Mercury's exosphere. However, Mercury's near‐sun location experiences an extremely high photodestruction rate that rapidly destroys water with a timescale of only ∼3.5 hr. Here, we use the PLANET direct simulation Monte Carlo (DSMC) code to investigate the fate of water from a single 1 km radius comet impact striking Mercury's North Pole (30 km/s at angle of 60°). We find that the evolving plume separates into four distinct phases: (a) an early plume phase in which ballistic escape and photodestruction reach their peaks, (b) a reentry phase in which water falling back toward the surface forms a self‐shielded shock‐topped atmosphere that migrates across the surface and ballistic loss ceases, (c) a quasi‐steady phase (QSP) in which a self‐shielding dawn atmospheric enhancement forms and drives a tenuous migration of exospheric water to the cold traps with a longitudinal dependence, and finally (d) a late phase in which self‐shielding ends and photodestruction dominates, effectively ending substantial water migration. In this work, we quantify the fates of the arriving water molecules, and describe some of the more important features of this highly unsteady, evolving three‐dimensional atmosphere. We find that 23% of the initial water is photodestroyed, 65% of the water ballistically escapes the system (of which, 79% photodissociates prior to reaching the Hill radius), and 14% ends up in Mercury's cold traps, which is significantly more than the ∼5% that migrates to the Moon's cold traps during an equivalent impact.

A comprehensive collisional-radiative model is developed and coupled with a one-dimensional two-temperature fluid model to study capacitively coupled radio-frequency argon discharges at pressures ranging from 0.5 to 5 Torr and electrode peak-to-peak voltages of 150 and 300 V. The fluid model is formulated using the drift-diffusion approximation. The CR model accounts for the argon ground state, 30 excited states in the 4 s, 4p, 3d, and 5 s manifolds, electrons, atomic (Ar⁺) and molecular (Ar 2 + ) ions, and argon excimer molecules (Ar 2 ∗ ). The processes included in the model are electron-impact excitation and ionization, radiation emission, heavy-particle collisions, radiative recombination, and processes involving Ar 2 + and Ar 2 ∗ molecules. Non-Maxwellian electron energy distribution functions, calculated using Bolsig+, are considered, with fine-structure collisional cross-sections acquired from the LXCat database. A detailed comparison is presented between numerical predictions and experimental measurements from a glow discharge device developed at UT Austin, focusing on populations of excited states in the 4p and 4 s manifolds, and electron number density. Additional comparisons are made with experimental data and PIC/Monte Carlo simulations from the study by (Donkó et al 2023 Plasma Sources Sci. Technol. 32 065002). While agreement with the Donkó et al data is somewhat improved—particularly with the PIC/Monte Carlo results—the CR-fluid model systematically overpredicts the experimental values. Possible contributing factors to these discrepancies and improvements to the model are discussed. In addition, the sensitivity of the results to the EEDF modeling is assessed by comparing simulations using Bolsig+-derived, Druyvesteyn, and Maxwellian distributions. The results show that predictions of electron temperature and plasma potential are strongly affected by the assumed EEDF shape. The inclusion of higher-lying excited states beyond the 4p manifold was found to have a minimal impact on discharge dynamics under the conditions studied. The code has been made publicly available on GitHub as open-source software.

This study investigates the water plumes of Saturn's moon, Enceladus, using Direct Simulation Monte Carlo (DSMC) modeling to analyze venting dynamics and plume structures. Building on prior research, we employ a parametrized DSMC approach to model water vapor and ice particle flows, leveraging Cassini spacecraft data from instruments such as the Ion and Neutral Mass Spectrometer and the Ultraviolet Imaging Spectrograph. The study explores whether vent conditions, such as mass flow rates, mixture temperatures, and particle sizes, can be inferred from observational data. We develop a computational framework to expand plume simulations beyond 10 km altitudes, incorporating gravitational and inertial forces in an Enceladus‐fixed reference frame. A sensitivity analysis correlates vent parameters with observed data, identifying critical contributors such as vent orientation and location, mass flow rate, exit temperature, and ice grain characteristics. This approach reduces the dimensionality of fitting procedures, enabling robust parameter constraints and a more detailed understanding of plume dynamics. Key findings include constrained values for mass flow rates, ice grain radii assuming single‐size particles, and exit temperatures (∼44–61 K), consistent with theoretical predictions. Additionally, variations in vent orientation and positional parameters were refined from the work of Porco et al. (2014, https://doi.org/10.1088/0004‐6256/148/3/45 ). These results highlight the importance of collision dynamics in shaping plume structures. This work establishes a computationally efficient methodology for analyzing cryovolcanic plumes applicable to future missions exploring icy moons such as Enceladus or Europa. By prioritizing sensitive parameters, the study offers insights for optimizing observational strategies to maximize scientific yield.

Optical emission spectroscopy is performed in a capacitively coupled 13.56 MHz radio-frequency argon plasma discharge with a 2 cm gap between parallel plate electrodes. Cycle-averaged measurements are made at the center of the discharge volume at pressures of 0.1 Torr to 10 Torr and peak-to-peak voltages ⩽ 1 kV. Spectra are calibrated for absolute spectral radiance, and populations of excited electronic states are inferred from the atomic transition lines. Measurements of all ten states comprising the Ar(4p) grouping are reported individually, and the population distributions analyzed as functions of both pressure and voltage. It is found that the population densities increase with applied voltage. The state distribution is highly non-equilibrium and the relative populations are insensitive to voltage or pressure at pressures ⩽ 1 Torr. At higher pressures, the relative population distribution becomes a function of both pressure and voltage. Uncertainty quantification is performed using a Bayesian approach, making the reported population densities useful for plasma modeling purposes and validation of numerical simulations.

We present an algorithm for solving the one-dimensional space collisional Boltzmann transport equation (BTE) for electrons in low-temperature plasmas (LTPs). Modeling LTPs is useful in many applications, including advanced manufacturing, material processing, and hypersonic flows, to name a few. The proposed BTE solver is based on an Eulerian formulation. It uses Chebyshev collocation method in physical space and a combination of Galerkin and discrete ordinates in velocity space. We present self-convergence results and cross-code verification studies compared to an in-house particle-in-cell (PIC) direct simulation Monte Carlo (DSMC) code. Boltzsim is our open source implementation of the solver. Furthermore, we use Boltzsim to simulate radio-frequency glow discharge plasmas (RF-GDPs) and compare with an existing methodology that approximates the electron BTE. We compare these two approaches and quantify their differences as a function of the discharge pressure. The two approaches show an 80x, 3x, 1.6x, and 0.98x difference between cycle-averaged time periodic electron number density profiles at 0.1 Torr, 0.5 Torr, 1 Torr, and 2 Torr discharge pressures, respectively. As expected, these differences are significant at low pressures, for example less than 1 Torr.

We developed a Monte‐Carlo‐based radiative heat transfer model capable of simulating solar exposure and subsequent warming of rough snow and ice surfaces on ice‐covered airless solar system bodies. The model accounts for wavelength‐dependent internal light scattering and heat conduction in the snow interior down to meter‐scale depths. We validated the model against analytical and experimental test cases with relevant applications to Europa, one of Jupiter's moons. We examined differential heating across the surface, from the centimeter to meter scale, to reveal potential patterns of preferential sublimation that could lead to rough ice morphologies, such as penitentes. An exploration of parameters such as penitente height‐width ratios, shape, size, snow grain size, and thermal properties revealed that taller, thinner, larger penitentes with sharper peaks, coarser snow grain sizes, and lower thermal inertias are more likely to grow in Europa's environment near the equator.