arXiv subjects

Leonardos Gkouvelis

Publications and source records attributed to Leonardos Gkouvelis.

9 recordsLinked to original sources

arXiv2026

Helium-poor winds do not require helium-poor planets

Observations of the metastable He,{\sc i},10830,Å triplet have revealed escaping exoplanet atmospheres whose inferred helium abundances range from nearly nebular compositions to strongly helium-depleted winds. Such depletion is commonly interpreted as evidence for atmospheric evolution, preferential escape, or departures from primordial composition. We present a closed-form analytic transport-retention theory for the atmospheric transition region between the homopause and the base of the planetary wind. The theory quantifies the competition between upward transport and molecular separation, demonstrating that transport physics alone can substantially deplete the helium abundance supplied to the escaping flow. Our solution yields a retention factor, $χ_{\rm He}$, that measures the fraction of helium supplied to the base of the hydrodynamic wind relative to the deep atmospheric abundance. Combining the analytic framework with numerical forward models of the He,{\sc i},10830,Å absorption and a sample of twelve helium-observed sub-Neptunes and mini-Neptunes, we find that observed systems span the full range of predicted retention states, from nearly complete retention to strong helium depletion. These forward models use the helium abundance supplied by the analytic framework as the lower-boundary condition to predict the corresponding He,{\sc i},10830,Å absorption. We further show that increasing helium retention systematically strengthens the expected absorption signal by increasing the helium reservoir available to the upper atmosphere. These results suggest that helium depletion does not necessarily imply intrinsically helium-poor atmospheres or evolutionary transitions toward secondary compositions, but may instead be a natural consequence of transport physics in escaping atmospheres.

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arXiv2026

Global evolution of electric fields during planet encircling dust storms on Mars

Planet-encircling dust storms fundamentally reshape Martian weather and the near-surface electrostatic environment. We investigate the generation and evolution of electric fields during global dust storms using bimodal dust size distributions from the NASA Ames Mars Global Climate Model, coupled with a triboelectric charging and electrostatic diagnostic scheme that links collisional charging to the local dynamical state of the atmosphere. Focusing on the dust-lifting and buildup phase and its subsequent evolution, we quantify the electric-field energy density and discharge characteristics, including onset thresholds, event frequency, and spatial clustering. The simulations reveal broad storm-active belts of enhanced electrification, with the most frequent threshold exceedances occurring in southern low-to-mid latitudes and secondary activity in northern low-to-mid latitudes. Modeled near-surface electric fields reach $10^{2}$--$10^{3}\ \mathrm{V\,m^{-1}}$ comparable to values inferred for smaller-scale dust phenomena. The results indicate that electric-field generation is controlled by the interplay between dust loading, turbulence-driven collisional activity, and conductivity-dependent charge relaxation, with diurnal conductivity variations strongly suppressing daytime electric-field buildup and most events remaining in the weak glow or Townsend discharge regime. While the model captures the large-scale distribution of electrically favorable conditions, the predicted spatial extent of activity likely represents an upper bound, as small-scale turbulent structures are not fully resolved. These results provide a quantitative framework to identify regions where electrostatic discharges are most likely during GDSs and to inform instrument design, power-system protection, and operations planning for future robotic and human missions.

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arXiv2026

A theory of transmission spectroscopy of hydrodynamic outflows from planetary atmospheres: Spectral-line saturation and limits on mass-loss constraints

Transmission spectroscopy is a key technique in the characterization of exoplanet atmospheres and has been widely applied to planets undergoing hydrodynamic escape. While a robust analytic theory exists for transmission spectra of hydrostatic atmospheres, the corresponding interpretation for escaping atmospheres has so far relied on numerical modeling, despite the growing number of observations of planetary winds. In this work, a theory of transmission spectroscopy in hydrodynamically escaping atmospheres is developed by coupling the standard transmission geometry to a steady-state, spherically symmetric, isothermal outflow. This approach yields closed-form expressions for the chord optical depth and effective transit radius of a planetary wind and allows the optical depth inversion problem to be examined. The analytic solution reveals that transmission spectroscopy of planetary winds naturally separates into two regimes. In an opacity-limited regime, transmission depths retain sensitivity to the atmospheric mass-loss rate $\dot{M}$. Beyond a critical threshold, however, spectral-line cores become saturated and no longer provide a unique constraint on the escape rate. This transition is marked by a sharp analytic boundary of the form $σ(λ)\,\dot{M} \le C_{\rm sat}$, where $σ(λ)$ is the line absorption cross-section and $C_{\rm sat}$ is a constant set by the thermodynamic and geometric properties of the wind. This condition specifies when the inversion between transmission depth and mass-loss rate admits a real solution. Once it is violated, the effective transit radius is no longer controlled by opacity or mass loss, but by the geometric extent of the absorbing wind.

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arXiv2026

A Closed-Form Analytical Theory of Non-Isobaric Transmission Spectroscopy for Exoplanet Atmospheres

Analytical models are essential for building physical intuition and guiding the interpretation of exoplanet observations by clarifying the dependencies that shape atmospheric signatures. We present a generalization of the classical isothermal, isobaric transmission model by allowing the opacity to vary with pressure as a power law, $κ\propto P^{n}$, and explicitly defining the reference opacity $κ_{0}$ at a chosen pressure $P_{0}$. By treating the slant optical depth as an Abel transform of the radial absorption coefficient, we derive a closed-form expression for the effective transit radius in a hydrostatic, isothermal atmosphere with pressure-dependent opacity. The solution provides a compact framework for exploring non-isobaric effects and explicitly links the vertical opacity gradient to observable spectral features. We benchmark the model against empirical transmission spectra of Earth and the hot Jupiter WASP-39b, finding a significantly improved fit relative to the isobaric formula. This generalized expression offers a physically interpretable foundation for analyzing high-precision spectra from JWST and upcoming ARIEL observations, and can serve as a basis for semi-analytical retrieval approaches optimized for computational efficiency.

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arXiv2025

Data availability and requirements relevant for the Ariel space mission and other exoplanet atmosphere applications

The goal of this white paper is to provide a snapshot of the data availability and data needs primarily for the Ariel space mission, but also for related atmospheric studies of exoplanets and brown dwarfs. It covers the following data-related topics: molecular and atomic line lists, line profiles, computed cross-sections and opacities, collision-induced absorption and other continuum data, optical properties of aerosols and surfaces, atmospheric chemistry, UV photodissociation and photoabsorption cross-sections, and standards in the description and format of such data. These data aspects are discussed by addressing the following questions for each topic, based on the experience of the "data-provider" and "data-user" communities: (1) what are the types and sources of currently available data, (2) what work is currently in progress, and (3) what are the current and anticipated data needs. We present a GitHub platform for Ariel-related data, with the goal to provide a go-to place for both data-users and data-providers, for the users to make requests for their data needs and for the data-providers to link to their available data. Our aim throughout the paper is to provide practical information on existing sources of data whether in databases, theoretical, or literature sources.

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arXiv2025

Limb Asymmetries on WASP-39b: A Multi-GCM Comparison of Chemistry, Clouds, and Hazes

With JWST, observing separate spectra of the morning and evening limbs of hot Jupiters has finally become a reality. The first such observation was reported for WASP-39b, where the evening terminator was observed to have a larger transit radius by about 400 ppm and a stronger 4.3 $μ$m CO$_2$ feature than the morning terminator. Multiple factors, including temperature differences, photo/thermochemistry, clouds and hazes, could cause such limb asymmetries. To interpret these new limb asymmetry observations, a detailed understanding of how the relevant processes affect morning and evening spectra grounded in forward models is needed. Focusing on WASP-39b, we compare simulations from five different general circulation models (GCMs), including one simulating disequilibrium thermochemistry and one with cloud radiative feedback, to the recent WASP-39b limb asymmetry observations. We also post-process the temperature structures of all simulations with a 2D photochemical model and one simulation with a cloud microphysics model. Although the temperatures predicted by the different models vary considerably, the models are remarkably consistent in their predicted morning--evening temperature differences. Several equilibrium-chemistry simulations predict strong methane features in the morning spectrum, not seen in the observations. When including disequilibrium processes, horizontal transport homogenizes methane, and these methane features disappear. However, even after including photochemistry and clouds, our models still cannot reproduce the observed ${\sim}2000$ ppm asymmetry in the CO$_2$ feature. A combination of factors, such as varying metallicity and unexplored parameters in cloud models, may explain the discrepancy, emphasizing the need for future models integrating cloud microphysics and feedback across a broader parameter space.

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arXiv2025

Interior redox state effects on the stability of secondary atmospheres and observational manifestations: LP 791-18 d as a case study for outgassing rocky exoplanets

Recent advances in space and ground-based facilities now enable atmospheric characterization of a selected sample of rocky exoplanets. These atmospheres offer key insights into planetary formation and evolution, but their interpretation requires models that couple atmospheric processes with both the planetary interior and the surrounding space environment. This work focuses on the Earth-size planet LP791 18d, which is estimated to receive continuous tidal heating due to the orbital configuration of the system; thus, it is expected to exhibit volcanic activity. We estimate the mantle temperature of 1680-1880 K. Our results show that the atmospheric mean molecular weight gradient is controlled by oxygen fugacity rather than bulk metallicity. Furthermore, we use the atmospheric steady-state solutions produced from the interior redox state versus surface pressure parameter space and explore their atmospheric stability. We find that stability is achieved only in highly oxidized scenarios while reduced interior states fall into the hydrodynamic escape regime with mass loss rates on the order of 10^5-10^8 kg/s. We argue that scenarios with reduced interior states are likely to have exhausted their volatile budget during the planets lifetime. Furthermore, we predict the atmospheric footprint of the planets interior based on its oxidation state and assess its detectability using current or forthcoming tools to constrain the internal and atmospheric composition. We show that the degeneracy between bare rock surfaces and thick atmospheres can be resolved by using three photometric bands to construct a color-color diagram that accounts for potential effects from photochemical hazes and clouds. Our modeling approach connects interior and atmospheric processes, providing a basis to explore volatile evolution and potential habitability.

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arXiv2025

High-resolution Ultraviolet-to-nearinfrared Characterization of Exoplanet Atmospheres

The Habitable Worlds Observatory (HWO) offers a unique opportunity to revolutionize our understanding of planetary formation and evolution. The goal of this Science Case Development Document (SCDD) is to investigate the physical and chemical processes that shape the composition and atmospheric mass loss in exoplanets. We review the key observables currently known as diagnostics of mass loss via transit observations, i.e., absorption lines of escaping hydrogen (Lyman-alpha), helium, and metals (Fe, Mg, C, O). We also explore the challenges to infer planetary formation processes based on atmospheric composition characterization. HWO could enable a broad, continuous coverage from far-ultraviolet to near-infrared spectroscopy (~100--1600 nm) at high resolution (R > 60, 000), which is essential to make these measurements, disentangle their planetary origin from stellar activity, and ultimately, contextualize the escape rates by simultaneously characterizing the composition, cloud predominance, and thermal structure of exoplanet atmospheres.

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arXiv2025

An Analytical Model of Wavelength-dependent Opposition Surge in Emittance and Reflectance Spectroscopy of Airless Rocky Exoplanets

Theoretical frameworks for reflection and emission spectroscopy of exoplanet surfaces are becoming increasingly important for the characterization of rocky exoplanets, especially with the rapid growth of the detected exoplanet population and observational capabilities. The Hapke theory of reflectance and emittance spectroscopy has been widely adopted in the exoplanet community, yet a key physical effect - the opposition surge enhancement at small phase angles - remains largely neglected. This phenomenon, driven by shadow hiding and coherent backscattering, introduces a significant brightening that depends on wavelength, particle size, and surface morphology. In this paper, I propose an alternative formulation for opposition surge modeling, ensuring a smooth-to-sharp transition at small phase angles, dictated by wavelength-dependent scattering properties. I evaluate the impact of opposition surge on phase curves and surface spectra, comparing a family of models with increasing simplifications, ranging from a full wavelength-dependent opposition effect to its complete omission. My results indicate that neglecting opposition effects can introduce systematic deviations in retrieved albedos, spectral features, and phase curves, with errors reaching up to 20%-30% in certain spectral bands. Upcoming JWST observations will probe phase angles below ~10° for rocky exoplanets around M dwarfs; thus, accounting for opposition effects is crucial for accurate surface characterization. Proper treatment of this effect will lead to improved retrievals of surface albedo, mineralogical composition, and roughness properties. This study establishes a physically consistent framework for exoplanet phase-curve modeling and provides a foundation for future retrieval algorithms aimed at interpreting exoplanet surfaces.

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