arXiv subjects

Laura Schaefer

Publications and source records attributed to Laura Schaefer.

At least 19 recordsLinked to original sources

arXiv2026

Coupled atmospHere Interior modeL Intercomparison (CHILI). I. Evolutionary Modelling -- Primordial Magma Oceans of Earth and Venus

Earth and Venus represent two evolutionary outcomes arising from initially molten 'magma ocean' periods, followed by lifetimes of chemical and geophysical divergence. Their physics is common to all rocky planets and is accessible to simulations that adopt coupled interior-atmosphere modelling approaches. Our understanding of planet histories and interpretation of current states is dependent on this modelling, yet existing codes vary in their approximations. Here, we present the first results from the Coupled atmospHere Interior modeL Intercomparison (CHILI) project; benchmarking planetary evolution codes in the context of Earth and Venus to identify key model sensitivities. Our 'nominal' Earth models predict magma ocean solidification timescales within 4 Myr of thermal evolution, and are consistent with empirical constraints on Earth's early history. Venus scenarios exhibit more diverse behaviours where prolonged magma ocean stages can be conditionally sustained for 50 Myr. Cooling timescales correlate with initial hydrogen and carbon budgets, but model-specific treatments of volatile partitioning and vertical energy transport introduce substantial inter-model variance. Different parametrisations of mantle geodynamics, convection, melting curves, rheological properties, and radiative transfer give rise to divergent evolutionary behaviours. Discrepancies in atmospheres generated by magma ocean outgassing underscore these differences, although C-H-O compositions with surface pressures exceeding 100 bar are favoured. This intercomparison identifies critical sensitivities in volatile partitioning, escape processes, mantle viscosity, and melting. Validating these treatments is essential for enabling deep insight into the early histories of the Solar System's terrestrial planets, and for drawing meaningful interpretations from ongoing observational exoplanet campaigns.

astro-ph.EP↗

arXiv2026

The dark and featureless surface of rocky exoplanet LHS 3844 b from JWST mid-infrared spectroscopy

JWST has opened a new era in the study of rocky exoplanets, enabling direct characterization of their surfaces with mid-infrared spectroscopy. Different types of rock have distinct spectral features that are diagnostic of the chemical composition and other physical properties like surface texture. Measurements of these features can provide valuable clues about a planet's geologic history and interior processes. Here we report a JWST 5-12 micron thermal emission spectrum for the rocky exoplanet LHS 3844 b. It is best matched by a dark, low-silica surface, such as basalt or other olivine-rich materials. The spectrum rules out fresh powder surfaces; however, space weathering can darken the powders and make them more consistent with the data. The data also disfavor trace concentrations of CO$_2$ or SO$_2$ gas (with 5-sigma and 3-sigma upper limits of 100 mbar and 10 microbar, respectively). Taken together, these results are well fit by an old, space-weathered surface with no evidence of accumulated volcanic gases.

astro-ph.EP↗

arXiv2025

Coupled atmospHere Interior modeL Intercomparison (CHILI) Protocol Version 1.0: A CUISINES Intercomparison Project of Magma Ocean Models

Spectroscopic characterization of rocky exoplanets with the James Webb Space Telescope has brought the origin and evolution of their atmospheres into the focus of exoplanet science. Time-evolved models of the feedback between interior and atmosphere are critical to predict and interpret these observations and link them to the Solar System terrestrial planets. However, models differ in methodologies and input data, which can lead to significant differences in interpretation. In this paper, we present the experimental protocol of the Coupled atmospHere Interior modeL Intercomparison (CHILI) project. CHILI is an (exo-)planet model intercomparison project within the Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet Studies (CUISINES) framework, which aims to support a diverse set of multi-model intercomparison projects in the exoplanet community. The present protocol includes the initial set of participating magma ocean models, divided into evolutionary and static models, and two types of test categories, one focused on Solar System planets (Earth & Venus) and the other on exoplanets orbiting low-mass M-dwarfs. Both test categories aim to quantify the evolution of key markers of the links between planetary atmospheres and interiors over geological timescales. The proposed tests would allow us to quantify and compare the differences between coupled atmosphere-interior models used by the exoplanet and planetary science communities. Results from the proposed tests will be published in dedicated follow-up papers. To encourage the community to join this comparison effort and as an example, we present initial test results for the early Earth and TRAPPIST-1 b, conducted with models differing in the treatment of energy transport in the planetary interior and atmosphere, surface boundary layer, geochemistry, and the in- and outgassing of volatile compounds.

astro-ph.EP↗

arXiv2025

Helium Depletion in Escaping Atmospheres of Sub-Neptunes: A Signature of Primary-to-Secondary Transition

Short-period sub-Neptunes are common in extrasolar systems. These sub-Neptunes are generally thought to have primary atmospheres of protoplanetary-disk gas origin. However, atmospheric escape followed by degassing from their interiors can lead to the transition to secondary atmospheres depleted in gases less-soluble to magma, such as helium. These primary and secondary atmospheres can potentially be distinguished from observations of escaping hydrogen and helium. This study aims to elucidate the impact of the primary-secondary transition on atmospheric compositions of short-period sub-Neptunes. We simulate their evolution with atmospheric escape driven by stellar X-ray and extreme ultraviolet irradiation and degassing of hydrogen, helium, and water from their rocky interiors, with a one-dimensional structure model. We show that the transition takes place for low-mass, close-in planets which experience extensive atmospheric escape. These planets show the depletion of helium and enrichment of water in their atmospheres, because of their low and high abundances in the planetary interiors, respectively. A compilation of our parameter survey (the orbital period, planetary mass, envelope mass, and mantle FeO content) shows a correlation between the planet radius and the helium escape rate. We suggest that the transition from primary to secondary atmospheres may serve an explanation for helium non-detection for relatively-small ($\lesssim 2.5\ R_\oplus$) exoplanets.

astro-ph.EP↗

arXiv2025

Oscillating Heat Transfer Prediction in Porous Structures Using Generative AI-Assisted Explainable Machine Learning

Predicting and interpreting thermal performance under oscillating flow in porous structures remains a critical challenge due to the complex coupling between fluid dynamics and geometric features. This study introduces a data-driven wGAN-LBM-Nested_CV framework that integrates generative deep learning, numerical simulation based on the lattice Boltzmann method (LBM), and interpretable machine learning to predict and explain the thermal behavior in such systems. A wide range of porous structures with diverse topologies were synthesized using a Wasserstein generative adversarial network with gradient penalty (wGAN-GP), significantly expanding the design space. High-fidelity thermal data were then generated through LBM simulations across various Reynolds (Re) and Strouhal numbers (St). Among several machine learning models evaluated via nested cross-validation and Bayesian optimization, XGBoost achieved the best predictive performance for the average Nusselt number (Nu) (R^2=0.9981). Model interpretation using SHAP identified the Reynolds number, Strouhal number, porosity, specific surface area, and pore size dispersion as the most influential predictors, while also revealing synergistic interactions among them. Threshold-based insights, including Re > 75 and porosity > 0.6256, provide practical guidance for enhancing convective heat transfer. This integrated approach delivers both quantitative predictive accuracy and physical interpretability, offering actionable guidelines for designing porous media with improved thermal performance under oscillatory flow conditions.

physics.flu-dyn↗

arXiv2024

A hybrid origin for the Martian atmosphere

The Martian isotopic record displays a dichotomy in volatile compositions. Interior volatiles from the mantle record a chondritic heritage (e.g., H, N, Kr, Xe) whereas the atmospheric reservoir of Kr and Xe - which do not currently experience escape - record heritage from a solar-like source. Motivated by disparate inferences on the source of Martian atmospheric volatiles (outgassed versus nebular captured), we consider hybrid-source accretionary atmospheres in which a high molecular weight (e.g., CO2-rich) outgassed component is mixed in with the low molecular weight H2-rich nebular atmosphere. We conduct calculations of nebular capture with and without a high molecular weight outgassed component mixed into the atmosphere during the lifetime of the solar nebula. Mixing an outgassed component into the nebular layer enhances the captured gas inventory by 1-3 orders of magnitude - depending on the outgassed inventory - relative to "pure" nebular capture. These observations and calculations suggest that the Martian atmosphere arose as a subequal mixture of outgassed and nebular-derived components, and provide a framework for assessing the role of various mechanisms of gas loss over the entire history of the planet.

astro-ph.EP↗

arXiv2024

Towards a self-consistent evaluation of gas dwarf scenarios for temperate sub-Neptunes

The recent JWST detections of carbon-bearing molecules in a habitable-zone sub-Neptune have opened a new era in the study of low-mass exoplanets. The sub-Neptune regime spans a wide diversity of planetary interiors and atmospheres not witnessed in the solar system, including mini-Neptunes, super-Earths, and water worlds. Recent works have investigated the possibility of gas dwarfs, with rocky interiors and thick H$_2$-rich atmospheres, to explain aspects of the sub-Neptune population, including the radius valley. Interactions between the H$_2$-rich envelope and a potential magma ocean may lead to observable atmospheric signatures. We report a coupled interior-atmosphere modelling framework for gas dwarfs to investigate the plausibility of magma oceans on such planets and their observable diagnostics. We find that the surface-atmosphere interactions and atmospheric composition are sensitive to a wide range of parameters, including the atmospheric and internal structure, mineral composition, volatile solubility and atmospheric chemistry. While magma oceans are typically associated with high-temperature rocky planets, we assess if such conditions may be admissible and observable for temperate sub-Neptunes. We find that a holistic modelling approach is required for this purpose and to avoid unphysical model solutions. We find using our model framework and considering the habitable-zone sub-Neptune K2-18 b as a case study that its observed atmospheric composition is incompatible with a magma ocean scenario. We identify key atmospheric molecular and elemental diagnostics, including the abundances of CO$_2$, CO, NH$_3$ and, potentially, S-bearing species. Our study also underscores the need for fundamental material properties for accurate modelling of such planets.

astro-ph.EP↗

arXiv2023

Outgassing Composition of the Murchison Meteorite: Implications for Volatile Depletion of Planetesimals and Interior-atmosphere Connections for Terrestrial Exoplanets

Outgassing is a central process during the formation and evolution of terrestrial planets and their atmospheres both within and beyond the solar system. Although terrestrial planets' early atmospheres likely form via outgassing during planetary accretion, the connection between a planet's bulk composition and its initial atmospheric properties is not well understood. One way to inform this connection is to analyze the outgassing compositions of meteorites, and in particular carbonaceous chondrites, because they are some of the most volatile-rich, primitive materials (in terms of their bulk compositions) that are available for direct study. In addition, they may serve as compositional analogs for the building block materials of terrestrial planets in our solar system and around other Sun-like stars. This study builds upon previous outgassing experiments that monitored the abundances of volatile species (e.g., H2O, CO, and CO2) released from the Murchison meteorite. To gain a more complete understanding of Murchison's outgassing composition, we perform a series of heating experiments under atmospheric pressure (1 bar) and vacuum (1E-9 bar) conditions on samples of the Murchison meteorite and subsequent bulk element analysis to inform the outgassing trends of a suite of major elements in Murchison (e.g., Fe, Mg, Zn, and S). Under both pressure conditions, sulfur outgases significantly at the highest temperatures (800C - 1000C). For the samples heated under vacuum conditions, we also detect outgassing of zinc. Combined with prior outgassing experiments, this study provides important insights into the volatile depletion patterns of undifferentiated planetesimals and the early outgassing compositions of terrestrial exoplanets.

astro-ph.EP↗

arXiv2023

No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c

Seven rocky planets orbit the nearby dwarf star TRAPPIST-1, providing a unique opportunity to search for atmospheres on small planets outside the Solar System (Gillon et al., 2017). Thanks to the recent launch of JWST, possible atmospheric constituents such as carbon dioxide (CO2) are now detectable (Morley et al., 2017, Lincowski et al., 2018}. Recent JWST observations of the innermost planet TRAPPIST-1 b showed that it is most probably a bare rock without any CO2 in its atmosphere (Greene et al., 2023). Here we report the detection of thermal emission from the dayside of TRAPPIST-1 c with the Mid-Infrared Instrument (MIRI) on JWST at 15 micron. We measure a planet-to-star flux ratio of fp/fs = 421 +/- 94 parts per million (ppm) which corresponds to an inferred dayside brightness temperature of 380 +/- 31 K. This high dayside temperature disfavours a thick, CO2-rich atmosphere on the planet. The data rule out cloud-free O2/CO2 mixtures with surface pressures ranging from 10 bar (with 10 ppm CO2) to 0.1 bar (pure CO2). A Venus-analogue atmosphere with sulfuric acid clouds is also disfavoured at 2.6 sigma confidence. Thinner atmospheres or bare-rock surfaces are consistent with our measured planet-to-star flux ratio. The absence of a thick, CO2-rich atmosphere on TRAPPIST-1 c suggests a relatively volatile-poor formation history, with less than 9.5 +7.5 -2.3 Earth oceans of water. If all planets in the system formed in the same way, this would indicate a limited reservoir of volatiles for the potentially habitable planets in the system.

astro-ph.EP↗

arXiv2022

A primordial atmospheric origin of hydrospheric deuterium enrichment on Mars

The deuterium-to-hydrogen (D/H or 2H/1H) ratio of Martian atmospheric water (~6x standard mean ocean water, SMOW) is higher than that of known sources, requiring planetary enrichment. A recent measurement by NASA's Mars Science Laboratory rover Curiosity of >3 Gyr clays yields a D/H ratio ~3x SMOW, demonstrating that most enrichment occurs early in Mars's history. As on Venus, Mars's D/H enrichment is thought to reflect preferential loss to space of 1H (protium) relative to 2H (deuterium), but the global environmental context of large and early hydrogen losses remain to be determined. Here, we apply a recent model of primordial atmosphere evolution to Mars, link the magma ocean of the accretion epoch with a subsequent water-ocean epoch, and calculate the behavior of deuterium for comparison with the observed record. We find that a ~2-3x hydrospheric deuterium-enrichment is produced if the Martian magma ocean is chemically reducing at last equilibration with the primordial atmosphere, making H2-CO the initially dominant species, with minor abundances of H2O-CO2. Reducing gases - in particular H2 - can cause greenhouse warming and prevent a water ocean from freezing immediately after the magma ocean epoch. Moreover, the pressure-temperature conditions are high enough to produce ocean-atmosphere H2O-H2 isotopic equilibrium such that surface H2O strongly concentrates deuterium relative to H2, which preferentially takes up protium and escapes from the primordial atmosphere. The proposed scenario of primordial H2-rich outgassing and escape suggests significant durations (>Myr) of chemical conditions on the Martian surface conducive to prebiotic chemistry immediately following Martian accretion.

astro-ph.EP↗

arXiv2021

The air over there: exploring exoplanet atmospheres

Atmospheric compositions for rocky exoplanets will depend strongly on the bulk planetary composition and the orbital position of the planet. Non-traditional gases may be present in the atmospheres of exceptionally hot planets. Atmospheres of more clement planets will depend on the abundances of volatiles acquired during planet formation and atmospheric removal processes, including escape, condensation, and reaction with the surface. While the observations of exoplanet atmospheres to date has focused on giant planets, a series of new space and ground-based observatories over the coming decade will revolutionize the precision and spectral resolution with which we are able to probe exoplanet atmospheres. This article consolidates lessons learned from the study of giant planet atmospheres, and points to the observations and challenges on the horizon for terrestrial planets.

astro-ph.EP↗

arXiv2021

Composition of Terrestrial Exoplanet Atmospheres from Meteorite Outgassing Experiments

Terrestrial exoplanets likely form initial atmospheres through outgassing during and after accretion, although there is currently no first-principles understanding of how to connect a planet's bulk composition to its early atmospheric properties. Important insights into this connection can be gained by assaying meteorites, representative samples of planetary building blocks. We perform laboratory outgassing experiments that use a mass spectrometer to measure the abundances of volatiles released when meteorite samples are heated to 1200 $^{\circ}$C. We find that outgassing from three carbonaceous chondrite samples consistently produce H$_2$O-rich (averaged ~66 %) atmospheres but with significant amounts of CO (~18 %) and CO$_2$ (~15 %) as well as smaller quantities of H$_2$ and H$_2$S (up to 1 %). These results provide experimental constraints on the initial chemical composition in theoretical models of terrestrial planet atmospheres, supplying abundances for principal gas species as a function of temperature.

astro-ph.EP↗

arXiv2021

Water on Hot Rocky Exoplanets

Data suggest that most rocky exoplanets with orbital period $p$ $<$ 100 d ("hot" rocky exoplanets) formed as gas-rich sub-Neptunes that subsequently lost most of their envelopes, but whether these rocky exoplanets still have atmospheres is unknown. We identify a pathway by which 1-1.7 $R_{Earth}$ (1-10 $M_{Earth}$) rocky exoplanets with orbital periods of 10-100 days can acquire long-lived 10-2000 bar atmospheres that are H$_2$O-dominated, with mean molecular weight $>$10. These atmospheres form during the planets' evolution from sub-Neptunes into rocky exoplanets. H$_2$O that is made by reduction of iron oxides in the silicate magma is highly soluble in the magma, forming a dissolved reservoir that is protected from loss so long as the H$_2$-dominated atmosphere persists. The large size of the dissolved reservoir buffers the H$_2$O atmosphere against loss after the H$_2$ has dispersed. Within our model, a long-lived, water-dominated atmosphere is a common outcome for efficient interaction between a nebula-derived atmosphere (peak atmosphere mass fraction 0.1-0.6 wt%) and oxidized magma ($>$5 wt% FeO), followed by atmospheric loss. This idea predicts that most rocky planets that have orbital periods of 10-100 days and that have radii within 0.1-0.2 $R_{Earth}$ of the lower edge of the radius valley still retain H$_2$O atmospheres. This prediction is imminently testable with JWST and has implications for the interpretation of data for transiting super-Earths.

astro-ph.EP↗

arXiv2020

Atmosphere Origins for Exoplanet Sub-Neptunes

Planets with 2 $R_{\oplus}$ < $R$ < 3 $R_{\oplus}$ and orbital period $<$100 d are abundant; these sub-Neptune exoplanets are not well understood. For example, $Kepler$ sub-Neptunes are likely to have deep magma oceans in contact with their atmospheres, but little is known about the effect of the magma on the atmosphere. Here we study this effect using a basic model, assuming that volatiles equilibrate with magma at $T$ $\sim$ 3000 K. For our Fe-Mg-Si-O-H model system, we find that chemical reactions between the magma and the atmosphere and dissolution of volatiles into the magma are both important. Thus, magma matters. For H, most moles go into the magma, so the mass target for both H$_2$ accretion and H$_2$ loss models is weightier than is usually assumed. The known span of magma oxidation states can produce sub-Neptunes that have identical radius but with total volatile masses varying by 20-fold. Thus, planet radius is a proxy for atmospheric composition but not for total volatile content. This redox diversity degeneracy can be broken by measurements of atmosphere mean molecular weight. We emphasise H$_2$ supply by nebula gas, but also consider solid-derived H$_2$O. We find that adding H$_2$O to Fe probably cannot make enough H$_2$ to explain sub-Neptune radii because $>$10$^3$-km thick outgassed atmospheres have high mean molecular weight. The hypothesis of magma-atmosphere equilibration links observables such as atmosphere H$_2$O/H$_2$ ratio to magma FeO content and planet formation processes. Our model's accuracy is limited by the lack of experiments (lab and/or numerical) that are specific to sub-Neptunes; we advocate for such experiments.

astro-ph.EP↗

arXiv2019

Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis

Transiting planets with radii 2-3 $R_\bigoplus$ are much more numerous than larger planets. We propose that this drop-off is so abrupt because at $R$ $\sim$ 3 $R_\bigoplus$, base-of-atmosphere pressure is high enough for the atmosphere to readily dissolve into magma, and this sequestration acts as a strong brake on further growth. The viability of this idea is demonstrated using a simple model. Our results support extensive magma-atmosphere equilibration on sub-Neptunes, with numerous implications for sub-Neptune formation and atmospheric chemistry.

astro-ph.EP↗

arXiv2019

Hydrogen isotopic evidence for early oxidation of silicate Earth

The Moon-forming giant impact extensively melts and partially vaporizes the silicate Earth and delivers a substantial mass of metal to Earth's core. Subsequent evolution of the magma ocean and overlying atmosphere has been described by theoretical models but observable constraints on this epoch have proved elusive. Here, we report calculations of the primordial atmosphere during the magma ocean and water ocean epochs and forge new links with observations to gain insight into the behavior of volatiles on the early Earth. As Earth's magma ocean crystallizes, it outgasses the bulk of the volatiles into the primordial atmosphere. The redox state of the magma ocean controls both the chemical composition of the outgassed volatiles and the hydrogen isotopic composition of water oceans that remain after hydrogen loss from the primordial atmosphere. Whereas water condenses and is retained, molecular hydrogen does not condense and can escape, allowing large quantities (~10^2 bars) of hydrogen - if present - to be lost from Earth in this epoch. Because the escaping inventory of H can be comparable to the hydrogen inventory in the early oceans, the corresponding deuterium enrichment can be large with a magnitude that depends on the initial H2 inventory. By contrast, the common view that terrestrial water has a carbonaceous chondrite source requires the oceans to preserve the isotopic composition of that source, undergoing minimal D-enrichment via H2 loss. Such minimal enrichment places upper limits on the amount of primordial H2 in contact with early water oceans (pH2<20 bars), implies oxidizing conditions for outgassing from the magma ocean, and suggests that Earth's mantle supplied the oxidant for the chemical resorption of metals during late accretion.

astro-ph.EP↗

arXiv2019

Semi-Lagrangian implicit Bhatnagar-Gross-Krook collision model for the finite-volume discrete Boltzmann method

A new implicit BGK collision model using a semi-Lagrangian approach is proposed in this paper. Unlike existing models, in which the implicit BGK collision is resolved either by a temporal extrapolation or by a variable transformation, the new model removes the implicitness by tracing the particle distribution functions (PDFs) back in time along their characteristic paths during the collision process. An interpolation scheme is needed to evaluate the PDFs at the traced-back locations. By using the first-order interpolation, the resulting model allows for the straightforward replacement of ${f_{\alpha}}^{eq,n+1}$ by ${f_{\alpha}}^{eq,n}$ no matter where it appears. After comparing the new model with the existing models under different numerical conditions (e.g. different flux schemes and time marching schemes) and using the new model to successfully modify the variable transformation technique, three conclusions can be drawn. First, the new model can improve the accuracy by almost an order of magnitude. Second, it can slightly reduce the computational cost. Therefore, the new scheme improves accuracy without extra cost. Finally, the new model can significantly improve the ${\Delta}t/{\tau}$ limit compared to the temporal interpolation model while having the same ${\Delta}t/{\tau}$ limit as the variable transformation approach. The new scheme with a second-order interpolation is also developed and tested; however, that technique displays no advantage over the simple first-order interpolation approach. Both numerical and theoretical analyses are also provided to explain why the new implicit scheme with simple first-order interpolation can outperform the same scheme with second-order interpolation, as well as the existing temporal extrapolation and variable transformation schemes.

physics.comp-ph↗

arXiv2019

Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b

Most known terrestrial planets orbit small stars with radii less than 60% that of the Sun. Theoretical models predict that these planets are more vulnerable to atmospheric loss than their counterparts orbiting Sun-like stars. To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve. Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point $-$ possibly indicative of atmospheric circulation. Here we report a phase curve measurement for the smaller, cooler planet LHS 3844b, a 1.3 Earth radius world in an 11-hour orbit around a small, nearby star. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of $1040\pm40$ kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are unstable to erosion by stellar wind. The data are well fitted by a bare rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.

astro-ph.EP↗