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arxiv: 2408.10863 · v2 · submitted 2024-08-20 · 🌌 astro-ph.EP · astro-ph.IM

LavAtmos 2.0: Incorporating Volatiles Species in Vaporization Models

Christiaan P.A. van Buchem , Mantas Zilinskas , Yamila Miguel , Wim van Westrenen This is my paper

Pith reviewed 2026-05-23 22:07 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.IM
keywords hot rocky exoplanetslava oceansatmospheric compositionvaporizationvolatile elementschemical equilibriumC/O ratio55 Cnc e
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The pith

Including volatile elements in lava vaporization models raises abundances of vaporized species and produces low C/O atmospheres that may trace surface lava oceans on hot rocky exoplanets.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper updates the LavAtmos code to model how lava oceans on hot rocky exoplanets interact with their atmospheres when volatile elements are included in the vaporization process. Earlier models considered only non-volatile species, but this work adds C-, H-, N-, S-, and P-bearing gases by coupling to FastChem for equilibrium calculations across 523 species. The results show higher partial pressures for vaporized rock components such as SiO, TiO, and Na in both simple and complex atmospheric compositions. Atmospheres above lava oceans consistently exhibit relatively low carbon-to-oxygen ratios. This low ratio emerges as a potential observable signature of exposed surface magma.

Core claim

By expanding LavAtmos to version 2.0 and integrating FastChem, the vaporization calculations now treat volatile species alongside rock-forming elements. This produces greater abundances of vaporized species such as SiO, TiO, and Na than in prior non-volatile models for all tested atmospheric compositions, including proposed cases for 55 Cnc e. The resulting atmospheres show a relatively low C/O ratio, indicating that volatile elements must be accounted for in comprehensive vaporization modeling and that this ratio could serve as a tracer for surface lava oceans.

What carries the argument

The coupling of the FastChem chemical equilibrium code into the LavAtmos vaporization routine to compute partitioning for 523 gas-phase species that include C, H, N, S, and P volatiles.

If this is right

  • Vaporised species such as SiO, TiO, and Na appear in greater abundances than in models that omit volatiles.
  • Partial pressures of vaporised species increase for every tested atmospheric composition once volatiles are included.
  • Atmospheres containing C, H, N, S, and P above a lava ocean exhibit a relatively low C/O ratio.
  • A low atmospheric C/O ratio can function as a tracer for the presence of surface lava oceans on hot rocky exoplanets.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Atmospheric retrievals from telescope data on hot rocky exoplanets may systematically misestimate composition if they ignore coupled lava-atmosphere chemistry.
  • The low C/O signature offers a target for future spectroscopic observations of known lava-ocean candidates.
  • Similar equilibrium modeling could be applied to volcanic outgassing on solar-system bodies to test consistency with observed gas ratios.

Load-bearing premise

The chemical equilibrium calculations performed by FastChem, when coupled to the lava vaporization routine, correctly capture the partitioning of all 523 species across the full range of pressures and temperatures relevant to lava-ocean atmospheres.

What would settle it

Transmission or emission spectra of a confirmed lava-ocean exoplanet that show a high C/O ratio together with lower-than-predicted SiO, TiO, or Na abundances would contradict the model's predictions.

Figures

Figures reproduced from arXiv: 2408.10863 by Christiaan P.A. van Buchem, Mantas Zilinskas, Wim van Westrenen, Yamila Miguel.

Figure 1
Figure 1. Figure 1: Diagram of the LavAtmos 2.0 equilibrium code: [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Mole fraction output comparison: Results from LavAtmos 2.0 are shown using solid lines and results from Charnoz et al. (2023) are shown using the dashed lines. A surface temperature of 2000 K and a BSE composition of the melt are assumed for both sets of calculations. K in as calculated using LavAtmos 2 (solid lines) alongside the mole fractions published in Charnoz et al. (2023) (dashed lines). These were… view at source ↗
Figure 3
Figure 3. Figure 3: Effect of pure volatile atmospheres on Si species: Shown here are the partial pressures of SiO2 (top row, panels a and b), SiO (middle row, panels c and d), and Si (bottom row, panels e and f) above a BSE lava ocean. The plots in the left column show partial pressures as a function of total volatile pressure at a fixed temperature of 3000 K. The plots in the right column show partial pressures as a functio… view at source ↗
Figure 4
Figure 4. Figure 4: Effect of a pure C atmosphere on a selection of vaporised species: The partial pressures of selected vaporised species (O2, SiO, TiO, Fe, Na, and K) in a C pure volatile atmosphere above a BSE lava ocean are shown using solid lines, while the dashed lines are used to indicate their partial pressures in volatile free atmospheres. The left panel (a) shows the partial pressures as a function of total volatile… view at source ↗
Figure 5
Figure 5. Figure 5: Effect of N-dominated complex atmospheres on Si species: Shown here are the partial pressure of SiO2, SiO, and Si as a function of total volatile pressure at a fixed BSE lava ocean surface temperature of 3000 K (panel a) and temperature at a fixed total volatile pressure of 1 bar (panel b). The solid lines indicate partial pressures in a complex volatile atmosphere (see ’N dominated’ in [PITH_FULL_IMAGE:f… view at source ↗
Figure 6
Figure 6. Figure 6: Behavior of K, Na, Si, and Ti species in S- and C-dominated complex atmospheres: [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Computed partial pressures of vaporised species in a 55-Cnc e like atmosphere: [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Computed partial pressures of volatile species in a 55-Cnc e like atmosphere: [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: C/O ratio in a 55-Cnc e like atmosphere: Shown here are the C/O ratios of two different volatile atmosphere compositions - carbon poor (grey) and carbon rich (black) as a function of total volatile pressure (panel a) at a fixed temperature of 2500 K and as a function of temperature at a fixed total volatile pressure of 10 bar (panel b). C/O ratios indicated using the solid lines are calculated using the me… view at source ↗
Figure 10
Figure 10. Figure 10: E [PITH_FULL_IMAGE:figures/full_fig_p010_10.png] view at source ↗
read the original abstract

Due to strong irradiation, hot rocky exoplanets are able to sustain lava oceans. Direct interaction between these oceans and overlying atmospheres can provide insight into planetary interiors. In order to fully understand how the composition of the atmosphere of such planets are affected by the properties of the oceans, comprehensive chemical equilibrium models are required. Thus far, most models have only taken non-volatile species into account when calculating lava vaporisation. We investigate the effect of including C-, H-, N-, S-, and P-bearing species in the equilibrium lava vaporisation calculations on the overall atmospheric composition of hot rocky exoplanets by expanding our LavAtmos code. In LavAtmos 2.0 we integrate the chemical equilibrium code FastChem to expand the considered gas phase species to 523. We apply this new approach to calculate the composition of "pure" atmospheres which contain only a single volatile element and more complex atmospheres which contain C, H, N, S, and P. We also test two proposed compositions for the atmosphere of 55-Cnc e. We find that the inclusion of volatile elements in vaporisation calculations increases the partial pressures of vaporised species for all tested atmospheric compositions. Our models indicate that the tested volatile atmospheres above a lava ocean have a relatively low C-O ratio. This demonstrates the utility of complex chemical models that better describe the chemical behavior of atmospheres across a wide range of pressures and temperatures. Volatile elements must be taken into account for comprehensive modeling of vaporisation from a surface lava ocean into a volatile atmosphere. Vaporised species such as SiO, TiO, and Na may be present in greater abundances than previously estimated. A low atmospheric C/O ratio may be able to function as a new tracer for the presence of surface lava oceans on hot rocky exoplanets.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript presents LavAtmos 2.0, an extension of the LavAtmos vaporization code that couples to the FastChem equilibrium solver to include C-, H-, N-, S-, and P-bearing volatiles, expanding the gas-phase network to 523 species. The updated model is applied to pure-volatile atmospheres, complex multi-volatile mixtures, and two proposed compositions for 55 Cnc e; the authors report that volatile inclusion raises partial pressures of vaporized species (SiO, TiO, Na) relative to prior non-volatile-only calculations and that the resulting atmospheres exhibit low C/O ratios, which they propose as a potential observational tracer for surface lava oceans on hot rocky exoplanets.

Significance. If the equilibrium calculations prove accurate, the work demonstrates that volatile species must be included for realistic vaporization modeling and supplies a concrete, observationally testable prediction (low C/O) that could help identify lava-ocean planets. The coupling of a dedicated vaporization routine to a large-network equilibrium code is a useful methodological advance for the field.

major comments (2)
  1. [Abstract] Abstract (final paragraph) and Methods (FastChem coupling description): the headline quantitative result—increased partial pressures of SiO, TiO, Na etc. once volatiles are admitted—is produced entirely by FastChem’s Gibbs-energy minimization on the 523-species network. No benchmark against JANAF tables, another equilibrium code, or high-T experimental data is reported for the expanded network at the relevant lava-ocean temperatures (>2000 K) and interface pressures; any systematic bias in partition functions or missing high-T species would directly alter the reported abundance increases and the low-C/O tracer claim.
  2. [Abstract] Abstract and § on 55 Cnc e test cases: the two atmospheric compositions tested are defined by chosen elemental abundances (free parameters listed in the model setup). No sensitivity analysis or error propagation on these abundances is described, yet the reported partial-pressure changes and C/O values are presented as robust outcomes of the volatile-inclusive calculation.
minor comments (2)
  1. The manuscript would benefit from an explicit statement of the pressure-temperature boundary conditions assumed at the lava-atmosphere interface when passing elemental abundances to FastChem.
  2. Figure captions and text should clarify whether the plotted partial pressures are equilibrium values at the surface or column-integrated quantities.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive review and the opportunity to respond to the major comments. We address each point below.

read point-by-point responses

  1. Referee: [Abstract] Abstract (final paragraph) and Methods (FastChem coupling description): the headline quantitative result—increased partial pressures of SiO, TiO, Na etc. once volatiles are admitted—is produced entirely by FastChem’s Gibbs-energy minimization on the 523-species network. No benchmark against JANAF tables, another equilibrium code, or high-T experimental data is reported for the expanded network at the relevant lava-ocean temperatures (>2000 K) and interface pressures; any systematic bias in partition functions or missing high-T species would directly alter the reported abundance increases and the low-C/O tracer claim.

    Authors: We agree that the manuscript does not report new benchmarks of the 523-species network against JANAF tables or high-temperature experiments at >2000 K. FastChem has been validated in its original publications for equilibrium chemistry across a range of conditions, and our primary result is the relative change in partial pressures when volatiles are added within the same modeling framework. We will revise the Methods section to include additional references to FastChem validations and a brief discussion of potential uncertainties arising from the thermodynamic data at lava-ocean temperatures. revision: partial

  2. Referee: [Abstract] Abstract and § on 55 Cnc e test cases: the two atmospheric compositions tested are defined by chosen elemental abundances (free parameters listed in the model setup). No sensitivity analysis or error propagation on these abundances is described, yet the reported partial-pressure changes and C/O values are presented as robust outcomes of the volatile-inclusive calculation.

    Authors: The two compositions for 55 Cnc e are adopted directly from specific literature proposals, as described in the model setup. While a dedicated sensitivity analysis on the elemental abundances was not included in the original manuscript, the low C/O ratio is recovered consistently across the pure-volatile, multi-volatile, and 55 Cnc e cases. We will add a short sensitivity test varying the key elemental abundances within plausible ranges in the revised manuscript to support the robustness of this finding. revision: yes

Circularity Check

0 steps flagged

No significant circularity; results are forward outputs of external equilibrium solver

full rationale

The paper couples its LavAtmos vaporization routine to the independent FastChem code and runs chemical-equilibrium calculations on chosen elemental inventories (pure-volatile and C/H/N/S/P mixtures). Reported partial-pressure increases and the low C/O ratio emerge directly as numerical outputs of that solver for the expanded 523-species network; no parameters are fitted to the reported abundances, no self-citation chain supplies a uniqueness theorem, and no ansatz or renaming reduces the central claim to its own inputs. The derivation is therefore a standard forward-model application and remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The model rests on the assumption that local chemical equilibrium holds at the lava-atmosphere interface and that FastChem's thermodynamic database covers all relevant species without missing important reactions or condensed phases. No new entities are postulated.

free parameters (1)
  • Atmospheric elemental abundances for 55 Cnc e test cases
    Two specific compositions are chosen and fed into the equilibrium solver; their exact values are not given in the abstract.
axioms (1)
  • domain assumption Chemical equilibrium is reached instantaneously and can be computed by minimizing Gibbs free energy for the 523 gas species.
    Invoked when FastChem is integrated to expand the species list beyond non-volatile rock elements.

pith-pipeline@v0.9.0 · 5876 in / 1384 out tokens · 41987 ms · 2026-05-23T22:07:00.452358+00:00 · methodology

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Reference graph

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