🔭 astrophysics

LavAtmos 2.0: Incorporating Volatiles Species in Vaporization Models

This paper introduces LavAtmos 2.0, an updated chemical equilibrium model that incorporates volatile species (C, H, N, S, P) into lava vaporization calculations, revealing that including these elements significantly increases the partial pressures of vaporized species and results in low atmospheric C/O ratios that could serve as a new tracer for surface lava oceans on hot rocky exoplanets.

Original authors: Christiaan P. A. van Buchem, Mantas Zilinskas, Yamila Miguel, Wim van Westrenen

Published 2026-04-21

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CC BY 4.0

Original authors: Christiaan P. A. van Buchem, Mantas Zilinskas, Yamila Miguel, Wim van Westrenen

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a planet so close to its star that its surface isn't just hot; it's a bubbling, churning ocean of molten rock, like a global lava lamp. This is what scientists think happens to "Ultra-Short Period" rocky planets. For a long time, researchers tried to figure out what the air (atmosphere) above these lava oceans looks like.

Here is the problem they were facing: They were trying to predict the air's recipe by only looking at the ingredients coming from the lava itself (like silicon, magnesium, and iron). They assumed the air was "dry" and empty of other stuff.

But, new telescopes (like the James Webb Space Telescope) are finding that some of these planets might actually have thick, "wet" atmospheres filled with volatile gases like water vapor, carbon dioxide, and hydrogen. The old models were like trying to bake a cake while ignoring the fact that someone just dumped a bucket of milk into the batter. The result? The recipe was wrong.

Enter "LavAtmos 2.0": The New Kitchen Simulator

The authors of this paper, led by C.P.A. van Buchem, built a new computer program called LavAtmos 2.0. Think of this program as a super-advanced kitchen simulator that finally lets you mix the "lava ingredients" with the "atmosphere ingredients" and see how they react together.

Here is how their new discovery works, using some simple analogies:

1. The "Oxygen Thief" Effect

In the old models, the lava ocean would release oxygen into the air, creating a lot of free oxygen gas ( $O_2$ ).

The Old Way: Imagine the lava ocean is a factory pumping out oxygen. The air gets full of it.
The New Way (LavAtmos 2.0): When you add volatile gases (like Carbon or Hydrogen) to the mix, they act like oxygen thieves. They grab onto that free oxygen and lock it up in new molecules (like Carbon Dioxide or Water).
The Result: Because the "oxygen thieves" steal the free oxygen, the pressure of free oxygen drops.

2. The "Pressure Cooker" Reaction

Why does stealing oxygen matter? Because the lava ocean reacts to how much free oxygen is in the air.

The Analogy: Imagine the lava ocean is a pressure cooker. If the air above it is full of free oxygen, the lava is "calm" and doesn't want to release much of its own rocky vapor. But if the air is "stolen" of its oxygen (low oxygen pressure), the lava gets excited and starts boiling over much more aggressively.
The Discovery: By adding those volatile "thieves," the lava ocean releases much more of its rocky vapor (like Silicon Monoxide, Sodium, and Potassium) than we previously thought. In fact, the amount of rocky vapor can increase by huge amounts—sometimes by a factor of 100 or 1,000!

3. The "55 Cancri e" Mystery

The team tested their new model on a famous planet called 55 Cancri e. This planet is a hot, rocky world that astronomers are trying to understand.

The Puzzle: Previous observations suggested 55 Cancri e might have a thick atmosphere, but the data was confusing. Some models said it had a lava ocean; others said it didn't.
The New Clue: The researchers found that if 55 Cancri e has a lava ocean and a volatile atmosphere, the chemistry changes in a specific way. The lava ocean acts like a giant sponge, soaking up oxygen from the air and changing the balance of Carbon to Oxygen (the C/O ratio).
The Takeaway: If we look at the light from this planet and see a low Carbon-to-Oxygen ratio, it might be a "smoking gun" that a lava ocean is sitting right underneath that atmosphere, feeding it oxygen.

Why This Matters

Before this paper, scientists thought that thick, gassy atmospheres would hide the signs of a lava ocean, making it impossible to see.

The Old Belief: "If there's a thick gas cloud, we can't see the lava."
The New Reality: "Actually, the gas cloud makes the lava boil harder, making the signs of the lava (like Sodium and Silicon) even brighter and easier to spot!"

Summary

Think of the atmosphere and the lava ocean as two people in a dance.

Old Model: They danced separately. The lava did its thing, and the gas did its thing, and they didn't really interact.
New Model (LavAtmos 2.0): They are now dancing together. The gas changes the music, and the lava changes its steps. The result is a much more energetic dance, where the lava releases more vapor, and the whole system looks different than we expected.

This new understanding helps astronomers know exactly what to look for when they point their giant telescopes at these scorching worlds, potentially solving the mystery of whether they are covered in oceans of fire.

1. Problem Statement

Ultra-short period (USP) rocky exoplanets are subjected to intense stellar irradiation, often maintaining surface temperatures high enough to sustain global lava oceans. These oceans act as a direct interface between the planetary interior and the atmosphere, allowing for thermochemical equilibrium.

Limitation of Previous Models: Existing models (e.g., LavAtmos 1.0, Piette et al. 2023) typically assume that volatile elements (Carbon, Hydrogen, Nitrogen, Sulfur, Phosphorus) are absent from the atmospheres of USP planets due to atmospheric erosion. Consequently, they calculate lava vaporization in a "volatile-free" environment and simply add volatile abundances to the atmosphere afterward.
The Gap: This approach fails to account for the chemical feedback loop where volatile species in the atmosphere react with oxygen released from the lava, altering the oxygen fugacity ( $fO_2$ ) and, consequently, the vaporization rates of rock-forming species (SiO, SiO $_2$ , Na, K, etc.). Recent observations of 55 Cnc e by JWST suggest the presence of a significant volatile atmosphere, necessitating a model that self-consistently couples lava vaporization with volatile chemistry.

2. Methodology

The authors developed LavAtmos 2.0, an updated open-source Python code that integrates the chemical equilibrium solver FastChem (Kitzmann et al. 2023) with the existing thermodynamic framework of LavAtmos.

Core Workflow:
1. Input: Melt composition (e.g., Bulk Silicate Earth), surface temperature ( $T$ ), total volatile pressure ( $P_{vol}$ ), and elemental abundances of volatiles (C, H, N, S, P).
2. Iterative Solution: The code solves for the partial pressure of oxygen ( $P_{O_2}$ $P_{O_{2}}$ ) that satisfies both the Law of Mass Action (for vaporization reactions) and Mass Conservation (for the total atmospheric budget).
  - Inner Loop: Guesses an atmospheric Oxygen abundance to run FastChem, which calculates gas-phase equilibrium species (including H $_2$ O, CO $_2$ , etc.).
  - Outer Loop: Adjusts the $P_{O_2}$ guess until the $P_{O_2}$ output by FastChem matches the $P_{O_2}$ required to satisfy the melt vaporization equilibrium constants (derived from JANAF tables and MELTS code).
3. Output: Partial pressures of all gas-phase species (523 species considered) in equilibrium with the melt.
Validation: The code was validated against Charnoz et al. (2023) for H-dominated atmospheres, showing close agreement in mole fractions, with minor discrepancies attributed to different Gibbs free energy minimization approaches between FastChem and the CEA/NASA code used in the previous study.

3. Key Contributions

Code Expansion: Expanded the LavAtmos framework from 31 gas species to 523, enabling the inclusion of complex volatile chemistry (C-H-N-S-P systems).
Self-Consistent Coupling: Moved beyond the "sum" method (adding volatiles post-calculation) to a fully coupled equilibrium model where volatiles actively influence the vaporization of the melt.
Systematic Testing:
- Tested "pure" volatile atmospheres (containing only one volatile element: C, H, N, S, or P).
- Tested "complex" atmospheres dominated by one element but containing traces of others.
- Applied the model to specific scenarios for 55 Cnc e based on recent JWST data.

4. Key Results

A. Effect on Vaporized Species (Rock Vapors)

Increased Partial Pressures: The inclusion of volatile elements significantly increases the partial pressures of vaporized rock species (SiO, SiO $_2$ , Na, K, Fe) compared to volatile-free models.
Mechanism: Volatile elements (especially C and H) react with O $_2$ released by the melt to form stable molecules (e.g., CO, CO $_2$ , H $_2$ O). This consumption of O $_2$ lowers the atmospheric $P_{O_2}$ . According to mass action laws (e.g., $P_{SiO} \propto P_{O_2}^{-1/2}$ ), a lower $P_{O_2}$ drives the vaporization equilibrium toward the gas phase, increasing the abundance of rock vapors.
Elemental Dependence: Carbon has the strongest effect (due to high affinity for oxygen forming CO/CO $_2$ ), followed by H, S, P, and N (which has the weakest effect).
Magnitude: In volatile-dominated regimes (pressures > 0.01 bar, temperatures < 3600 K), the partial pressures of SiO and SiO $_2$ can be orders of magnitude higher than predicted by volatile-free models.

B. Effect on Volatile Species and C/O Ratio

Total Oxygen Budget: Contrary to the intuition that O $_2$ is "removed," the model shows that the total elemental oxygen abundance in the atmosphere increases significantly (up to 3 orders of magnitude) when volatiles are included. This is because the lowered $P_{O_2}$ allows the melt to release more oxygen into the atmosphere to satisfy equilibrium.
C/O Ratio: The presence of a lava ocean in equilibrium with a volatile atmosphere leads to a lower C/O ratio compared to models that ignore the lava-ocean feedback. The lava ocean acts as a massive reservoir of oxygen, buffering the system and oxidizing carbon species (converting CO to CO $_2$ ).
55 Cnc e Application:
- When testing two atmospheric compositions for 55 Cnc e (Carbon-poor vs. Carbon-rich), the LavAtmos 2.0 model showed that a Carbon-rich atmosphere in equilibrium with a lava ocean could reproduce the C/O ratios observed in JWST data.
- A Carbon-poor atmosphere failed to match the observed C/O ratios under the same conditions.
- The model suggests that a low C/O ratio in a USP planet's atmosphere could be a signature of a surface lava ocean.

5. Significance and Implications

Observational Detectability: Previous studies suggested that volatile atmospheres would suppress the spectral signatures of rock vapors (SiO, SiO $_2$ ). LavAtmos 2.0 reverses this conclusion: volatile atmospheres may actually enhance the detectability of these species by increasing their partial pressures. This makes SiO, SiO $_2$ , Na, and K viable tracers for detecting lava oceans on USP planets.
Atmospheric Structure: The increased abundance of short-wave absorbers (Na, K) and the altered thermal structure (due to changes in opacity from CO $_2$ /H $_2$ O) could lead to thermal inversions, impacting the interpretation of emission spectra.
Interpretation of Exoplanet Data: The paper argues that interpreting the atmospheres of Hot Rocky Exoplanets (HREs) requires self-consistent modeling of the melt-atmosphere interaction. Ignoring the feedback of volatiles on vaporization leads to systematic underestimations of rock vapor abundances and incorrect C/O ratios.
Future Directions: The authors note that while the model assumes the lava ocean is the dominant oxygen reservoir, future work must address the solubility of volatiles (H $_2$ O, CO $_2$ ) in the melt and the depth required for the ocean to effectively buffer the atmosphere.

In summary, LavAtmos 2.0 demonstrates that volatile species are not passive components but active drivers of lava ocean chemistry, fundamentally altering the predicted atmospheric composition and observability of ultra-short period rocky exoplanets.