Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited
Recently, the James Webb Space Telescope has enabled detailed spectroscopic characterization of sub-Neptune atmospheres. With detections of carbon- and oxygen-bearing species such as CO, CO _2 , CH _4 , and H _2 O, a central question is whether the atmospheric C/O ratio, commonly used to trace forma...
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2025-01-01
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| Online Access: | https://doi.org/10.3847/2041-8213/adf185 |
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| author | Aaron Werlen Caroline Dorn Hilke E. Schlichting Simon L. Grimm Edward D. Young |
| author_facet | Aaron Werlen Caroline Dorn Hilke E. Schlichting Simon L. Grimm Edward D. Young |
| author_sort | Aaron Werlen |
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| description | Recently, the James Webb Space Telescope has enabled detailed spectroscopic characterization of sub-Neptune atmospheres. With detections of carbon- and oxygen-bearing species such as CO, CO _2 , CH _4 , and H _2 O, a central question is whether the atmospheric C/O ratio, commonly used to trace formation location in giant planets, can serve a similar diagnostic role for sub-Neptunes. We use the global chemical equilibrium framework of H. E. Schlichting & E. D. Young to quantify how magma ocean–atmosphere interactions affect the atmospheric C/O ratio. We find that the resulting C/O ratios range from several orders of magnitude below solar to a few times solar. The atmospheric C/O ratio in sub-Neptunes is therefore not inherited from the protoplanetary disk, but instead emerges from chemical equilibrium between the atmosphere and the underlying magma ocean. Planetary mass, atmospheric mass fraction, and thermal state all strongly influence the atmospheric C/O ratio. In addition, carbon partitioning into the metal phase typically reduces the atmospheric C/O ratio substantially, particularly for atmospheric mass fractions less than a few percent. Finally, we couple the deep equilibrium compositions to 1D atmospheric models that self-consistently solve for the pressure–temperature structure and chemical composition, including photochemistry. We find that the C/O ratio varies with altitude under low vertical mixing conditions ( K _zz = 10 ^4 cm ^2 s ^−1 ) but remains constant under strong mixing ( K _zz = 10 ^7 cm ^2 s ^−1 ). Our results imply that observed C/O ratios of sub-Neptunes can be used to probe their interiors. Specifically, C/O ratios much lower than host star values would imply an underlying magma ocean with iron metal having sequestered significant amounts of carbon. |
| format | Article |
| id | doaj-art-bcb76c1c765d4556aa93ae74f323feab |
| institution | DOAJ |
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| language | English |
| publishDate | 2025-01-01 |
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| series | The Astrophysical Journal Letters |
| spelling | doaj-art-bcb76c1c765d4556aa93ae74f323feab2025-08-20T03:18:06ZengIOP PublishingThe Astrophysical Journal Letters2041-82052025-01-019882L5510.3847/2041-8213/adf185Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than InheritedAaron Werlen0https://orcid.org/0009-0005-1133-7586Caroline Dorn1https://orcid.org/0000-0001-6110-4610Hilke E. Schlichting2https://orcid.org/0000-0002-0298-8089Simon L. Grimm3https://orcid.org/0000-0002-0632-4407Edward D. Young4https://orcid.org/0000-0002-1299-0801Institute for Particle Physics and Astrophysics , ETH Zurich, CH-8093 Zurich, Switzerland ; awerlen@ethz.chInstitute for Particle Physics and Astrophysics , ETH Zurich, CH-8093 Zurich, Switzerland ; awerlen@ethz.chDepartment of Earth, Planetary, and Space Sciences, University of California , Los Angeles, CA 90095, USAInstitute for Particle Physics and Astrophysics , ETH Zurich, CH-8093 Zurich, Switzerland ; awerlen@ethz.ch; Department of Astrophysics, University of Zurich , CH-8057 Zurich, SwitzerlandDepartment of Earth, Planetary, and Space Sciences, University of California , Los Angeles, CA 90095, USARecently, the James Webb Space Telescope has enabled detailed spectroscopic characterization of sub-Neptune atmospheres. With detections of carbon- and oxygen-bearing species such as CO, CO _2 , CH _4 , and H _2 O, a central question is whether the atmospheric C/O ratio, commonly used to trace formation location in giant planets, can serve a similar diagnostic role for sub-Neptunes. We use the global chemical equilibrium framework of H. E. Schlichting & E. D. Young to quantify how magma ocean–atmosphere interactions affect the atmospheric C/O ratio. We find that the resulting C/O ratios range from several orders of magnitude below solar to a few times solar. The atmospheric C/O ratio in sub-Neptunes is therefore not inherited from the protoplanetary disk, but instead emerges from chemical equilibrium between the atmosphere and the underlying magma ocean. Planetary mass, atmospheric mass fraction, and thermal state all strongly influence the atmospheric C/O ratio. In addition, carbon partitioning into the metal phase typically reduces the atmospheric C/O ratio substantially, particularly for atmospheric mass fractions less than a few percent. Finally, we couple the deep equilibrium compositions to 1D atmospheric models that self-consistently solve for the pressure–temperature structure and chemical composition, including photochemistry. We find that the C/O ratio varies with altitude under low vertical mixing conditions ( K _zz = 10 ^4 cm ^2 s ^−1 ) but remains constant under strong mixing ( K _zz = 10 ^7 cm ^2 s ^−1 ). Our results imply that observed C/O ratios of sub-Neptunes can be used to probe their interiors. Specifically, C/O ratios much lower than host star values would imply an underlying magma ocean with iron metal having sequestered significant amounts of carbon.https://doi.org/10.3847/2041-8213/adf185Exoplanet atmospheric compositionExoplanet atmospheric structureExoplanet structure |
| spellingShingle | Aaron Werlen Caroline Dorn Hilke E. Schlichting Simon L. Grimm Edward D. Young Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited The Astrophysical Journal Letters Exoplanet atmospheric composition Exoplanet atmospheric structure Exoplanet structure |
| title | Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited |
| title_full | Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited |
| title_fullStr | Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited |
| title_full_unstemmed | Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited |
| title_short | Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited |
| title_sort | atmospheric c o ratios of sub neptunes with magma oceans homemade rather than inherited |
| topic | Exoplanet atmospheric composition Exoplanet atmospheric structure Exoplanet structure |
| url | https://doi.org/10.3847/2041-8213/adf185 |
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