{"paper":{"title":"Sub-Neptunes as Soot Factories: Deep Atmosphere Hydrocarbon Formation and Quenching as the Origin of Sub-Neptune Aerosol Trends","license":"http://creativecommons.org/licenses/by/4.0/","headline":"Sub-Neptune atmospheres produce peak amounts of soot-forming hydrocarbons around 600 K through deep-atmosphere quenching.","cross_cats":[],"primary_cat":"astro-ph.EP","authors_text":"Arjun B. Savel, Eliza M.-R. Kempton, Jeehyun Yang","submitted_at":"2026-04-13T18:07:10Z","abstract_excerpt":"Recent population-level studies of sub-Neptune atmospheres have identified a tentative parabolic trend in transmission spectrum amplitude for planets with Teq ~ 500-800 K. While the trend has been commonly attributed to hydrocarbon aerosols, we lack a first-principles explanation of its underlying chemical mechanism. Previous work has focused on the role of methane photolysis and subsequent polymerization, but with limited reaction networks that truncated at C2-species and couldn't reproduce the observed parabolic trend. In this work, enabled by a computer-automated, rate-based chemical networ"},"claims":{"count":4,"items":[{"kind":"strongest_claim","text":"We find that the predicted abundances of PAHs peak near 600 K, and fall off toward higher and lower Teq, consistent with the observed muted-spectra regime suggested in observational studies by HST and JWST.","source":"verdict.strongest_claim","status":"machine_extracted","claim_id":"C1","attestation":"unclaimed"},{"kind":"weakest_assumption","text":"That the automated rate-based network generator produces a sufficiently complete reaction set and that deep-atmosphere quenching dominates the observable-layer abundances without photochemistry or other processes substantially altering the PAH levels across the full Teq range.","source":"verdict.weakest_assumption","status":"machine_extracted","claim_id":"C2","attestation":"unclaimed"},{"kind":"one_line_summary","text":"Deep hydrocarbon chemistry and quenching in sub-Neptunes create PAH abundances peaking at 600 K, providing a first-principles explanation for the muted transmission spectra trend between 500-800 K.","source":"verdict.one_line_summary","status":"machine_extracted","claim_id":"C3","attestation":"unclaimed"},{"kind":"headline","text":"Sub-Neptune atmospheres produce peak amounts of soot-forming hydrocarbons around 600 K through deep-atmosphere quenching.","source":"verdict.pith_extraction.headline","status":"machine_extracted","claim_id":"C4","attestation":"unclaimed"}],"snapshot_sha256":"fec3241b4139675dc8ead6a71fae4031bf2eec69de972262ad7971e4ba9dd14d"},"source":{"id":"2604.11919","kind":"arxiv","version":2},"verdict":{"id":"e9524088-8cd6-42cb-b57d-8606630247a4","model_set":{"reader":"grok-4.3"},"created_at":"2026-05-10T15:44:38.450453Z","strongest_claim":"We find that the predicted abundances of PAHs peak near 600 K, and fall off toward higher and lower Teq, consistent with the observed muted-spectra regime suggested in observational studies by HST and JWST.","one_line_summary":"Deep hydrocarbon chemistry and quenching in sub-Neptunes create PAH abundances peaking at 600 K, providing a first-principles explanation for the muted transmission spectra trend between 500-800 K.","pipeline_version":"pith-pipeline@v0.9.0","weakest_assumption":"That the automated rate-based network generator produces a sufficiently complete reaction set and that deep-atmosphere quenching dominates the observable-layer abundances without photochemistry or other processes substantially altering the PAH levels across the full Teq range.","pith_extraction_headline":"Sub-Neptune atmospheres produce peak amounts of soot-forming hydrocarbons around 600 K through deep-atmosphere quenching."},"integrity":{"clean":true,"summary":{"advisory":0,"critical":0,"by_detector":{},"informational":0},"endpoint":"/pith/2604.11919/integrity.json","findings":[],"available":true,"detectors_run":[],"snapshot_sha256":"c28c3603d3b5d939e8dc4c7e95fa8dfce3d595e45f758748cecf8e644a296938"},"references":{"count":16,"sample":[{"doi":"10.3847/2041-8213/add010","year":2025,"title":"2025a, ApJL, 985, L10, doi: 10.3847/2041-8213/add010","work_id":"64bbbc65-7f72-4ba0-ab61-0445ae8b1111","ref_index":1,"cited_arxiv_id":"","is_internal_anchor":false},{"doi":"","year":2023,"title":"(2023); Roy et al","work_id":"5d65c2d5-2482-4614-91d8-2b29220247ce","ref_index":2,"cited_arxiv_id":"","is_internal_anchor":false},{"doi":"","year":2024,"title":"(2024) GJ 1214 b (HST; Kreidberg et al","work_id":"1cf36070-6c40-4ff9-ad95-e1705af2aae0","ref_index":3,"cited_arxiv_id":"","is_internal_anchor":false},{"doi":"","year":2024,"title":"(2024); Ohno et al","work_id":"56c78a60-ee32-4c1d-bfc2-a84e0924e437","ref_index":4,"cited_arxiv_id":"","is_internal_anchor":false},{"doi":"","year":2022,"title":"(2022); Ahrer et al","work_id":"77c99e55-90be-4198-97a1-61f74491e6da","ref_index":5,"cited_arxiv_id":"","is_internal_anchor":false}],"resolved_work":16,"snapshot_sha256":"07322d1df365eb0fdb59485eb89da33af6e6d08147551156a6288dd295cdc86b","internal_anchors":0},"formal_canon":{"evidence_count":2,"snapshot_sha256":"73d9286c3fdd955b54bb18b9ab8d0dd3f2a293c6eb0da6f17d53c65601781efa"},"author_claims":{"count":0,"strong_count":0,"snapshot_sha256":"258153158e38e3291e3d48162225fcdb2d5a3ed65a07baac614ab91432fd4f57"},"builder_version":"pith-number-builder-2026-05-17-v1"}