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Abstract Tidal heating on Io due to its finite eccentricity was predicted to drive surface volcanic activity, which was subsequently confirmed by the Voyager spacecraft. Although the volcanic activity in Io is more complex, in theory volcanism can be driven by runaway melting in which the tidal heating increases as the mantle thickness decreases. We show that this runaway melting mechanism is generic for a composite planetary body with liquid core and solid mantle, provided that (i) the mantle rigidity,μ, is comparable to the central pressure, i.e.,μ/(ρgRP) ≳ 0.1 for a body with densityρ, surface gravitational accelerationg, and radiusRP; (ii) the surface is not molten; (iii) tides deposit sufficient energy; and (iv) the planet has nonzero eccentricity. We calculate the approximate liquid core radius as a function ofμ/(ρgRP), and find that more than 90% of the core will melt due to this runaway forμ/(ρgRP) ≳ 1. From all currently confirmed exoplanets, we find that the terrestrial planets in the L 98-59 system are the most promising candidates for sustaining active volcanism. However, uncertainties regarding the quality factors and the details of tidal heating and cooling mechanisms prohibit definitive claims of volcanism on any of these planets. We generate synthetic transmission spectra of these planets assuming Venus-like atmospheric compositions with an additional 5%, 50%, and 98% SO2component, which is a tracer of volcanic activity. We find a ≳3σpreference for a model with SO2with 5–10 transits with JWST for L 98-59bcd.
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This content will become publicly available on February 13, 2026
Evidence for a Volcanic Atmosphere on the Sub-Earth L 98-59 b
Abstract Assessing the prevalence of atmospheres on rocky planets around M-dwarf stars is a top priority of exoplanet science. High-energy activity from M dwarfs can destroy the atmospheres of these planets, which could explain the lack of atmosphere detections to date. Volcanic outgassing has been proposed as a mechanism to replenish the atmospheres of tidally heated rocky planets. L 98-59 b, a sub-Earth transiting a nearby M dwarf, was recently identified as the most promising exoplanet to detect a volcanic atmosphere. We present the transmission spectrum of L 98-59 b from four transits observed with JWST NIRSpec G395H. Although the airless model provides an adequate fit to the data based on itsχ2, an SO2atmosphere is preferred by 3.6σover a flat line in terms of the Bayesian evidence. Such an atmosphere would likely be in a steady state where volcanism balances escape. If so, L 98-59 b must experience at least eight times as much volcanism and tidal heating per unit mass as Io. If volcanism is driven by runaway melting of the mantle, we predict the existence of a subsurface magma ocean in L 98-59 b extending up toRp ∼ 60%–90%. An SO2-rich volcanic atmosphere on L 98-59 b would be indicative of an oxidized mantle with an oxygen fugacity offO2 > IW + 2.7, and it would imply that L 98-59 b must have retained some of its volatile endowment despite its proximity to its star. Our findings suggest that volcanism may revive secondary atmospheres on tidally heated rocky planets around M dwarfs.
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- Award ID(s):
- 2303553
- PAR ID:
- 10591731
- Publisher / Repository:
- AAS
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 980
- Issue:
- 2
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L26
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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