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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 toR_p ∼  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. 

Abstract MESSENGER observations suggest a magma ocean formed on proto-Mercury, during which evaporation of metals and outgassing of C- and H-bearing volatiles produced an early atmosphere. Atmospheric escape subsequently occurred by plasma heating, photoevaporation, Jeans escape, and photoionization. To quantify atmospheric loss, we combine constraints on the lifetime of surficial melt, melt composition, and atmospheric composition. Consideration of two initial Mercury sizes and four magma ocean compositions determines the atmospheric speciation at a given surface temperature. A coupled interior–atmosphere model determines the cooling rate and therefore the lifetime of surficial melt. Combining the melt lifetime and escape flux calculations provides estimates for the total mass loss from early Mercury. Loss rates by Jeans escape are negligible. Plasma heating and photoionization are limited by homopause diffusion rates of ∼10⁶kg s⁻¹. Loss by photoevaporation depends on the timing of Mercury formation and assumed heating efficiency and ranges from ∼10^6.6to ∼10^9.6kg s⁻¹. The material for photoevaporation is sourced from below the homopause and is therefore energy limited rather than diffusion limited. The timescale for efficient interior–atmosphere chemical exchange is less than 10,000 yr. Therefore, escape processes only account for an equivalent loss of less than 2.3 km of crust (0.3% of Mercury’s mass). Accordingly, ≤0.02% of the total mass of H₂O and Na is lost. Therefore, cumulative loss cannot significantly modify Mercury’s bulk mantle composition during the magma ocean stage. Mercury’s high core:mantle ratio and volatile-rich surface may instead reflect chemical variations in its building blocks resulting from its solar-proximal accretion environment. 

Abstract The recent inference of sulfur dioxide (SO₂) in the atmosphere of the hot (approximately 1,100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations^1–3suggests that photochemistry is a key process in high-temperature exoplanet atmospheres⁴. This is because of the low (<1 ppb) abundance of SO₂under thermochemical equilibrium compared with that produced from the photochemistry of H₂O and H₂S (1–10 ppm)^4–9. However, the SO₂inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 μm and, therefore, the detection of other SO₂absorption bands at different wavelengths is needed to better constrain the SO₂abundance. Here we report the detection of SO₂spectral features at 7.7 and 8.5 μm in the 5–12-μm transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS)¹⁰. Our observations suggest an abundance of SO₂of 0.5–25 ppm (1σrange), consistent with previous findings⁴. As well as SO₂, we find broad water-vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 μm. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy-element content (metallicity) for WASP-39b of approximately 7.1–8.0 times solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.