Diversity in the haziness and chemistry of temperate sub-Neptunes

Abstract: Recent transit observations of K2-18b and TOI-270d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (T$\mathrm{eq}$=250-400K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO$_2$ abundances on both planets, implying bulk compositions with high water mass fractions. However, it remains unclear whether these findings hold true for all temperate sub-Neptunes. Here, we present the JWST NIRSpec/PRISM 0.7-5.4$\mathbfμ$m transmission spectrum of a third temperate sub-Neptune, the 2.4R$\oplus$ planet LP 791-18c (T$_\mathrm{eq}$=355K), which is even more favorable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite LP 791-18c's radius, mass, and equilibrium temperature being in between those of K2-18b and TOI-270d, we find a drastically different transmission spectrum. While we also detect methane on LP 791-18c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO$_2$ absorption. Overall, we infer a deep metal-enriched atmosphere (246-415$\times$solar) for LP 791-18c, with a CO$_2$-to-CH$_4$ ratio smaller than 0.07 (at 2$σ$), indicating less H$_2$O in the deep envelope of LP 791-18c and implying a relatively dry formation inside the water ice-line. These results show that sub-Neptunes that are near-analogues in density and temperature can show drastically different aerosols and envelope chemistry, and are intrinsically diverse beyond a simple temperature dependence.