标题： 由JWST揭示的温带亚海王星K2-18 b富含水的内部 显示英文标题

标题： A water-rich interior in the temperate sub-Neptune K2-18 b revealed by JWST

摘要： 温暖的亚海王星是探测太阳系外液态水海洋的有吸引力的目标。 如果富含水且缺乏大质量氢-氦包层，这些行星尽管尺寸大于地球，仍可能在其大气层下维持液态层。 对温暖的亚海王星K2-18 b的先前观测显示，其大气以H2为主，富含CH4，并有中等证据表明存在CO2以及二甲基硫醚（DMS）的初步迹象。 在此，我们呈现K2-18 b的四次新的JWST/NIRSpec凌日观测。 结果得到的高精度透射光谱稳健地检测到CH4和CO2，精确测量了它们的丰度，并牢固确立了该行星的富水性质：要么是体积占比超过10%的H2O厚包层，要么是位于液态水海洋之上的薄大气层。 光谱未检测到可识别的H2O、NH3或CO。 大气中水蒸气的缺失表明存在高效的冷阱，而NH3和CO的未检测结果支持了一个小的H2丰富大气层覆盖液态储层的情景。 然而，包含这些气体的替代模型也可以在不确定范围内再现光谱，突显了需要更深入的观测。 光谱仅包含DMS、甲基硫醇（CH3SH）和一氧化二氮（N2O）的微弱信号，没有任何信号超过3 sigma，在不施加强烈超瑞利霾的情况下，所有信号均低于约2 sigma。 同时，我们的自洽光化学模型显示，在高金属量的大量H2丰富大气中，DMS和CH3SH可能非生物形成，因此考虑它们作为生物标志物的潜在用途时需要额外的指示器。 K2-18 b是一个寒冷且富水的世界，作为探索非地球类行星中液态水环境出现最有希望的温暖亚海王星之一，促使进一步表征其大气和内部结构。 显示英文摘要

摘要： Temperate sub-Neptunes are compelling targets for detecting liquid-water oceans beyond the Solar System. If water-rich and lacking massive hydrogen-helium envelopes, these planets could sustain liquid layers beneath their atmospheres despite sizes larger than Earth. Previous observations of the temperate sub-Neptune K2-18 b revealed an H2-dominated atmosphere rich in CH4, with moderate evidence for CO2 and tentative signs of dimethyl sulfide (DMS). Here we present four new JWST/NIRSpec transit observations of K2-18 b. The resulting high-precision transmission spectrum robustly detects both CH4 and CO2, precisely measuring their abundances and firmly establishing the planet's water-rich nature: either a thick envelope with >10% H2O by volume or a thin atmosphere above a liquid-water ocean. The spectrum reveals no detectable H2O, NH3, or CO. The absence of atmospheric water vapor suggests an efficient cold trap, while the nondetections of NH3 and CO support the scenario of a small H2-rich atmosphere overlying a liquid reservoir. However, alternative models that include these gases can also reproduce the spectrum within uncertainties, highlighting the need for deeper observations. The spectrum only contains marginal signals of DMS, methyl mercaptan (CH3SH), and nitrous oxide (N2O), with none exceeding 3 sigma in model preference and all falling below ~2 sigma without imposing a strong super-Rayleigh haze. Meanwhile, our self-consistent photochemical models show that DMS and CH3SH may form abiotically in massive H2-rich atmospheres of high metallicity, making it important to consider additional indicators for their potential use as biosignatures. K2-18 b, a cool, water-rich world, stands out as one of the most promising temperate sub-Neptunes for exploring the emergence of liquid-water environments in non-Earth-like planets, motivating further characterization of its atmosphere and interior.