Evidence for a volcanic atmosphere on the sub-Earth L 98-59 b

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 its χ 2 , an SO2 atmosphere is preferred by 3.6σ over 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 to Rp „ 60 ´ 90%. An SO2 -rich volcanic atmosphere on L 98-59 b would be indicative of an oxidized mantle with an oxygen fugacity of fO2 ą 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.

评估M型矮星周围岩质行星大气的普遍程度是系外行星科学的首要任务。M型矮星的高能活动可能破坏这些行星的大气，这或许能解释迄今为止大气探测结果匮乏的原因。火山排气（volcanic outgassing）被认为是一种补充潮汐加热（tidally-heated）岩质行星大气的机制。L 98-59 b是一颗环绕邻近M型矮星运行的亚地球行星（sub-Earth），最近被认定为探测火山大气最具前景的系外行星。我们展示了利用JWST NIRSpec G395H观测到的L 98-59 b四次凌星的透射光谱（transmission spectrum）。尽管无大气模型基于χ²统计量能充分拟合数据，但从贝叶斯证据（Bayesian evidence）来看，SO₂大气模型比平坦线模型更优，差值达3.6σ。这种大气可能处于火山活动与大气逃逸平衡的稳态。若情况属实，L 98-59 b单位质量的火山活动和潮汐加热量必须至少是木卫一（Io）的八倍。若火山活动由地幔失控熔融驱动，我们预测L 98-59 b存在一个延伸至行星半径（Rₚ）约60–90%的地下岩浆海洋。L 98-59 b富含SO₂的火山大气表明其地幔已氧化，氧逸度（oxygen fugacity）fO₂ > IW+2.7；这也意味着尽管L 98-59 b靠近其母星，但仍保留了部分挥发性物质。我们的研究结果表明，火山活动可能使M型矮星周围潮汐加热岩质行星的次生大气（secondary atmosphere）得以再生。