标题： 潮汐锁定系外行星上的气候不稳定 显示英文标题

标题： Climate instability on tidally locked exoplanets

摘要： 可能使同步自转的岩石行星气候不稳定的反馈机制可能出现在具有强烈昼夜表面温度对比的行星上。 类地宜居带（HZ）行星在地质时间尺度上能够维持稳定的地表液态水。 这需要风化作用对温室气体消耗的速率与其它过程对温室气体的补充之间达到平衡。 已发现的小半径系外行星以及预期的M型矮星宜居带岩石行星预计会处于潮汐锁定状态。 我们研究了两种可能使潮汐锁定行星气候不稳定的反馈机制。 (1) 如果压力的小幅变化改变了行星表面的温度分布，使得当压力降低时风化速率增加，则会在恒星点附近出现风化速率增加、压力降低和恒星点表面温度升高的正反馈。 (2) 当压力降低增加了高于熔点的表面积（通过减少恒星点的平流冷却），并且相应的液体体积增加导致大气的净溶解，从而进一步降低压力。 我们使用一个简化的能量平衡模型来绘制这些不稳定性可能发生的情况。 风化失控可能会缩小宜居带，并在宜居带内引起地质学上快速的10^3倍压力变化。 火星在过去可能经历过风化失控。 恒星点溶解通常是对压力变化的负反馈或弱正反馈。 如果大气具有高辐射效率，这两种不稳定性都会被抑制。 我们的结果最适用于薄大气且温室气体辐射效率低的情况。 这些结果揭示了一种新的途径，使得宜居带行星可能发生快速的气候变化并变得无法居住。 显示英文摘要

摘要： Feedbacks that can destabilize the climates of synchronously-rotating rocky planets may arise on planets with strong day-night surface temperature contrasts. Earth-like habitable-zone (HZ) planets maintain stable surface liquid water over geological time. This requires equilibrium between the temperature-dependent rate of greenhouse-gas consumption by weathering,and greenhouse-gas resupply by other processes. Detected small-radius exoplanets, and anticipated M-dwarf HZ rocky planets, are expected to be tidally locked. We investigate two feedbacks that can destabilize climate on tidally-locked planets. (1) If small changes in pressure alter the temperature distribution across a planet's surface such that the weathering rate increases when the pressure decreases, a positive feedback occurs involving increasing weathering rate near the substellar point, decreasing pressure, and increasing substellar surface temperature. (2) When decreases in pressure increase the surface area above the melting point (through reduced advective cooling of the substellar point), and the corresponding increase in volume of liquid causes net dissolution of the atmosphere, a further decrease in pressure occurs. We use an idealized energy balance model to map out the conditions under which these instabilities may occur. The weathering runaway can shrink the habitable zone, and cause geologically rapid 10^3-fold pressure shifts within the HZ. Mars may have undergone a weathering runaway in the past. Substellar dissolution is usually a negative feedback or weak positive feedback on changes in pressure. Both instabilities are suppressed if the atmosphere has a high radiative efficiency. Our results are most relevant for atmospheres that are thin and have low greenhouse-gas radiative efficiency. These results identify a new pathway by which HZ planets can undergo rapid climate shifts and become uninhabitable.