CLIMATE INSTABILITY ON TIDALLY LOCKED EXOPLANETS
CLIMATE INSTABILITY ON TIDALLY LOCKED EXOPLANETS
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DOI：
10.1088/0004-637x/743/1/41
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发表时间：
2011-12-10
影响因子：
4.9
通讯作者：
Manga, Michael
中科院分区：
文献类型：
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作者：
Kite, Edwin S.;Gaidos, Eric;Manga, Michael
在昼夜表面温度反差强烈的行星上，可能会出现可能破坏同步旋转岩石行星气候稳定的反馈。类地宜居行星在地质时期内保持稳定的表面液态水。这需要风化造成的温室气体消耗与温度相关的速率与其他过程产生的温室气体补给之间达到平衡。检测到的小半径系外行星和预期的 M 矮星宜居带岩石行星预计将同步旋转（潮汐锁定）。在本文中，我们研究了两种可能破坏同步旋转行星气候稳定的假设反馈。 （1）如果压力的微小变化改变了行星表面的温度分布，使得当压力下降时风化率上升，就会出现失控的正反馈，包括恒星下点附近的风化率增加、压力降低和恒星下表面温度升高。我们将这种反馈称为增强型亚恒星风化不稳定性（ESWI）。 (2) 当压力降低时，熔点以上的表面积分数增加（通过减少星下点的平流冷却），并且液体体积的相应增加导致大气的净溶解，压力将进一步降低。这种亚恒星溶解反馈也会导致失控的气候变化。我们使用理想化的能量平衡模型来绘制这些不稳定性可能发生的条件。在这个简化模型中，风化失控可以缩小宜居带，并导致宜居带内的地质压力快速变化 10(3) 倍。火星过去可能经历过风化失控。次星溶解通常是对大气压变化的负反馈或弱正反馈。它只能引起小而深的海洋和高度可溶的大气气体的失控变化。如果大气具有高辐射效率，这两种不稳定性都会受到抑制。我们的结果与稀薄、温室气体辐射效率低且主要温室气体也是大气主要成分的大气最相关。 ESWI 还需要星下点附近的土地，并且需要进行构造重铺（火山作用、造山）以实现压力的大幅跃升。这些结果确定了一条新途径，通过该途径，宜居带行星可以经历快速的气候变化并变得不适宜居住。
Feedbacks that can destabilize the climates of synchronously rotating rocky planetsmay arise on planets with strong day-night surface temperature contrasts. Earth-like habitable planets maintain stable surface liquid water over geologic 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 habitable-zone rocky planets, are expected to be in synchronous rotation (tidally locked). In this paper, we investigate two hypothetical feedbacks that can destabilize climate on planets in synchronous rotation. (1) If small changes in pressure alter the temperature distribution across a planet's surface such that the weathering rate goes up when the pressure goes down, a runaway positive feedback occurs involving increasing weathering rate near the substellar point, decreasing pressure, and increasing substellar surface temperature. We call this feedback enhanced substellar weathering instability (ESWI). (2) When decreases in pressure increase the fraction of 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 will occur. This substellar dissolution feedback can also cause a runaway climate shift. We use an idealized energy balance model to map out the conditions under which these instabilities may occur. In this simplified model, the weathering runaway can shrink the habitable zone and cause geologically rapid 10(3)-fold atmospheric pressure shifts within the habitable zone. Mars may have undergone a weathering runaway in the past. Substellar dissolution is usually a negative feedback or weak positive feedback on changes in atmospheric pressure. It can only cause runaway changes for small, deep oceans and highly soluble atmospheric gases.Both instabilities are suppressed if the atmosphere has a high radiative efficiency. Our results are most relevant for atmospheres that are thin, have low greenhouse-gas radiative efficiency, and have a principal greenhouse gas that is also the main constituent of the atmosphere. ESWI also requires land near the substellar point, and tectonic resurfacing (volcanism, mountain-building) is needed for large jumps in pressure. These results identify a new pathway by which habitable-zone planets can undergo rapid climate shifts and become uninhabitable.