Runaway greenhouse effect and habitability of planets : revisited

之前大多数关于宜居带的研究都隐含地假设了一颗被海洋覆盖的“水”行星，它像现在的地球一样拥有大量液态水。然而，有可能存在一个宜居的“陆地”星球，它被广阔的干燥沙漠覆盖，但当地有丰富的水。液态水的存在条件对于少水的陆地行星和水生行星来说可能是不同的，因为导致完全冻结的冰反照率反馈和导致完全蒸发的失控温室都是由水的相变引起的。 1. 我们使用大气环流模型 CCSR/NIES AGCM5.4g 进行了实验，并确定了导致水从表面完全蒸发的最小太阳通量。 2. 如果地下水的分布完全受大气环流控制，则地下水集中在高纬度地区。然后，液态水存在于失控温室的临界通量之外的表面。 3.我们发现了多种平衡状态。 4. 平流层中的水蒸气丰度保持在非常低的水平，而表面则存在水。因此，这种状态对于大气逃逸非常稳定。 5. 水丰度太大，大气无法控制水的分布。那么，液态水的稳定性受到水行星的稳定性的限制。这表明，与直觉相反，对于水较少的“陆地”行星来说，液态水的稳定范围比水丰富的“水”行星更宽。

Most of the previous studies on the habitable zone implicitly assume an ocean-covered 'aqua' planet that has a large amount of liquid water like the present Earth. However, there is a possibility of a habitable 'land' planet that is covered by vast dry desert but has locally abundant water. The conditions for the existence of liquid water can be different for a less water land planet from that of an aqua planet, because, both the ice-albedo feedback, which causes the complete freezing, and the runaway greenhouse, which causes the complete evaporation, are caused by the phase change of water. 1. We performed experiments using a general circulation model CCSR/NIES AGCM5.4g and determined the minimum solar flux that cause the complete evaporation of water from the surface. 2. If the distribution of the ground water is completely controlled by the atmospheric circulation, the ground water is localized in high latitude area. Then, the liquid water exists on the surface beyond the critical flux of the runaway greenhouse. 3. We found multiple equilibrium states. 4. Vapor abundance in the stratosphere is kept very low while water exists on the surface. Thus, this state is very stable against the atmospheric escape. 5. Water abundance which is too large will not allow the atmospheric control of water distribution. Then, the stability of liquid water is limited by that of an aqua planet. This suggests, against the intuition, that the stable range of liquid water is wider for less water "land" planets than water abundant "aqua" planets.