Runaway greenhouse effect and habitability of planets : revisited

以前大多数关于宜居带的研究都隐含地假设了一个海洋覆盖的“水”行星，它像现在的地球一样拥有大量的液态水。然而，有可能存在一个可居住的“陆地”行星，它被广阔的干燥沙漠覆盖，但当地有丰富的水。液态水存在的条件对于一个水少的陆地行星和一个水行星是不同的，因为，导致完全冻结的冰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式CCSR/NIES AGCM 5.4g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的控制，则地下水分布在高纬度地区。在失控温室临界通量以上，地表存在液态水。3.我们发现了多个平衡态。4.平流层中的蒸汽丰度保持非常低，而水存在于表面。因此，这种状态对大气逃逸非常稳定。5.水的丰度太大将不允许大气控制水的分布。然后，液态水的稳定性受到水行星的限制。这表明，与直觉相反，液态水的稳定范围在水较少的“陆地”行星上比水丰富的“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.