Rainbows, polarization, and the search for habitable planets
Rainbows, polarization, and the search for habitable planets
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DOI：
10.1089/ast.2006.0039
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发表时间：
2007-04-01
期刊：
影响因子：
4.2
通讯作者：
Bailey, Jeremy
中科院分区：
文献类型：
--
作者：
Bailey, Jeremy
目前关于太阳系外地行星特征的提议主要依赖于光谱技术的使用。虽然光谱学在探测行星大气中的气体成分方面是有效的，但它不提供检测液态水存在的方法，而液态水是宜居行星的定义特征。在这篇文章中，我研究了一种使用偏振来表征行星大气的替代技术的潜力。通过寻找“初级彩虹”散射角的偏振峰，就有可能探测到行星大气中液滴的存在，并通过其折射率来限制液体的性质。单次散射计算表明，在所有可能的云滴大小范围内都存在一个明确的彩虹散射峰，并清楚地将液滴的存在与冰或尘埃等固体颗粒的存在区分开来。过去曾使用彩虹散射来确定金星大气中云滴的性质，并使用地球反射率的偏振和方向性(POLDER)仪器来区分地球大气中的液态云和冰云。虽然液态水云的存在并不能保证表面存在水，但这项技术可以补充光谱技术，以表征潜在的宜居行星的大气。对于两种不同的云层情况，地球的圆盘综合彩虹峰值估计为12.7%或15.5%的偏振度。对这一彩虹峰的观测被证明是可行的，利用拟议的陆地行星探测仪日冕记录仪任务，总积分时间与光谱表征所需的时间相似。
Current proposals for the characterization of extrasolar terrestrial planets rest primarily on the use of spectroscopic techniques. While spectroscopy is effective in detecting the gaseous components of a planet's atmosphere, it provides no way of detecting the presence of liquid water, the defining characteristic of a habitable planet. In this paper, I investigate the potential of an alternative technique for characterizing the atmosphere of a planet using polarization. By looking for a polarization peak at the "primary rainbow" scattering angle, it is possible to detect the presence of liquid droplets in a planet's atmosphere and constrain the nature of the liquid through its refractive index. Single scattering calculations are presented to show that a well-defined rainbow scattering peak is present over the full range of likely cloud droplet sizes and clearly distinguishes the,presence of liquid droplets from solid particles such as ice or dust. Rainbow scattering has been used in the past to determine the nature of the cloud droplets in the Venus atmosphere and by the POLarization and Directionality of Earth Reflectances (POLDER) instrument to distinguish between liquid and ice clouds in the Earth atmosphere. While the presence of liquid water clouds does not guarantee the presence of water at the surface, this technique could complement spectroscopic techniques for characterizing the atmospheres of potential habitable planets. The disk-integrated rainbow peak for Earth is estimated to be at a degree of polarization of 12.7% or 15.5% for two different cloud cover scenarios. The observation of this rainbow peak is shown to be feasible with the proposed Terrestrial Planet Finder Coronograph mission in similar total integration times to those required for spectroscopic characterization.