Bailey Jeremy
Australian Centre for Astrobiology, Macquarie University, Sydney, New South Wales, Australia.
Astrobiology. 2007 Apr;7(2):320-32. doi: 10.1089/ast.2006.0039.
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.
当前关于系外类地行星特征描述的提议主要基于光谱技术的应用。虽然光谱学在探测行星大气的气态成分方面很有效，但它无法探测液态水的存在，而液态水是宜居行星的决定性特征。在本文中，我研究了一种利用偏振来描述行星大气特征的替代技术的潜力。通过在“主虹”散射角寻找偏振峰，可以探测行星大气中液滴的存在，并通过其折射率来确定液体的性质。本文给出的单次散射计算表明，在可能的云滴大小的整个范围内都存在一个明确的虹散射峰，并且能清楚地将液滴的存在与冰或尘埃等固体颗粒区分开来。过去，虹散射已被用于确定金星大气中云滴的性质，以及被地球反射率的偏振和方向性（POLDER）仪器用于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地表存在水，但这种技术可以补充光谱技术，用于描述潜在宜居行星的大气特征。对于地球，在两种不同的云量情况下，盘面积分虹峰的偏振度估计分别为12.7%或15.5%。结果表明，在与光谱特征描述所需的总积分时间相似的情况下，利用提议的类地行星发现者日冕仪任务来观测这个虹峰是可行的。
