蓝色、白色和红色海洋行星：通量和偏振颜色轨道变化的模拟
Blue, white, and red ocean planets - Simulations of orbital variations in flux and polarization colors
摘要：
一颗系外行星的可居住性将在很大程度上取决于液态水的存在。对系外行星反射的星光的通量和/或偏振测量可能有助于识别系外海洋。我们调查的宽带光谱特征的通量和偏振相位函数的反射星光唯一识别外海洋。我们计算总通量F和偏振通量Q的星光反射的无云和（部分）多云的系外行星，波长从350到865 nm。海洋表面有由菲涅尔反射波面和白浪组成的波浪，并包括水体内的散射。海洋行星的总通量F、偏振通量Q和偏振度P在相位角α范围内从蓝色、白色到红色变化，F的相位角α范围为134-108度，Q的相位角α范围为123-157度，云覆盖率fc在F的相位角α范围内从0.0增加到1.0，Q的相位角α范围内从0.98增加到0.98。P的颜色变化仅在fc范围为0.03-0.98时发生，颜色交叉角α范围为88-161度。一个多云的、表面为零的海洋行星的总通量F也可以改变颜色，并且对于fc=0.0，海洋行星的F不会因为表面压力ps > 8 bar而改变颜色。一个零表面的行星的偏振通量Q不会因为任何fc而改变颜色。由系外行星反射的星光P的颜色变化，从蓝色，通过白色，到红色，α值在88度以上增加，似乎可以识别出（部分）多云的系外海洋。偏振通量Q的颜色变化与增加的α 123度以上似乎唯一地识别一个外海洋，独立于表面压力或云分数。在颜色变化的相位角，恒星和它的行星之间的角距离比在反射光中出现闪烁的相位角大得多。因此，偏振的颜色变化为探测外海洋提供了更好的前景。
英文摘要：
An exoplanet's habitability will depend strongly on the presence of liquid water. Flux and/or polarization measurements of starlight that is reflected by exoplanets could help to identify exo-oceans. We investigate which broadband spectral features in flux and polarization phase functions of reflected starlight uniquely identify exo-oceans. We compute total fluxes F and polarized fluxes Q of starlight reflected by cloud-free and (partly) cloudy exoplanets, for wavelengths from 350 to 865 nm. The ocean surface has waves composed of Fresnel reflecting wave facets and whitecaps, and scattering within the water body is included. Total flux F, polarized flux Q, and degree of polarization P of ocean planets change color from blue, through white, to red at phase angles alpha ranging from 134-108 deg for F, and from 123-157 deg for Q, with cloud coverage fraction fc increasing from 0.0 to 1.0 for F, and to 0.98 for Q. The color change in P only occurs for fc ranging from 0.03-0.98, with the color crossing angle alpha ranging from 88-161 deg. The total flux F of a cloudy, zero surface albedo planet can also change color, and for fc=0.0, an ocean planet's F will not change color for surface pressures ps > 8 bars. Polarized flux Q of a zero surface albedo planet does not change color for any fc. The color change of P of starlight reflected by an exoplanet, from blue, through white, to red with increasing alpha above 88 deg, appears to identify a (partly) cloudy exo-ocean. The color change of polarized flux Q with increasing alpha above 123 deg appears to uniquely identify an exo-ocean, independent of surface pressure or cloud fraction. At the color changing phase angle, the angular distance between a star and its planet is much larger than at the phase angle where the glint appears in reflected light. The color change in polarization thus offers better prospects for detecting exo-oceans.
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