Clouds in the atmospheres of extrasolar planets II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds
Clouds in the atmospheres of extrasolar planets II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds
复制标题
太阳系外行星大气层中的云II。
DOI：
--
复制
发表时间：
2011
期刊：
影响因子：
--
通讯作者：
H. Rauer
中科院分区：
文献类型：
--
作者：
D. Kitzmann;A. Patzer;P. V. Paris;M. Godolt;H. Rauer
目标。我们研究了多层云（低层水和高层冰云）对绕不同类型恒星运行的类地行星热发射光谱的影响。由于云具有波长依赖的吸收和散射特性，因此对这种行星发射光谱有重要影响。我们还研究了云对从低分辨率光谱中获得行星表面温度信息的能力的影响。方法。我们使用先前开发的基于地球大气观测的参数云模型，与一维辐射对流稳态气候模型相结合。本文应用该模型研究了云对类地系外行星热发射光谱的影响与中心恒星类型的关系。结果。云层的存在通常导致行星红外光谱的减少，这与光谱吸收特征的衰减有关，例如O3的9.6µm吸收带。这种阻尼作用不仅限于源自云层以下的吸收特征，而且还发现了形成于云层之上的特征。当只考虑单个云层时，两种云对光谱的影响基本相同，但底层的物理过程明显不同。对于考虑多层云的模式情景，其覆盖范围产生地球表面平均温度，低层云对热发射光谱只有很小的影响。在这些情况下，主要的差异是由高空冰云造成的。影响最大的是一颗围绕f型恒星运行的行星，在高云层覆盖的低分辨率发射光谱中没有吸收特征。然而，对于大多数中心恒星，即使在高云层覆盖下，行星大气吸收带也存在。当将黑体辐射曲线拟合到红外发射光谱的光谱形状时，云也会影响低分辨率光谱对地表温度的推导。随着高层云量的增加，导出的温度越来越低估了实际的行星表面温度。因此，云可以显著地改变行星的测量表观温度以及红外特征光谱特征的可探测性。因此，在对云的存在进行进一步调查之前，不应该过早地从潜在宜居行星的名单中抛弃那些观测得出的表面温度稍低的行星。
Aims. We study the impact of multi-layered clouds (low-level water and high-level ice clouds) on the thermal emission spectra of Earth-like planets orbiting different types of stars. Clouds have an important influence on such planetary emission spectra due to their wavelength dependent absorption and scattering properties. We also investigate the influence of clouds on the ability to derive information about planetary surface temperatures from low-resolution spectra. Methods. We use a previously developed parametric cloud model based on observations in the Earth’s atmosphere, coupled to a onedimensional radiative-convective steady state climate model. This model is applied here to study the effect of clouds on the thermal emission spectra of Earth-like extrasolar planets in dependence of the type of central star. Results. The presence of clouds lead in general to a decrease of the planetary IR spectrum associated with the dampening of spectral absorption features such as the 9.6 µm absorption band of O3 for example. This dampening is not limited to absorption features originating below the cloud layers but was also found for features forming above the clouds. When only single cloud layers are considered, both cloud types exhibit basically the same effects on the spectrum but the underlying physical processes are clearly different. For model scenarios where multi-layered clouds have been considered with coverages which yield mean Earth surface temperatures, the low-level clouds have only a small influence on the thermal emission spectra. In these cases the major differences are caused by highlevel ice clouds. The largest effect was found for a planet orbiting the F-type star, where no absorption features can be distinguished in the low-resolution emission spectrum for high cloud coverages. However, for most central stars, planetary atmospheric absorption bands are present even at high cloud coverages. Clouds also affect the derivation of surface temperatures from low-resolution spectra when fitting black-body radiation curves to the spectral shape of the IR emission spectra. With increasing amount of high-level clouds the derived temperatures increasingly under-estimate the real planetary surface temperatures. Consequently, clouds can alter significantly the measured apparent temperature of a planet as well as the detectability of the characteristic spectral signatures in the infrared. Therefore, planets with observationally derived somewhat lower surface temperatures should not be discarded too quickly from the list of potential habitable planets before further investigations on the presence of clouds have been made.