标题： 映射部分多云系外行星的表面很难 显示英文标题

标题： Mapping the Surface of Partially Cloudy Exoplanets is Hard

摘要： 反射光度测量法可以揭示类地系外行星上的海洋和大陆的存在，从而直接限制这些世界当前和长期的宜居性。 从观测到的光曲线推断行星的反照率图是一项挑战，因为不同的地图可能产生无法区分的光曲线。 云的变化加剧了这种不确定性。 之前有人建议，跨越数天的全盘积分测光可以结合起来以获得一颗系外行星的无云表面地图。 我们通过同时拟合行星的固定表面地图和覆盖其上的时间变化云层，展示了这种方法作为贝叶斯检索的一部分。 我们在合成数据上测试了这种方法，然后将其应用于地球的实际全盘积分观测。 我们发现连续八天的合成观测就足以重建一个忠实的低分辨率表面反照率图，而无需对云物理做出假设。 对于具有可忽略的测光不确定性的光曲线，位置处大气顶层的最小反照率是一个很好的表面反照率估计值。 当应用于来自DSCOVR航天器上地球多色成像相机的观测时，我们的方法仅去除了少量的云。 我们将这一困难归因于观测的全相位几何结构以及地球云的短相关长度。 对于气候类似地球的系外行星，可能很难做得比云平均地图更好。 我们推测，对于近四分之一相位成像且拥有大型云系统的系外行星，云移除将是成功的。 显示英文摘要

摘要： Reflected light photometry of terrestrial exoplanets could reveal the presence of oceans and continents, hence placing direct constraints on the current and long-term habitability of these worlds. Inferring the albedo map of a planet from its observed light curve is challenging because different maps may yield indistinguishable light curves. This degeneracy is aggravated by changing clouds. It has previously been suggested that disk-integrated photometry spanning multiple days could be combined to obtain a cloud-free surface map of an exoplanet. We demonstrate this technique as part of a Bayesian retrieval by simultaneously fitting for the fixed surface map of a planet and the time-variable overlying clouds. We test this approach on synthetic data then apply it to real disk-integrated observations of the Earth. We find that eight days of continuous synthetic observations are sufficient to reconstruct a faithful low resolution surface albedo map, without needing to make assumptions about cloud physics. For lightcurves with negligible photometric uncertainties, the minimal top-of-atmosphere albedo at a location is a good estimate of its surface albedo. When applied to observations from the Earth Polychromating Imaging Camera aboard the DSCOVR spacecraft, our approach removes only a small fraction of clouds. We attribute this difficulty to the full-phase geometry of observations combined with the short correlation length for Earth clouds. For exoplanets with Earth-like climatology, it may be hard to do much better than a cloud-averaged map. We surmise that cloud removal will be most successful for exoplanets imaged near quarter phase that harbour large cloud systems.