论文标题
来自卡西尼（Cassini）的伽利亚卫星的反射光观测：冷陆生期的测试床
Reflected Light Observations of the Galilean Satellites from Cassini: a testbed for cold terrestrial exoplanets
论文作者
论文摘要
对于具有较薄气氛或没有气氛的陆生外行星，表面为行星的反射光信号贡献了光。测量太阳系中物体的各种磁盘集成亮度以及具有照明和波长的变化对于计划成像观察直接成像的外部行星的观察以及解释最终数据集都是必不可少的。在这里，我们测量了加利利卫星的亮度的变化，这是中心经度，照明相角和波长的函数。数据范围从400-950nm到一系列波长，主要相位角度从0-25度开始，一些约束观测值接近60-140度。尽管卫星之间的大小和密度相似，但表面不均匀性导致磁盘集成反射率与行星经度和相位角度的显着变化。我们发现这些变化足以确定月球的旋转周期。我们还发现，在低相角中，表面可以产生8-36％的反射率变化，而有限的高相角观察结果表明，变化将在较高角度以比例更大的幅度。此外，所有伽利略卫星都比理想化的兰伯特模型在最有可能通过直接成像任务观察到的阶段所预测的要黑。如果地球大小的系外行星的表面与加利利卫星相似，我们发现未来的直接成像任务将需要达到小于0.1 \，ppb的精确度。如果要达到必要的精度，将来的系外行星观测可以利用类似的观测方案来推断表面变化，确定旋转周期并可能推断出表面组成。
For terrestrial exoplanets with thin atmospheres or no atmospheres, the surface contributes light to the reflected light signal of the planet. Measurement of the variety of disk-integrated brightnesses of bodies in the Solar System and the variation with illumination and wavelength is essential for both planning imaging observations of directly imaged exoplanets and interpreting the eventual datasets. Here we measure the change in brightness of the Galilean satellites as a function of planetocentric longitude, illumination phase angle, and wavelength. The data span a range of wavelengths from 400-950nm and predominantly phase angles from 0-25 degrees, with some constraining observations near 60-140 degrees. Despite the similarity in size and density between the moons, surface inhomogeneities result in significant changes in the disk-integrated reflectivity with planetocentric longitude and phase angle. We find that these changes are sufficient to determine the rotational periods of the moon. We also find that at low phase angles the surface can produce reflectivity variations of 8-36% and the limited high phase angle observations suggest variations will have proportionally larger amplitudes at higher phase angles. Additionally, all the Galilean satellites are darker than predicted by an idealized Lambertian model at the phases most likely to be observed by direct-imaging missions. If Earth-size exoplanets have surfaces similar to that of the Galilean moons, we find that future direct imaging missions will need to achieve precisions of less than 0.1\,ppb. Should the necessary precision be achieved, future exoplanet observations could exploit similar observation schemes to deduce surface variations, determine rotation periods, and potentially infer surface composition.
