水Ice颗粒的偏振测量提供了对冰冷行星表面上晶粒尺寸和烧结程度的了解
Polarimetry of Water Ice Particles Providing Insights on Grain Size and Degree of Sintering on Icy Planetary Surfaces
摘要：
行星表面散射光的偏振测量是一个强有力的工具，以提供其微观结构的约束。为了改善对冰表面偏振数据的解释，我们开发了冰样品偏振仪（POLICES），以补充伯尔尼大学冰实验室的测量设施。新的装置使用高精度斯托克斯偏振计来测量在中等相位角（从1.5到30°）的表面散射的可见光的偏振度。我们提出的光度和偏振相位曲线测量的各种表面上的纯水冰粒具有良好的控制的大小和形状（球形，粉碎，霜）。结果表明，负极化分支的幅值和形状随冰的粒径和变质程度的变化而变化。我们发现，新鲜的霜形成的水冷凝在冷表面上有一个相位曲线的共振（米氏振荡），表明霜胚是透明的微米大小的颗粒，具有窄的尺寸分布和球形。将这些测量结果与太阳系冰冷卫星的偏振观测结果进行比较，表明木卫二可能被相对较粗（约40-400 μm）的烧结颗粒覆盖，比土卫二和土卫五更可能被平均数微米的霜状颗粒覆盖。极化对颗粒大小和烧结程度的高度敏感性使其成为探测冰冷行星表面正在进行的过程的理想工具，例如冰火山活动。
英文摘要：
The polarimetry of the light scattered by planetary surfaces is a powerful tool to provide constraints on their microstructure. To improve the interpretation of polarimetric data from icy surfaces, we have developed the POLarimeter for ICE Samples (POLICES) complementing the measurement facilities of the Ice Laboratory at the University of Bern. The new setup uses a high precision Stokes polarimeter to measure the degree of polarization in the visible light scattered by surfaces at moderate phase angles (from 1.5 to 30°). We present the photometric and polarimetric phase curves measured on various surfaces made of pure water ice particles having well-controlled size and shape (spherical, crushed, frost). The results show how the amplitude and the shape of the negative polarization branch change with the particles sizes and the degree of metamorphism of the ice. We found that fresh frost formed by water condensation on cold surfaces has a phase curve characterized by resonances (Mie oscillations) indicating that frost embryos are transparent micrometer-sized particles with a narrow size distribution and spherical shape. Comparisons of these measurements with polarimetric observations of the icy satellites of the Solar System suggest that Europa is possibly covered by relatively coarser (~40-400 μm) and more sintered grains than Enceladus and Rhea, more likely covered by frost-like particles of few micrometers in average. The great sensitivity of polarization to grain size and degree of sintering makes it an ideal tool to detect hints of ongoing processes on icy planetary surfaces, such as cryovolcanism.
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