Gypsum, a typical hydrous calcium sulfate, has been found in some carbonaceous chondrites with aqueous alterations and has also been identified on Mars using visible and near-infrared (VNIR) reflectance spectra measured by orbiting and in-situ sensors. Therefore, gypsum may exist on more extraterrestrial bodies that have undergone hydration processes. Due to the dependence of spectral features on illumination and viewing conditions for packed particulate grains, understanding the photometric properties of gypsum grains with various shapes is crucial for aiding in the remote detection of gypsum grains. We conducted measurements of the spectral bi-directional reflectance distribution functions (BRDF) for three gypsum varieties: fibrous Satin spar, crystal-like Selenite, and fine-grained aggregate Alabaster, each with two particle size distributions. We found that the scattering properties of Satin spar exhibit unusual saddle-shaped BRDF patterns rarely seen in the measured BRDF of most particulate samples, including snow and ice particles. The measurements on Satin spar with the smaller size distribution (0–45 µm) can be well-reproduced by a ray-tracing model of circular cylinders combined with radiative transfer calculations.

中文翻译：

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填充纯石膏颗粒的光谱双向反射率测量


石膏是一种典型的水合硫酸钙，在一些具有水蚀变的碳质球粒陨石中被发现，并且在火星上也通过轨道传感器和原位传感器测量的可见光和近红外 (VNIR) 反射光谱进行了识别。因此，石膏可能存在于更多经历过水合过程的外星天体上。由于光谱特征依赖于堆积颗粒颗粒的照明和观察条件，了解各种形状的石膏颗粒的光度特性对于帮助远程检测石膏颗粒至关重要。我们对三种石膏品种的光谱双向反射率分布函数 (BRDF) 进行了测量：纤维状缎晶石、晶体状亚硒酸盐和细粒骨料雪花石膏，每种石膏都有两种粒度分布。我们发现，缎晶石的散射特性表现出不寻常的马鞍形 BRDF 图案，这在大多数颗粒样品（包括雪和冰颗粒）的测量 BRDF 中很少见。具有较小尺寸分布（0-45 µm）的缎晶石的测量可以通过圆柱体的射线追踪模型与辐射传输计算相结合来很好地再现。