月球极区雷达异常撞击坑分布特征与成因分析

Radar detection of the potential water ice in the permanently shadowed regions near the lunar poles is one of the hottest research topics in lunar science, and interpretation of the anomalous radar echoes over the lunar polar region is also remained as an important unresolved challenge. In this project, we will conduct a global search for the anomalous craters over the Moon in multi-source radar data, including Miniature Radio Frequency (Mini-RF) radar, Miniature Synthetic Aperture Radar (Mini-SAR), and Earth-based Arecibo radar. Statistical characteristics of the radar echoes for the anomalous craters will be analyzed, which will be employed to judge whether polar anomalous craters are over-abundance or not. Next, previous radar scattering model will be improved to include scattering from large rocks, multiple scattering, coherent backscatter opposition effect (CBOE), and scattering from partially buried rocks perched on a rough lunar surface. With the improved radar scattering model, we will study how radar echo strength and polarization status vary with the key parameters of the lunar regolith layer, such as surface roughness, dielectric constant, size and abundance of rocks, and regolith thickness. The compact polarization theory will be revised to include the effect of local incidence angle of radar wave, and will be utilized to analyze potential factors for the enhanced circular polarization ratio (CPR) for the anomalous craters. In combination with surface roughness, dielectric constant, size and abundance of buried rocks, and regolith thickness obtained from other remote sensing data, potential factors for the elevated radar CPR in the interior of anomalous craters will be analyzed using the improved radar scattering model and compact polarization scattering theory. Furthermore, we will study whether there is ice deposit over the lunar polar regions, and estimate the inventory of water ice if there is. Finally, geological reasons for the occurrence of the anomalous craters will be investigated using impact cratering and space weathering theory.

雷达对月球两极永久阴影坑内潜在水冰的探测是月球科学领域的一个热点问题，对月球极区雷达回波异常的解译也是一个难点问题。本研究将由Mini-RF、Mini-SAR、Arecibo等多源雷达数据统计整个月球表面雷达异常撞击坑的分布，分析异常撞击坑的雷达回波特征，判断月球极区异常撞击坑数量是否偏多。在申请人前期工作的基础上，发展包括大石块散射、多次散射、后向散射增强、粗糙面与半掩埋石块复合散射的月表雷达散射模型，分析月表关键性特征参数对雷达回波的影响。进一步地，发展纳入局部入射角的简缩极化散射理论，分析引起雷达圆极化比偏高的可能因素。借助月球多源数据对月表粗糙度、介电常数、石块丰度、月壤厚度的估算结果，由雷达散射模型与简缩极化散射理论分析引起月表极区雷达回波异常的因素，判断月球极区是否有大量水冰的存在，如存在则估算水冰含量。最后，结合陨石撞击与空间风化理论，分析月表异常雷达撞击坑形成的地质原因。

为解决地基雷达对月球两极永久阴影坑内水冰探测存在的争议问题，印度“月船”1号、美国月球侦查轨道器上分别搭载了微型合成孔径雷达（Mini-SAR和Mini-RF），用于对月球进行高分辨率成像观测，搜寻月球极区水冰存在的证据。Mini-RF雷达团队在月球两极发现一种雷达回波异常撞击坑，其边缘以内雷达圆极化比高但边缘外低，该类撞击坑密度随纬度升高而增加，被认为是水冰存在的证据，但缺少对雷达回波异常的物理解释。本项目对整个月球表面6818轨Mini-RF原始数据进行了预处理，生成了全月表雷达回波功率与圆极化比镶嵌图。基于月表高分辨率数字高程数据，由水文学的水体淹没模型，搜索并构建了直径在2 km到20 km之间的撞击坑数据库。在此基础上，分析了直径大于2 km的月表撞击坑雷达回波特征，给出了月球表面雷达异常撞击坑分布图，研究了雷达异常撞击坑密度随纬度的变化，发现月球极区雷达异常撞击坑密度并没有偏多。根据撞击坑内外雷达回波的强弱差异，结合Diviner对月表石块丰度的反演结果，发现月表雷达回波圆极化比主要与表面石块有关。建立了月表二元混合散射模型，可以很好地解释月表雷达回波强度与石块丰度之间的关系。最后，结合月表光学图像、Diviner石块丰度、高程与坡度等，系统性研究了月表撞击坑的演化规律，发现新鲜撞击坑、雷达异常撞击坑、年老撞击坑是撞击坑演化的时间序列，这可以很好地解释Mini-RF雷达与Diviner热红外对撞击坑的观测结果。本项目研究结果表明，Mini-RF团队所认为的月球雷达异常撞击坑，其密度在月球两极并非偏多，雷达回波异常主要是由于撞击坑退化过程中边缘内外、表面与次表面石块非均匀退化所引起的，否定了月球极区存在大量水冰的结论。