As of 24th September Thursday 2020, The CNSA –China National Space Administration Belt and Road Initiative Lunar Mission on the One Hundred Kilometer diameter Von Karman Crater Chang’e 4 lander and the “Yutu 2” lunar rover the Chang’e-4 lander and the “Yutu-2” lunar rover… Chang’e 4 lander and “Yutu 2” lunar rover awakened autonomously and entered the 22nd day of work] Today, the Chang’e 4 lander and “Yutu 2” lunar rover, which have been working on the back of the moon for 630 days….. In following.. The Chang’e-4 lander and the “Yutu-2” lunar rover will finish the 22nd month day work at 7:30 on September 24 and 23:18 on the 23rd, and complete the moon night mode setting according to ground instructions, and enter moon night sleep. Up to now, Chang’e-4 has spent 630 Earth days on the back of the moon and travelled 547.17 meters cumulatively.
Based on the 21st month day panoramic camera stitched images, DOM images and other data, the “Yutu 2” lunar rover mainly travels during the 22nd month day, successively in the impact crater and reflection about 1.3km northwest of the landing site Areas with higher rates were detected.
Researchers have made use of data such as panoramic camera ring-shot detection, infrared imaging spectrometer calibration detection, and simultaneous detection of lunar radar during driving, and obtained a number of scientific results, which were recently published in the International Journal of Nature Astronomy.
The scientific team conducted in-depth research on the radar detection data and obtained important discoveries about the lunar soil and shallow structures in the landing zone. Based on the characteristics of low-frequency radar signals, as shown in Figure 1, the shallow structure of the landing area is divided into three basic units, from top to bottom there are strong reflection units (unit 1), weak reflection units (unit 2), and medium reflection units. Unit (Unit 3). Combining basic constraints such as regional geology and the spatial distribution of large-scale impact craters, the results of the geological interpretation are as follows: Unit 1 (total thickness of about 130m) is the accumulation of sputtering materials near multiple impact craters (including Finsen, Alder, and von Carmen). Impact craters such as L and L’) and the basalt breccia layer at the bottom; unit 2 (total thickness about 110 m) is a basalt layer with multiple eruptions; unit 3 (thickness not less than 200 m) is Leibniz in the north of the landing zone Spatter from impact craters. The high-frequency radar signal further gives the fine structure of the upper part of the unit 1, as shown in Figure 2, which is characterized by the presence of a 12m thick lunar soil layer on the top, which basically does not contain large rocks, and the bottom is a strip of 22m thick Sputters, they are all projectiles from the Finsen impact crater, with a total thickness of 34m.
Fig.1 The detection profile and interpretation result of the low-frequency channel of the lunar radarFig.1 The detection profile and interpretation result of the low-frequency channel of the lunar radar
The lunar radar carried by the “Yutu-2” lunar rover can obtain the geological section below the driving path and reveal the layered structure of the underground. Because the lunar radar is directly based on the lunar surface for detection, the reflected signal detected by it has large energy and clear characteristics, and the effect is far better than that of spaceborne radars more than 100km away from the lunar surface. Moreover, due to the use of a frequency much higher than 5MHz of the spaceborne radar, its resolution advantage is also very obvious. The main frequencies of the two channels of the lunar radar are 60MHz and 500MHz, the spatial resolution is 10m and 0.3m, and the detection depth is about 50m and 500m. The high-frequency channel is used to detect the high-resolution structure of the shallow lunar soil and its underlying sputter, and the low-frequency channel is used to detect the layered structure of the deep sputter and basalt.
Figure 2 The detection profile and interpretation result of the high-frequency channel of the lunar radar
The shallow structural profile obtained by the lunar radar shows that the lunar material detected by “Yutu 2” comes from the Finsen impact crater, not from the filling basalt of the von Karman impact crater itself; at the same time, the radar profile also reveals the landing area has experienced multiple impacts, sputtering accumulation and multiple basalt magma eruptions filling. These new discoveries are of great significance for understanding the evolution of the Moon’s South Pole-Aiken Basin, and have an important guiding role for the subsequent exploration and study of the composition and structure of the Moon’s internal material.
嫦娥四号着陆器和“玉兔二号”月球车分别于9月24日7时30分、23日23时18分结束第22月昼工作，按地面指令完成月夜模式设置，进入月夜休眠。截至目前，嫦娥四号已在月球背面度过630个地球日，累积行驶547.17米。
基于第21月昼全景相机拼接影像、DOM影像等数据情况，“玉兔二号”月球车在第22月昼期间主要以行驶为主，先后在距离着陆点西北方向约1.3km的撞击坑和反射率较高的区域进行了探测。
科研人员利用全景相机环拍探测、红外成像光谱仪定标探测、测月雷达行驶过程中同步探测等数据，取得多项科学成果，近期发表在Nature Astronomy国际期刊上。
科学团队对雷达探测数据开展了深入研究，获得了着陆区月壤和浅层结构的重要发现。基于低频雷达信号特征，如图1所示，将着陆区的浅层结构划分为三大基本单元，由上往下依次为强反射单元（单元1）、弱反射单元（单元2）和中等反射单元（单元3）。结合区域地质和大型撞击坑的空间分布等基本约束，地质解译结果如下：单元1（总厚度约130m）为临近多个撞击坑的溅射物堆积（包括芬森、阿尔德、冯·卡门L和L’等撞击坑）和底部的玄武岩角砾层；单元2（总厚度约110 m）为多次喷发的玄武岩层；单元3（厚度不小于200 m）为着陆区北部莱布尼兹撞击坑的溅射物。高频雷达信号进一步给出单元1上部的精细结构，如图2所示，其特征为顶部存在厚达12m的月壤层，基本不含大石块，其下为厚达22m的条带状溅射物，它们均是来自芬森撞击坑的抛射物，总厚度达34m。
图1 测月雷达低频通道的探测剖面及解译结果图1 测月雷达低频通道的探测剖面及解译结果
“玉兔二号”月球车搭载的测月雷达能够获取行驶路径下方的地质剖面，揭示地下的分层结构。由于测月雷达直接基于月面进行探测，因此，其探测到的反射信号能量大，特征清晰，效果远优于距月面100km以上的星载雷达探测。而且，由于采用远高于星载雷达5MHz的主频，其分辨率优势也十分明显。测月雷达两个通道的主频分别为60MHz和500MHz，空间分辨分别为10m和0.3m，探测深度分别约50m和500m。高频通道用于探测浅部月壤及其下伏溅射物的高分辨结构，低频通道用于探测深部溅射物和玄武岩等分层结构。
图2 测月雷达高频通道的探测剖面及解译结果图2 测月雷达高频通道的探测剖面及解译结果
测月雷达所获取的浅层结构剖面表明“玉兔二号”所探测的月面物质来自于芬森撞击坑，而不是来自冯·卡门撞击坑自身的充填玄武岩；同时，该雷达剖面还揭示了着陆区经历多期次的撞击溅射堆积和多期次玄武岩浆喷发充填。这些新发现对于认识月球南极-艾肯盆地的演化具有非常重要的意义，对于月球内部物质组成和结构的后续探测与研究有重要指导作用。
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