First of all, the authors conduct a review on the research of lunar water-ice exploration. Since the 1990s, many lunar exploration missions have explored and studied water-ice in the permanent shadow, such as using radar (Earth-based radar included), neutron spectrum, visible and near-infrared spectrum, laser, impact experiment and other detection technologies to carry out remote sensing exploration, and laboratory analysis of Apollo samples and lunar meteorites. In radar detection, circular polarization ratio (CPR) obtained from radar can be used to characterize the physical properties of the lunar surface (CPR of water-ice > 1 while CPR of rough lunar surface is generally between 0.5 and 1). Thus, the radar on the lunar exploration satellite can be used to detect the water-ice at lunar polar regions. In neutron detection, the neutron detector reflects the content of hydrogen in a certain area through the relative level of different types of neutrons counting rates. Combined with the analysis and judgment of the existing form of hydrogen, it is inferred whether there is water-ice in this area. Spectral detection can only detect from the surface down to a few millimeters; thus, the detected water should exist in the dust on lunar surface, which exists in the form of crystalline water with lunar minerals. Laser detection is also applied, via conducting a comprehensive analysis in combination with Lunar Orbital Laser Altimeter albedo and annual maximum surface temperature.
Then, authors introduce current and future international lunar water-ice exploration missions. The current way to detect the existence of lunar water-ice is indirect. Three CubeSats, namely Lunar IceCube, Lunar Flashlight, and LunaH-Map, are built by Morehead State University in Kentucky, a team from the Jet Propulsion Laboratory and Marshall Space Flight Center, and Arizona State University. Direct measurement asks for landing and positioning exploration in the permanent shadow of the lunar polar regions. The Volatiles Investigating Polar Exploration Rover (VIPER) is designed by NASA for collecting all kinds of soil environmental data under different light and temperature conditions. Russia plans to carry out 3 lunar exploration missions in the next 3 years, namely, Luna-25, Luna-26, and Luna-27. India’s Chandrayaan-3 lunar exploration mission was also announced by the Indian Space Research Organization, which consists of a propulsion module, a lander, and a lunar rover.
At last, authors discuss the scientific Missions of China’s Future Lunar Water-Ice Exploration. Chang’E-7 is scheduled to carry out detailed scientific surveys in the south pole of the Moon, one of whose scientific objectives is to probe the source, content, and distribution of lunar water-ice. Three scientific payloads are deployed to accomplish the objective: Lunar Microwave Imaging Radar (LMIR), Lunar Neutron Gamma Spectrometer (LNGS), and Lunar Water Molecular Analyzer (LWMA). Specifically, the LMIR is installed on the orbiter to remotely detect the water distribution on the global lunar surface and image the permanent shadow areas near the lunar polar landing area with high precision to identify the possible forms and distribution of water-ice. The LNGS, which is installed on the orbiter, can realize the highly sensitive detection of fast neutrons and epithermal neutrons on the global lunar surface, especially in the polar shadow area, so as to obtain the distribution and content of hydrogen in the subsurface. The LWMA is installed on the mini-flying probe. It can in situ detect the existence, content, form, and location of water-ice in the permanent shadow areas of the lunar south pole; study the source, injection mechanism, and surface process of water-ice; detect the volatile; and study the existence and content of organic molecules.
