Lunar Regolith Parameters Retrieval Using Radiative Transfer Simulation And Look-up Technique

The retrieval of the lunar regolith parameters is one of the important scientific aims for the lunar study. The knowledge about the distribution of the lunar regolith parameters is essential to explore and develop the lunar resources. This is also one of the four scientific objectives of our lunar project. However, it is difficult to simulate the lunar regolith microwave emission and to retrieve the lunar regolith parameters for the complexity of the lunar media component. The retrieval results gained by previous studies showed great difference from each other. The key problem is that the lunar parameters are unknown and need to be assumed or approximated.In this paper, we thoroughly generalize the distribution features, related models and other study results about such lunar regolith parameters as density, dielectric constants, (FeO+TiO2) content, surface roughness, temperature, thickness and water-ice content. Based on this, the complete lunar regolith model is constructed based on the related materials. In the model, the dielectric constant and the temperature are changing with depth and position, the surface is rough, and the regolith-rock surface is slantwise. Then the influences of the regolith thermal grads, media dielectric parameters, surface roughness and the parallel or not of the two surfaces on the microwave radiation of the lunar regolith are studied using the advanced radiative transfer equation. The results show that the regolith thermal grads, the dielectric constants and the roughness of the free space-regolith surface have great influences on the microwave radiation of the lunar regolith. The results also indicate that the roughness and the obliquity of the surface between regolith and rock have little effect on the simulation results, and they can be neglected in the following study. Therefore, a perfect lunar regolith model is constructed, which has thermal grads, changeable dielectric constants and rough upper surface.Then the lunar regolith parameters are retrieved using the data from the Chang'e-1 project and other missions. The (FeO+TiO2) content distribution is gained based on the model constructed by Lucey et al.[34][35] and Shkuratov et al.[37] with the data from the imaginary spectrum instrument. Meanwhile, the regolith dielectric constants is retrieved using the exponential density model, the filling model and the (FeO+TiO2) content. The spatial resolution of microwave radiometer image is 57×57km2, 35×35 km2, 35×35 km2, 35×35 km2 for 3.0, 7.8, 19.35, 37 GHz, respectively, which comprise 64 (3 GHz) or 25 (7.8, 19.35, 37 GHz) pixels from the laser elevation data. Therefore, the roughness parameters for every pixel in the microwave radiometer image can be calculated and the surface effective reflectivity can be obtained with Q/H model. The surface thermal grads, which takes the position and different time into account, is constructed with the surface temperature model provided by Wang Z Z[111] and the regolith thermal grads model designed by Ji Yaqiu, et al[17][19]. Thereafter, according to the dielectric constant distribution, thermal grads model and the surface effective reflectivity, the look-up table between lunar regolith parameters and the brightness temperature is constructed using the radiative transfer simulation.At last, the lunar regolith thickness map is gained with the look-up table and the microwave radiometer data from the Chang'e orbitor. The comparison is done between the retrieval results and the result for Apollo landing sites with multi-frequency electromagnetic probe and active moonquake methods, the evaluated result using the methods based on the shape and size frequencies of the lunar craters, the retrieval results using radar data onboard the earth and the result with multi-channel method. The results show that the look-up table method has a high accuracy within the latitude 70 o N and 70 o S, but rather low at other place.The study on the lunar water-ice content is also one of the important fields for lunar study. The existence of the water or ice can apparently change the dielectric constant of lunar regolith. So we do a physical experiment to test the effect of the bulk moisture content in the dense medium on its dielectric constant with Dobson model. Thereafter, we do abnormal analysis for the lunar regolith dielectric constants, and find 575 pixels'dielectric constants are abnormal. Then based on the advanced Dobson model, we gain that the water-ice content at permanent area of the lunar surface is about 1.269%, and the total area is about 9.19×104 km2. The method to evaluate the bulk moisture content is also provided.To verify the accuracy of the retrieval results, the BP neural network is introduced in our paper. Based on the passive microwave radiation transfer model and the lunar regolith parameters estimated by Shkuratov et al. [37], the vertical and horizontal polarized brightness temperatures are calculated when the frequencies used are 3.0,7.8,19.35,37.0GHz, respectively. Then, the simulated brightness temperatures of Mare Serenitatis, Mare Tranquillitatis, Mare Crisium, Mare Frecunditatis, Mare Frigoris, Mare Imbrium, Mare Humorum, Mare Nectaris and Mare Vaporum are used as the input of the BP neural network, and the output of the network is density, dielectric constant and thickness of the lunar regolith. The brightness temperatures of Mare Nubium, Mare Cognitum, Mare Roris, Mare Iridium, Mare Sinus and Mare Oceanus are input into the trained BP neural network to retrieve the lunar regolith parameters. The BP algorithm can avoid this complexity and provides a new method to retrieve the regolith parameters.The study results of the paper present that the look-up table between lunar regolith thickness and the surface microwave brightness temperature can be constructed perfectly through the radiation transfer simulation. It is feasible to retrieve the lunar regolith thickness using the look-up table method. The lunar regolith thickness map shows good accordance with the results gained by other methods within the latitude 70 o N and 70 o S. And the highest accuracy is up to 83.27%. That is to say, the look-up table method is better than the evaluation method based on the shape and size frequencies of the lunar craters. The water-ice content information gained with the dielectric abnormal analysis is reliable.The look-up table also shows bad accuracy at the southern and northern pole area. This is related to the selected regolith parameter models. The knowledge about such area is rarely. And the following study should be strengthened on the distributions of the regolith parameters. At the same time, the study on the lunar rock distribution should be paid more attention, which can provide great information on the lunar regolith density and the regolith dielectric constants.