Retrieval Of Heat Flow In Lunar Regolith Using CE-2 MRM Data

The heat flow in lunar regolith is defined as the rate at which the Moon loses its interior heat through its surface,which contains two endogenic sources,radiogenic heat flow and lunar mantle cooling heat flow.The heat flow in lunar regolith can quantify the energy lost inside the moon,which is of great significance for understanding the interior composition,structure,and evolution of the moon.The current studies on the heat flow in lunar regolith basically adopt the Apollo heat flow experiment data,the measured data of the lunar regolith samples,and the remote sensing data such as thermal infrared and optical data.Compared with the thermal infrared and optical data,the longer electromagnetic wavelength of the microwave data means greater penetration depth,which is beneficial for obtaining the temperature structure in the lunar regolith.The largest penetration depth of the microwave brightness temperature(TB)data obtained by the microwave radiometer(MRM)of Chang'e 1 or 2(CE-1/2)is about 2m,and the data are very sensitive to the temperature and the heat flow in lunar regolith,which can be used as a good data source for the retrieval of the heat flow in a large region such as the whole Moon.In this study,the lunar TB map was generated based on the CE-2 MRM data;the influence mechanism of the heat flow in lunar regolith on TB is studied based on the radiative transfer model and the thermal conduction equation;the retrieved model of the heat flow in lunar regolith was constructed based on the simulated normalized TB(n TB)and the CE-2 MRM observation n TB;furthermore,combined with the 3.0 GHz TB data at night,the heat flow in lunar regolith was retrieved based on the retrieved model.The main results are as follows:(1)The high-quality TB map was generated based on the 3D interpolation method.For solving the stripes problem in the TB map caused by the insufficient amount of CE-2MRM data,the 3D interpolation method based on Delaunay tetrahedralization is used for the first time to generate the TB map.(2)The influence mechanism of the heat flow in lunar regolith on TB is studied based on the radiative transfer model and the thermal conduction equation.In this study,the corresponding temperature profiles of the different heat flow in lunar regolith were obtained by solving the thermal conduction equation with real-time solar illumination and the heat flow in lunar regolith as boundary conditions.Then,the relationship between the heat flow in lunar regolith and TB is established by substituting the temperature profile into the radiative transfer model.(3)The retrieved model of the heat flow in lunar regolith was constructed based on the simulated n TB and the CE-2 MRM observation n TB.For avoiding the influence of the global calibration issue of the CE-2 MRM data on retrieved results,the simulated n TB and the observation n TB were used for the first time to retrieve the heat flow in lunar regolith.The retrieved scheme is as follows:First,the simulated n TB values and its corresponding heat flow values are used at each TB simulation position,and the functional relationship between the heat flow in lunar regolith and the n TB is obtained based on polynomial fitting.Second,the observation n TB is substituted into the functional relationship to retrieve the heat flow in lunar regolith.Finally,the heat flow map within 60°S to 60°N of the Moon is generated after removing abnormal values.(4)The distribution features of the retrieved heat flow in lunar regolith.The retrieved results are in good agreement with radioactive heat-producing elements.The high-value regions of the heat flow in lunar regolith mainly include the southwest of Oceanus Procellarum,Mare Frigoris,Mare Nubium,Vallis Schr?teri,the peripheries of Mare Imbrium,and Copernicus crater in the Procellarum KREEP Terrane(PKT).The heat flow in lunar regolith is mainly in the range of 14-22 m W m^-2,with an average of 18.1 m W m^-2.The heat flow in lunar regolith at Apollo 15 and 17 landing sites are 20.0 m W m^-2 and13.4 m W m^-2,respectively,which is less than 5%different from the results estimated by the Apollo heat flow experiments.