Inversion Method Of High Resolution Radioactive Element Content In Lunar Surface Based On CE2-GRS Data

The distribution of radioactive elements on the lunar surface plays an important role in helping us understand the formation and evolution of the moon,and is an essential scientific objective for all lunar exploration projects.The Chang’e-2 lunar exploration satellite launched by China carries a gamma-ray spectrometer(CE2-GRS)with relatively high energy variability and high stability.Its main scientific task is to conduct effective exploration of the lunar surface for seven months in a 100 km orbit around the moon,so as to obtain the composition of materials on the lunar surface and draw the distribution map of the lunar element content.This is the first time that China has received fully global,high-quality radioactive data from the moon.In this paper,by using Chang’e-2 gamma spectrum 2C data and through data reduction processes such as data screening,cosmic ray correction,background deduction and lunar surface radioactive element content inversion,content distribution maps of U,Th and K elements based on CE2-GRS data at the resolution of 1°×1° are obtained for the first time.The work of this paper is mainly divided into the following aspects:1.Convert CE2-GRS 2C time series data into monthly table 1°×1° high-resolution regional grid data.The process includes line data screening,gain and zero drift correction and energy spectrum accumulation.First,the CE2-GRS 2C data were screened according to the solar activity influence,the total value range of spectral lines and the status of mass markers.The filtered data accounts for about 98% of the total data.The position of positron annihilation peak and other obvious characteristic peaks is used to correct the gain and zero drift of spectral lines,so that the characteristic peaks of all the target elements on the spectral lines are in the same energy channel,and the errors caused by the inconsistency of peak positions in the calculation of cumulative energy spectrum average and characteristic peak area of each characteristic peak count are reduced.Finally,the columnar mapping method was used to convert the total time series data of the monthly table into the cumulative average data of the monthly table grid with a resolution of 1°×1°.2.The background composition of the gamma spectrum of Chang’e-2 is analyzed in detail.According to different background sources,different background deduction methods are adopted: the influence of 138 La on K element is deducted by cruise spectrum during the lunar flight;SNIP algorithm is used to subtract the scattering background of characteristic peaks of spectral lines.In view of the influence of cosmic rays in the detection process of CE2-GRS,two methods were used to normalize the cosmic rays with the energy segment of 6.13 MeV oxygen element and the energy segment of 8 ～ 8.75 MeV high radioactivity as the standard.In the absence of good results,EMD mode decomposition method is used to deduct the effects of all environmental backgrounds including cosmic rays.This is the first time that the EMD method is applied to the background deduction of lunar exploration gamma spectrum data,and the results are satisfactory.3.Select different noise reduction methods to deal with the statistical fluctuation problem of energy spectrum curve and the noise problem of 1°×1° high-resolution element count rate map.Noise reduction by discrete wavelet analysis of a single spectral line can effectively reduce the noise fluctuation of spectral line while preserving the characteristic peak-peak value to the maximum extent,which has an important contribution to improving the solving accuracy of the element characteristic peak area.A high precision and high resolution distribution map of U,Th and K elements is obtained by using two-dimensional median filtering and denoising.4.Using linear fitting and absolute content inversion model proposed by Lawrence et al.,two inversion methods of lunar surface radioactive element content were used to invert and compare the values of U,Th and K elements obtained from CE2-GRS data.The inversion results were compared with the contents of U,Th and K elements in the whole monthly table based on LP-GRS data.The overall distribution of radioactive elements obtained from the three kinds of data showed a high consistency.However,the large resolution mapping of LP-GRS made the count in the small area of radioactive element enrichment average with that in the surrounding area of low element abundance.The abundance obtained from analytic inversion decreases with the increase of map resolution.The 1°×1° high-resolution element content distribution map obtained in this paper retains the details of the content distribution of elements U,Th and K in the lunar table.5.The distribution characteristics of radioactive elements on the lunar surface were analyzed based on the content distribution map of U,Th and K elements at 1°×1°resolution obtained from CE2-GRS data inversion.The mechanism of enrichment of radioactive elements on the lunar surface is analyzed and some possible locations of KREEP volcanism are deduced.The abundance maps of major elements and radioactive elements and their geochemical interpretations are given.