Retrieving volume FeO and TiO<inf>2</inf> abundances of lunar regolith with CE-2 CELMS data using BPNN method

Abstract

© 2019 National Astronomical Observatories, CAS and IOP Publishing Ltd. The volume FeO and TiO2 abundances (FTAs) of lunar regolith can be more important for understanding the geological evolution of the Moon compared to the optical and gamma-ray results. In this paper, the volume FTAs are retrieved with microwave sounder (CELMS) data from the Chang'E-2 satellite using the back propagation neural network (BPNN) method. Firstly, a three-layered BPNN network with five-dimensional input is constructed by taking nonlinearity into account. Then, the brightness temperature (TB ) and surface slope are set as the inputs and the volume FTAs are set as the outputs of the BPNN network. Thereafter, the BPNN network is trained with the corresponding parameters collected from Apollo, Luna, and Surveyor missions. Finally, the volume FTAs are retrieved with the trained BPNN network using the four-channel TB derived from the CELMS data and the surface slope estimated from Lunar Orbiter Laser Altimeter (LOLA) data. The rationality of the retrieved FTAs is verified by comparing with the Clementine UV-VIS results and Lunar Prospector (LP) GRS results. The retrieved volume FTAs enable us to re-evaluate the geological features of the lunar surface. Several important results are as follows. Firstly, very-low-Ti (<1.5 wt.%) basalts are the most spatially abundant, and the surfaces with TiO2 > 5 wt.% constitute less than 10% of the maria. Also, two linear relationships occur between the FeO abundance (FA) and the TiO2 abundance before and after the threshold, 16 wt.% for FA. Secondly, a new perspective on mare volcanism is derived with the volume FTAs in several important mare basins, although this conclusion should be verified with more sources of data. Thirdly, FTAs in the lunar regolith change with depth to the uppermost surface, and the change is complex over the lunar surface. Finally, the distribution of volume FTAs hints that the highlands crust is probably homogeneous, at least in terms of the microwave thermophysical parameters.