| Cryptomare is a type of early lunar mare basalt material covered by high-albedo crater or basin ejecta.It represents early lunar volcanic activity.Understanding the distribution and composition of the cryptomare deposits can provide important information about the early thermal evolution and volcanic history of the Moon.Currently,the study of surface materials of cryptomare primarily relies on visible data.The presence of cryptomare deposits are determined through the identification of impact craters with dark halo features,spectral mixture analysis,and geochemical anomalies in surface materials.However,visible data only reflects the composition of the topmost layer of lunar regolith and is heavily influenced by space weathering and the coverage of impact ejecta.As a result,the study of surface materials of cryptomare based on visible techniques has its limitations.This paper focuses on the analysis of surface material composition characteristics and potential geological value of cryptomare using microwave radiometer data from the Chang’e-2 satellite.First,a lunar regolith radiative transfer model suitable for studying surface materials of cryptomare is constructed based on the formation and evolution processes of cryptomare.Then,the microwave radiometer data from Chang’e-2 is utilized to create brightness temperature maps of cryptomare region by selecting raw brightness temperature points and employing linear interpolation with Delaunay triangulation.To mitigate the influence of latitude variations in brightness temperature,normalized brightness temperature mapping and brightness temperature difference mapping are performed.Based on the brightness temperature characteristics of the Balmer-Kapteyn region,the presence of mare-like cryptomare deposits,referred to as"mare-like deposits,"is proposed.The detection capability of microwave radiometer data for this deposits is evaluated using data dimensionality reduction and regression analysis techniques.Additionally,the support vector machine method is applied for the extraction of mare-like deposits in the cryptomare regions,followed by an analysis of its potential geological applications.The main research content of the paper includes the following four aspects:(1)Theoretical framework for surface materials of cryptomare:Based on the radiative transfer equation,heat conduction equation,traditional lunar regolith-lunar rock two-layer model,and lunar dust-lunar regolith-lunar basalt three-layer model,a three-layer model consisting of plagioclase-rich lunar regolith,ilmenite-rich regolith,and lunar rock is constructed to represent the surface evolution process of cryptomare deposits.Subsequently,considering the research findings in the Balmer-Kapteyn region and the formation and evolution mechanisms of cryptomare deposits,one-layer infinite depth lunar regolith model is developed,treating lunar regolith as a mixture of highland debris and mare deposits.Numerical simulation results demonstrate the high sensitivity of microwave radiometer data to the proportion of mare deposits in the cryptomare region,indicating its potential for evaluating the compositional characteristics of surface materials in the region.(2)Discovery and definition of mare-like cryptomare deposits in the Balmer region based on microwave radiometer data:Through comparative analysis with normal regions’brightness temperatures,the presence of high iron oxide+titanium dioxide material is identified in the Balmer-Kapteyn region,exhibiting higher brightness temperatures during the daytime and lower brightness temperatures at night,consistent with the brightness temperature behavior of mare basalt deposits.Based on the formation and evolution processes of cryptomare deposits,this deposits are defined as mare-like cryptomare deposits.Utilizing existing visible and near-infrared remote sensing research as sample data,an adaptive extraction method for mare-like cryptomare deposits using principal component analysis and data fusion is established to validate the strong detection capability of microwave radiometer data for mare-like cryptomare deposits.The training samples are first optimized using the K-means algorithm,and then the random forest algorithm is employed to select the feature bands.Finally,the support vector machine method is utilized to construct an inversion model for mare-like cryptomare deposits.Based on the microwave radiometer data,information on mare-like cryptomare deposits is extracted.The results demonstrate the presence of mare-like cryptomare deposits in the Dewar,Lomonosov-Fleming,Schiller-Schickard,and Balmer-Kapteyn cryptomare regions.The distribution area of mare-like cryptomare deposits in the Dewar region is approximately 3101.1 km~2,59078.3 km~2 in the L-F region,173097.3 km~2 in the S-S region,and 38791.6 km~2 in the B-K region.These areas account for 64.9%,52.3%,76.4%,and 64%of their respective regions,indicating the widespread presence of mare-like cryptomare deposits in the cryptomare regions.It is important to note that the range of mare-like cryptomare deposits identified based on microwave radiometer data only represents a portion of the results obtained from visible extraction.Combining this information with the lunar impact history,it is likely that the varying proportions of highland debris and mare deposits,caused by differences in the covered basaltic titanium-iron content,have resulted in the formation of mare-like cryptomare deposits.(3)A new perspective on surface evolution in the cryptomare region is proposed,considering the presence of mare-like cryptomare deposits.Two hypotheses,namely the thickness hypothesis and the composition hypothesis,are put forward.Through comprehensive comparisons of brightness temperatures in the four cryptomare regions,it is determined that mare-like cryptomare deposits likely represents the compositional characteristics of underlying basaltic materials.Age estimation based on brightness temperature differences indicates that the age statistics in the region only represent the later surface modification age.These research findings are of great significance for further understanding the early magmatic evolution history of lunar maria.(4)New discoveries related to the influence of rock abundance:An improved radiative transfer model is developed to account for the impact of rock abundance.Numerical simulations are conducted to analyze the brightness temperature characteristics under the combined influence of rock and composition.Based on the brightness temperature behavior in the Lomonosov-Fleming region,an understanding of rock abundance anomalies is proposed.In the Balmer-Kapteyn region,considering the regional rock abundance anomalies and findings from visible research,the existence of potential hidden volcanic structures is suggested.An unknown and distinct material is found in the shallow lunar regolith of the Balmer-Kapteyn region,which exhibits low brightness temperature differences.This material is distributed in the northwest and southwest regions outside of the mare-like cryptomare deposits,as well as within the mare units of the Vendelinus impact crater.In summary,based on microwave radiometer data,the discovery of mare-like cryptomare deposits is made for the first time.An abnormality in rock abundance,distinct from previous findings,is identified,and the presence of an unknown material causing low brightness temperature differences is also observed.These research findings not only hold significant importance for deepening the understanding of surface material composition evolution of cryptomare but also contribute to further investigations into the thermal evolution history and magmatic evolution history of the Moon.Thus,these findings carry important scientific value. |
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