Quantitative Chemical Compositons And Pedogenesis Of Martian Soils By LIBS Studies

As the topmost layer of loose material on the Surface of Mars,the Martian soil is the critical zone between lithosphere,atmosphere and hydrosphere.Its chemical composition records the interaction between soil,atmosphere and hydrosphere over time.The particle size,color,and chemical composition of soils on the surface of Mars have been studied by remote sensing and in situ.The ChemCam laser induced breakdown spectrometer(LIBS)carried by the Curiosity rover was the first LIBS payload to be used for the exploration of extraterrestrial planet,and subsequent Mars2020 and Tianwen 1 missions also used LIBS technology.LIBS has the characteristics of fast detection speed,no sample preparation,multiple laser ablation at the same position for profile study,and small laser spot(～300-500 μm),which can be used for small area analyses.Based on the above advantages,LIBS technology is a powerful tool for the study of Martian soils.Quantitative analysis the chemical composition of complex samples is a difficult issue faced by LIBS.In this thesis,Elastic Net model is established for quantitative analyses the major elements of Martian soils.Elastic Net model is a sparse regression model with the sparsity of Lasso model and the stability of ridge regression.The ChemCam calibration targets is used to train Elastic Net model and optimize model parameters.The prediction results of the 8 major elements(Si,Ti,Al,Fe,Mg,Ca,Na,K)of the test set show that the root mean square error of predictions(RMSEP)of Elastic Net model is smaller than that of commom linear regrassion models.The accuracy of the model is tested with the ChemCam calibration targets,and the cross validation of the quantitative results of alpha particle X-ray spectrometer(APXS)and LIBS is carried out on the target with uniform surface.The validation results show that the Elastic Net model has high accuracy and can be used for quantitative analysis the chemical composition of geological targets on the Martian surface.The results of 23,805 LIBS detection points in Gale crater by the quantitative model established in this thesis are consistent with those by the ChemCam team,and the two predictions have a good correlation.Based on LIBS data,the Martian soils of Gale crater,Jezero crater and the southern part of Utopian Plain were analyzed.The Martian soils of Gale crater have been classified according to particle size:fine soil,coarse soil,fine soil and pebble mixture,and pebble.The coarse grained soils are richer in Si,Al,Na and K,while the fine grained soils are richer in Mg and Ca.It is found that the chemical composition of the first～2 laser ablation materials in the four types of soil is consistent,and these surface materials represents the dust of Mars.The chemical composition of pebble ablative by～6 laser waves changes obviously,and there is weathering crust on pebble surface compared with fine soil surface.The soils of the other two landing zones showed no significant difference from that of gale crater.The chemical composition of the Martian soils at Gale crater,Jezero crater and other landing zones is basically the same.The chemical composition of most Martian soils is close to the average chemical composition of the Martian crust.A few Martian soils have limited chemical alteration,with a chemical alteration index(CIA)of more than 65%.The results of principal component analysis(PCA)on LIBS data of Tianwen 1 landing site show that the soil composition in this area falls between basalt and andesite,which is consistent with the results of remote sensing analysis.The origin of Martian soil is complex and the evolution time is long,which mainly includes the following processes:soil formation by impact and physical weathering.Limited chemical alteration with the participation of water,acid,etc.Regional mixing of felsic endmember,mafic endmember,Mars dust and local addition of rock alteration materials.