An updated constraint on the local stratigraphy at the Chang'E-4 landing site

The Chang’E-4 mission has been exploring the lunar farside.Two scientific targets of the rover onboard are(1)resolving the possible mineralogy related to the South Pole-Aitken basin and(2)understanding the subsurface processes at the lunar farside.Publications to date that are based on the reflectance spectra and radar data obtained by the rover have shown a persistent inconsistency about the local stratigraphy.To explain both the abnormal surface topography at the landing site and the unexpected radargram observed by the rover,the Alder crater has been frequently reported to be older than the mare basalts at that landing site.However,this argument is not supported by earlier geological mapping nor recent crater statistics.Resolving this controversy is critical for a full understanding of the geological history of the landing area and for correct interpretations of the scientific data returned.Employing detailed crater statistics,rigorous statistical analyses,and an updated crater chronology function,this study is determined to resolve the relative ages of the Alder crater,Finsen crater,and the mare basalts on the floor of Von Kármán.Our results reveal that while background secondaries and local resurfacing have widely occurred in the study area,affecting age determinations,the statistics are significant enough to conclude that the Alder crater is the oldest among the three targets.This independent constraint is consistent with both the crosscutting relationships of different terrains in this area and global stratigraphic mapping.Our results exclude Alder as a possible contributor of the post-mare deposits at the landing site,appealing for a more systematic stratigraphy study to resolve the provenances of these deposits.

POLARIZED SPECKLE-PHOTOELASTICITY FOR 3-D STRESS ANALYSIS

Combining speckle interferometry with photoelasticity,the absolute retardation fringes in 3-D model are obtained by interference of the polarized rays reflected from the two surfaces of a sandwiched plate inside the model.This method can avoid the troubles introduced by Poisson's ratio μ=0.5 and stress-optical constants ratio A/B=-2 of the photoelastic material in a stress-frozenstate,and yield the re- sults which provide necessary supplementary information for the completely-experimental solution of 3-D stress state.A comparison of experimental results with theoretical ones shoves the feasibility of the method proposed in the present paper.

Geological Characteristics and Model Ages of Marius Hills on the Moon

Marius Hills is a volcanic plateau on the nearside of the Moon. It is of great interest for its high concentration of volcanic features, including domes, cones, ridges, and rilles. However, the morphological and chronological characteristics of this plateau were not well studied due to the low resolution of early mission data. This study describes the detailed morphology of the volcanic features using the latest high spatial resolution images of the Terrain Camera （TC） onboard Selene-1 （10 m/pix） and Narrow Angle Camera （NAC） onboard the Lunar Reconnaissance Orbiter （LRO） （0.5 m/pix）. We report here some new structures such as skylights and remnants of lava tubes. We have divided spectrally homogenous areas with Clementine UVVIS data and did crater size frequency distribution （CSFD） measurements with Lunar Orbiter （LO） IV and TC images in every spectral unit. We first report absolute model ages of 1.10 Ga for Marius basalt 1, 1.49 Ga for Fiamsteed basalt, and 1.46 Ga for Schiaparelli Basalt. In addition, we have identified several younger lava events： they are Marius basalt 2 （814 Ma）, medium to low titanium basalt （949 Ma）, and undifferentiated medium titanium basalt （687 Ma）. Finally, we propose a mantle plume scenario for the formation of Marius Hills, which could solve the inconsistency of previous models.

Evaluation of Different Predictor Models for Detailed Soil Particle-Size Distribution

An accurate mathematical representation of soil particle-size distribution(PSD) is required to estimate soil hydraulic properties or to compare texture measurements using different classification systems. However, many databases do not contain full PSD data,but instead contain only the clay, silt, and sand mass fractions. The objective of this study was to evaluate the abilities of four PSD models(the Skaggs model, the Fooladmand model, the modified Gray model GM(1,1), and the Fredlund model) to predict detailed PSD using limited soil textural data and to determine the effects of soil texture on the performance of the individual PSD model.The mean absolute error(MAE) and root mean square error(RMSE) were used to measure the goodness-of-fit of the models, and the Akaike's information criterion(AIC) was used to compare the quality of model fits. The performance of all PSD models except the GM(1,1) improved with increasing clay content in soils. This result showed that the GM(1,1) was less dependent on soil texture.The Fredlund model was the best for describing the PSDs of all soil textures except in the sand textural class. However, the GM(1,1) showed better performance as the sand content increased. These results indicated that the Fredlund model showed the best performance and the least values of all evaluation criteria, and can be used using limited soil textural data for detailed PSD.