No abstract

No abstract

Exploring the subsurface structure and stratification of Mars advances our understanding of Martian geology, hydrological evolution and palaeoclimatic changes, and has been a main task for past and continuing Mars exploration missions1–10. Utopia Planitia, the smooth plains of volcanic and sedimentary strata that infilled the Utopia impact crater, has been a prime target for such exploration as it is inferred to have hosted an ancient ocean on Mars11–13. However, 45 years have passed since Viking-2 provided ground-based detection results. Here we report an in situ ground-penetrating radar survey of Martian subsurface structure in a southern marginal area of Utopia Planitia conducted by the Zhurong rover of the Tianwen-1 mission. A detailed subsurface image profile is constructed along the roughly 1,171 m traverse of the rover, showing an approximately 70-m-thick, multi-layered structure below a less than 10-m-thick regolith. Although alternative models deserve further scrutiny, the new radar image suggests the occurrence of episodic hydraulic flooding sedimentation that is interpreted to represent the basin infilling of Utopia Planitia during the Late Hesperian to Amazonian. While no direct evidence for the existence of liquid water was found within the radar detection depth range, we cannot rule out the presence of saline ice in the subsurface of the landing area.

On the basis of salinity and organic geochemical analyses of mudstones, this paper analyzed the relationship between salinity and sedimentary facies of different strata; studied influences of salinity on organic matter richness, types, and thermal maturities of source rocks; and built the organic enrichment accumulation model for the Paleogene and Neogene saline lakes of the western Qaidam Basin. In the western Qaidam Basin, saline lake depocenters and high salinity areas migrated eastward, and chloride ion (Cl − ) concentration of source rocks showed an increasing trend from the Paleocene to Pliocene. The relationship between organic matter content and salinity of the Paleocene and Pliocene source rocks shows a three-interval model: both TOC (total organic carbon) content and chloroform bitumen content have low values at the low salinity stage, first increase to the peak and then decrease to the lowest values at the medium salinity stage, and sustain the lowest values at the super saline stage because of the interaction between organic productivity and reduction−oxidation in the saline lake of the western Qaidam Basin. The relationship between organic matter type and salinity of these source rocks suggests that the kerogen increasingly corresponds to a sapropelic and planktonic origin with increasing salinity. The organic matter enrichment model of the Paleogene and Neogene saline lakes of the western Qaidam Basin showed that saline lakes had high productivity and reducing conditions favorable for organic matter accumulation and preservation at their middle evolution stage. Through improving their apparent activation energy, salinity slowed down thermal evolution of source rocks, making the Paleogene and Neogene source rocks at low maturity to the immature stage in the western Qaidam Basin.