Geologic Characteristics Of The Paleolakes In Martian Southern Highland: Implications For Martian Paleo-climate And Paleo-environment

Water plays an important role in the martian geologic evolution and is important in shaping the surface of Mars.A great number of surface features that could be related to water have been recognized,such as valley networks,outflow channels,gullies,lakes and so on.Among them,paleolake is one of the hottest research fields as it contains information on the history of martian geology and environment change.The geomorphology of paleolakes is an indicator of surface water activities.The mineralogy of paleolakes can reveal the paleo-climate and paleo-environment.The age and distribution of paleolakes indicate the span and distribution of surface water activities on Mars.In addition,martian paleolakes are also places of high possibility to preserve the traces of life,and also places of high values in the searching of inhabiting environments.Therefore,paleolakes on Mars have been always selected as candidate landing sites of landing and rover missions and studies on paleolakes are of great significance in understanding the martian paleo-climate and paleo-environment,and in the searching of lives on Mars.Previous studies have identified more than 400 paleolakes on the martian surface.However,their results are not completed owing to the limitation of the data resolution.Therefore,the distribution of the martian paleolakes and their indications on the paleo-climate and paleo-environment may not be reliable.Besides,the study on the sedimentary landforms of the paleolakes and the types,distributions,origins,and the post-formation modifications are not sufficient,and the ages of the modifications are not well-constrained.Therefore,to shed light on the above problems,we carried out our research in the following aspects:1.Identification and geomorphological characteristics of the paleolakesWe performed detailed identification and investigation of paleolakes on martian southern highland,and updated the database of martian paleolakes.On this basis,we analyzed the geomorphology of the paleolakes and discussed the characteristics and origins of the landforms in paleolakes,such as deltas,layer deposits and sinuous ridges.In addition,we used high-resolution topographic data to acquire the geomorphic parameters of the paleolakes,such as diameter and surface area.2.Mineral types of the paleolakesWe analyzed the mineral compositions of selected areas based on spectral data.In this process,we paid attention to aqueous minerals such as chlorides,sulfates,carbonates and phyllosilicates.We analyzed the characteristics and distributions of mineral assemblages in the paleolakes.3.Post-formation modifications of the paleolakesWe analyzed the types of post-formation modifications of the paleolakes and identified the landforms related to modification processes such as impact cratering,glacial activity,volcanism,wind action,etc.We also analyzed the geologic characteristics and distributions of the post-formation modifications.4.Chronology of the paleolakesTwo aspects were emphasized in the chronological studies of the paleolakes: ages of original lakes and ages of post-formation modification events.To obtain the ages of the original lakes,we performed crater counting on the lake deposits,and to obtain the ages of modification events,we performed crater counting on the units that have undergone resurfacing events.5.Distribution of paleolakes and the significance to paleo-climate and paleo-environmentWe produced maps and discussed the distributions,ages,and sedimentary landforms of the paleolakes.We also studied the geomorphology and mineralogy of the paleolakes in selected areas to reveal the characteristics of paleo-climate and paleo-environment.On the basis of the above studies,we obtained the following conclusions:1.Identification and geologic characteristics of the paleolakes in the southern plateau of MarsIn this study,a total of 922 paleolakes were identified on the surface of Mars,of which 546 were newly discovered.On this basis,we studied the distribution and origin of lake deposits and post-formation modification landforms.By performing crater-size distribution measurements on deltas,we found the lakes were mainly active in Noachian and drained during the period from Noachian-Heperian boundary(around 3.7 Ga)to upper and lower Hesperian boundary(around 3.4 Ga).This indicated that significant climate change happens in Hesperian.Besides,dating results and geomorphologic studies also revealed that in Amazonian,there were still local and temporary surface water activities that led to the formation of lakes.These lakes may be formed by water from ice melting induced by impact or volcanic events.2.Geologic characteristics and origin of the sinuous ridges in the Tharsis plateauIn the identification and analysis of the paleolakes,we found several sinuous ridges in the paleolakes.They are generally considered to be related to water or glaciers,which have origins of inverted channel or esker.However,by analyzing the sinuous ridges in the Tharsis plateau,we found that sinuous ridges can be also formed in volcanic activities.We identified 38 sinuous ridges on southeast Tharsis plateau for the first time and proposed that they have a lava-tube origin.These lava tubes are important in the formation of Tharsis volcanic province and play key roles in long-distance transportation of lava.3.Geologic characteristics and indications of the paleolakes in northwest HellasThe Hellas basin is one of the largest and most ancient basins on the southern plateau of the Mars.Paleolakes around the basin are recorders of the complex geologic and climatic evolutionary history of the region.In this paper,we selected the northwestern part of the Hellas region as our research area,and studied the distributions,geomorphologic features,mineral compositions,post-formation modifications and the ages of the paleolakes in detail.The results showed a high concentration of phyllosilicates in the study area,which indicates a warm and wet climate condition.Besides,we also found carbonates and chlorides that may be formed by evaporation.Overall,evaporite minerals are rare in this region,which may be caused by fast climate change and/or short active period of the paleolakes.In addition,the study of the distribution and geomorphologic characteristics of the paleolakes showed that the area below the contour line of 0 m in northern Hellas may have been underwater,supporting the existence of a "Hellas Ocean" whose sea level can reach 0 m.Age determinations of the paleolakes showed that this region has undergone a complicated geologic history.First of all,climate change happened around Noachian-Hesperian boundary,resulting in the draining of paleolakes around 3.65 Ga.Subsequently,in the early Hesperian,volcanic activities led to the resurfacing of the basin floor of the paleolakes.At about 1 Ga ago,glacial activities led to the formation of glacial landforms in the paleolakes south of 25° S in this region,which indicates a cold and dry climate condition.In summary,this study made a detailed study on the geologic characteristics of the paleolakes on the martian southern highland,which has enriched the current understanding of the temporal and spatial distribution of the paleolakes and is of great significance in understanding the geologic evolution and climate change of the Mars.With the implementation of the Mars exploration mission in China,this study will provide supports for the determination of scientific goals and the selection of landing sites.