火星乌托邦平原浅表层结构与物质组成:来自SHARAD雷达数据的约束

Shallow subsurface structure and material composition of Martian Utopia Planitia:Constraints from SHARAD data

乌托邦平原是火星北方大平原的重要组成部分,中国“祝融号”火星车着陆在乌托邦平原南部,研究乌托邦平原表层结构和物质组成对于深入认识火星北半球的地质演化历史具有重要意义.本文以乌托邦平原内中亚马逊纪低地单元(mAl)、亚马逊纪和西方纪火山单元(AHv)、晚西方纪低地单元(lHl)、早亚马逊纪盆地单元(eAb)、亚马逊纪火山单元(Av)以及亚马逊纪和西方纪撞击单元(AHi)6个主要地质单元为研究对象,使用火星轨道器激光高度计(MOLA)、浅表层雷达(SHARAD)和火星小型侦察成像光谱仪(CRISM)数据,计算各个地质单元的地形粗糙度,反演地层结构、介电常数与损耗角正切,并估算浅层物质组成、识别矿物类型,依此推演乌托邦平原的地质演化历史.结果表明,(1)AHv和Av地质单元表层物质是火山熔岩,其余地质单元表层物质属于北方荒原物质(VBF);(2)AHv和Av地质单元火山熔岩的厚度约为50 m,mAl,lHl,eAb和AHi地质单元VBF的厚度约为71-96 m,82-92 m,121-141 m,57-64 m;(3)在3 km×3 km的均方根高度计算窗口内,AHi地质单元地形最为粗糙,AHv和Av地质单元表面地形由火山作用主导,l Hl地质单元表面风化和沉积作用明显,mAl地质单元的粗糙度与lHl地质单元同纬度粗糙度特征相似,e Ab地质单元地形平坦,主要受沉积作用影响.区域地质作用是各地质单元次表层结构与表层物质形成的关键因素,火山、构造、撞击、沉积和风化作用对各地质单元具有不同程度的影响,本研究从多角度分析了各地质单元经历的地质过程,可以为“祝融号”火星车巡视区表面土壤成分以及来源认识提供背景信息,有望为研究乌托邦平原复杂的地质过程提供有效约束.

In May 2021,China’s Zhurong rover successfully landed in southern Utopia Planitia,an important region of the Martian northern lowlands.The surface structure and composition of Utopia Planitia are crucial for understanding the geological evolution history of the northern hemisphere of Mars.This study investigated the subsurface structure and chemical components of materials in six major geological units of Utopia Planitia,including the middle Amazonian lowland(mAl)unit,Amazonian and Hesperian volcanic(AHv)unit,late Hesperian lowland(lHl)unit,early Amazonian basin(eAb)unit,Amazonian volcanic(Av)unit,and Amazonian and Hesperian impact(AHi)unit.We calculated the surface roughness,obtained the stratigraphic structure,estimated the dielectric constant and loss tangent,and analyzed the composition of subsurface materials using the data obtained from the Mars Orbiter Laser Altimeter,Shallow Radar(SHARAD),and Compact Reconnaissance Imaging Spectrometer for Mars.The principal outcomes of the analyses are as follows:(1)The surface materials of the AHv and Av units comprise volcanic lava,whereas the other geological units are predominantly composed of the Vastitas Borealis Formation(VBF).(2)The thicknesses of volcanic lavas in the AHv and Av units are estimated to be approximately 50 m,whereas that of VBF in the mAl,lHl,eAb,and AHi units are 71-96,82-92,121-141,and 57-64 m,respectively.(3)According to the root mean square height results,the AHi unit in Utopia Planitia has the most rugged terrain and the surface terrain of the AHv and Av units is predominantly influenced by volcanism.Moreover,the lHl unit exhibits evidence of weathering and sedimentation processes.The roughness of the mAl unit is comparable to that of the lHl geological unit at a similar latitude,while the eAb unit features a relatively flat topography,primarily due to sedimentation processes.The geological units of Utopia Planitia were influenced by various regional geological processes,such as volcanic activity,tectonic movement,impact events,sedimentation,and weathering.This study comprehensively analyzed the geological processes experienced by each geological unit from multiple perspectives.The present results provide valuable information for understanding the composition and origin of the surface materials in the exploration region of the Zhurong rover and useful constraints for investigating the complex geological processes that have occurred in Utopia Planitia.