国际火星探测科学目标演变与未来展望

The evolution of scientific goals for Mars exploration and future prospects

火星探测是当前太阳系探测和行星科学的焦点.经过近60年的发展,火星成为除地球外,探测和研究程度最高的太阳系行星体,派生出火星空间环境、火星大气、火星表层/次表层物质组成、形貌构造、撞击历史、冰川和冰冻层、气候变化、火星内部结构等多个研究领域.火星陨石研究和实验室模拟研究(实验模拟、数值模拟等)也得以快速发展.火星的重大科学发现包含早期和现代的水活动证据、地质环境多样性、现代地质过程监测、甲烷和有机物的发现、大气组成和演化、当前和近期气候变化、重力场和表面辐射环境等.重大科学成果的取得得益于科学目标规划的指引,也影响着未来科学目标的制订.通过梳理美国火星探测项目分析组(Mars Exploration Program Analysis Group,MEPAG)近20年火星探测科学目标(生命、气候、地质、载人)的演变,展现出国际火星探测思路及未来探测重点.未来10年的火星探测将进一步认知火星内部结构、开启火星生命探测的新阶段和开展火星和火星卫星样品返回.中国开展的火星探测任务也将为国际火星科学发展做出贡献.当前火星仍有诸多重大科学问题未有解答,这些问题与太阳系的重大科学问题紧密融合,突显出火星探测在太阳系形成演化以及太阳系行星宜居性的形成演化研究中不可替代的重要地位.

Mars is currently in the spotlight of solar system exploration and planetary science study.The scientific questions of Mars are closely integrated with big enigmas of the solar system,highlighting the centrality of Mars in understanding the formation and evolution of our solar system.After nearly six decades of exploration,Mars is easily the most studied planet in the solar system besides Earth,with a profusion of research across space environment,atmosphere,surface-subsurface compositions,topography and structure,impact history,glaciers and cryosphere,climate change,and internal structure.Martian meteorite and laboratory simulation studies(experimental and numerical)are also developing rapidly.Key discoveries in the past 20 years include evidence of past and current aqueous activity,geological environment diversity,modern geological processes,methane emissions and preserved organics,atmospheric composition and evolution,current and recent climate change,gravity fields and surface radiation environments,etc.Such scientific achievements,underpinned by peer-reviewed research goals developed by the planetary community,in turn shape future goals and targets.We review how the Mars exploration goals and targets(e.g.,life,climate,geology,preparation for human exploration)have evolved in the past 20 years,and show priorities and focii for future international Mars exploration.For example,the Mars exploration strategy evolved thematically from"follow the water"to"understand Mars as a system","understand the longterm evolution of habitability on Mars",and"exploration by humans on Mars".In the next 10-20 years,Mars exploration will further characterize the internal structure of Mars,start a new era of life detection,and return samples from Mars and its satellites.China’s first Mars mission will contribute to the international Mars science community with an orbiter and a rover exploring the northern lowlands with advanced payloads.New findings such as the structure of the Martian critical zone(i.e.,vertical zone of interaction between the atmosphere and crust with immediate relevance for habitability),local and global characteristics of the residual crustal magnetism,volcanic-geothermal evolution and the cyclicity of glaciations will clarify the still poorly known provenance and evolution of the topographic dichotomy.Hypotheses such as the formation of the global dichotomy by an oblique impact or an ancient ocean-sustaining climate may be tested and constrained.Internationally planned sample-return from Mars and its satellites,combined with study of Martian meteorites,experimental and numerical simulation of Martian processes,and study of terrestrial analog sites and samples will build absolute geochronology of Mars and constrain the timing and duration of critical events,such as cessation of the global magnetic field,quiescence of volcanism across the Noachian-Hesperian temporal boundary,transition to a single-plate planet,aqueous and sedimentary processes,and global climate change.