金星:认识早期地球的窗口

Venus:A planetary window into early Earth

无论在行星大小、质量还是轨道速度等方面,金星都是太阳系中与地球最相似的行星.自1960年代初期开始,金星一直是人类深空探测的重要目标.本文简要地回顾了人类探索金星的历史,总结了对金星已有的认识,梳理了金星的主要科学问题,最后介绍了未来的国际探测计划,并建议了我国的金星探测目标.早期对金星的探测以苏联的金星计划(Венера)和美国的水手系列(Mariner)为代表,后期的探测器以欧盟、日本等国家的“金星快车(Venus Express)”、“拂晓号(Akatsuki)”为代表.这些探测结果为我们认识金星大气成分、地表地形和内部结构提供了重要的数据.金星的大气组成以CO_(2)为主,含少量N2,与现在地球的大气组成显著不同,类似早期地球的大气组成.虽然金星地表目前没有液态水,但部分理论模拟工作表明金星地表可能曾经有液态水.一系列探测器对金星地表成分的分析表明,金星地表主要由玄武岩组成.在地形地貌方面,由于金星特殊的地表环境,金星表面风化作用对地表地貌影响很小.金星的地表主要受控于比较年轻的火山作用,发育了许多不同于地球的地貌特征,主要包括区域平原、盾状火山平原、冕状地形以及瓦片状地形等,其动力学机制可能是地幔柱—岩石圈相互作用或地幔对流,至今未发现与板块构造相关的地貌.现阶段金星没有太多大型的、活跃的火山热点,虽然无法估测准确的火山活动速率,但相比地球来说火山活动速率小很多.在内部结构方面,金星具有与地球类似的核幔壳结构.金星的内部组成也与地球类似,例如金星地幔很可能是与地球相似的橄榄岩成分.不存在内部磁场和缺乏板块构造是金星区别于地球的两个重要特征.关于金星为什么没有自身磁场,主流观点是金星地核缺乏对流,无法演化出磁场.而针对金星为什么没有演化出板块构造,目前认为主要有三个可能的原因:地表温度过高,没有软流圈,金星缺乏液态水,其中液态水的缺乏接受度最广.从大气组成、地表岩石组合、构造作用等角度来看,金星都与早期地球非常相似,是我们理解类地行星演化的天然实验室.研究金星和地球为什么会朝不同方向演化,是深入理解包括系外行星在内的行星的宜居性形成与演变的重要途径。因此,金星一直是优先级别最高的深空探测目标之一.近几年,美国、俄罗斯以及欧洲等国家和地区分别针对金星目前主要的科学问题,例如金星是否存在早期海洋、金星的宜居性以及结构和重力场等,先后提出各自的金星探测计划.我国在新的国际竞争中应该、也必然有所作为.

Venus is the most similar planet to the Earth in the solar system in terms of planet size,mass and or-bital velocity.Since the early 1960s,Venus has been an important target for human deep space exploration.In this contribution,the exploration history of Venus,the known and unknown of Venus,the main scientific problems of Venus,and the future international exploration plan of Venus are briefly reviewed.The early exploration of Venus is represented by the"Венераplan"of the Soviet Union and the"Mariner series"of the United States,whereas more recent missions are represented by the"Venus Express"and"Akatsuki"launched for other countries.These missions provide valuable data which significantly enhanced our understanding of Venus atmosphere composition,surface topography and internal structure.The atmosphere of Venus is dominated by CO2 and N2,which is diffe-rent from the atmospheric composition of the present Earth,similar to that of the early Earth.Some modeling re-sults suggest that Venus was once covered by ocean although no liquid water exists any longer on Venus.The ana-lyses of the surface compositions of Venus by a series of explorers show that Venus surface is mainly composed of basalt.In terms of topography,due to the special surface environment of Venus,the surface weathering of Venus is very weak and does not affect surface topography.The topographies of Venus are mainly controlled by volcanic re-surfacing and differ with the Earth significantly.The dominant topographies include regional plains,shield plains,corona and tessera,which are produced by mantle plume-lithosphere interaction or mantle convection.No topo-graphy related to plate tectonics has been found on Venus.At present,Venus lacks many large and active volcanic hotspots.Although it is difficult to estimate the accurate volcanism rate,the volcanism rate is much smaller than that of the current earth.In terms of internal structure,Venus has a core-mantle-crust structure similar to that of Earth,but there is no internal magnetic field on Venus.In addition,Venus and Earth have similar densities and have similar compositions,for example,the mantle of Venus is likely to be composed of peridotite which is similar to that of Earth.No internal magnetic field and lack of plate tectonics are two important characteristics of Venus.The most popular explanation for the absence of magnetic field on Venus is that Venus lacks convection in its core and cannot develop its own dynamo.Three explanations were proposed to account for the absence of plate tectonics on Venus,the extremely hot surface,the lack of asthenosphere,and the lack of water,among which the last one is the most widely accepted.Considering many aspects including atmosphere composition,surface rock assemblages and tectonics,Venus is very similar to the early Earth.Venus provides the best natural laboratory for us to understand the evolution of the Earth and other terrestrial planets.Understanding why Venus and Earth evolve into different directions is one of the most important means to study the genesis and evolution of planet habitability and has signi-ficant bearings for studying the potential habitability of exoplanets.Venus has been one of the highest priority of deep space exploration targets.In recent years,the United States,Russia,Europe and other countries have put for-ward their own Venus exploration plans for above scientific problems,such as whether there was a sea on the early Venus,the unhabitability of Venus,and the structure and gravity field.China should and must do something in the new international competition.