太阳系生命信息探测

A review of the search for life in our Solar System

寻找地外生命是人类开展太阳系探测的根本出发点。火星是太阳系中最可能存在生命的天体之一。尽管海盗号开展的生物科学实验没有获得存在生命的确凿证据,但是近二十年的探测利用高分辨成像、光谱、质谱、雷达、中子分析等多方面的探测手段,获得了河流侵蚀地貌、古湖泊河流沉积物、水成矿物、极地冰盖、大气中水蒸气组分等一系列反映火星上有水的证据。这些发现暗示了火星过去或现在存在适宜生命繁衍的环境特征。大气甲烷、火星样品中有机物是下一步火星生命探测的重点。木星和土星的天然卫星,包括木卫二、土卫二和土卫六等在地外生命的研究中引起极大关注。木卫二上的盐水海洋环境被认为可能孕育生命,海洋中可能存在类似地球上的水热地质作用。土卫二南极存在细粒冰晶的喷射,且喷射物含有对于生命非常关键的液态水、氨气和其他碳氢化合物等。碳质球粒陨石存在大量可溶性有机物,其中包括多种构成蛋白质的氨基酸。星尘号和深度撞击任务的探测结果证实彗星所代表原始物质中含有大量的有机化合物,部分类型的氨基酸在碳质陨石和星际尘埃IDP颗粒相同,进一步支持了彗星与地球生命的起源之间存在联系。文中分析了太阳系生命探测中亟须解决的关键问题,包括系统建立地外生物标志化合物、发展适合太阳系探测的有机物分析技术、开展地球极端生命研究。最后呼吁我国在未来火星等天体探测任务中能增加有机物探测的相关载荷,及时介入太阳系生命探测,开拓深空领域。

Searching for extraterrestrial life addresses a fundamental problem in solar system exploration. In our Solar System, the celestial bodies of interest for searching for life are Mars, Europa, Enceladus and, non- water liquid, Titan. To detect life on Mars, Viking landers carried Gas Chromatograph-Mass Spectrometer （GC/MS） and conducted three biology experiments aiming to search for metabolic activity in Martian samples. However, no organic molecules were found at the parts per billion （ppb） and the biology experiments results suggested no life present in the Martian soil. During the past decade, the systematic investigations, like high resolution camera, VIS-NIR spectrometer, thermal infrared spectromenter, APXS, GC/MS and radar, have produced a detailed knowledge of Mars＇ habitable environments, including river valleys and channel, lakes and deltas, aqueous minerals, polar ice caps, and water vapor in the atmosphere. Methane in the Martian atmosphere implies the existence of the mechanism（s） of production and/or release of methane on Mars. Future missions on Mars will focus on organic compound in Martian soil. Europa is a priority of exploration target for extraterrestrial life. The main reason is the presence of a water ocean beneath the icy crust and possible biosignatures in it. For Enceladus, Cassini data have revealed about a dozen or so jets of fine icy particles emerging from the south polar region. The vapor contained simple organic compounds, which may be of biological origin and also indicated potential habitable environments on Enceladus. Carbonaceous meteorites contained a wide range of soluble compounds such as amino acids, which is considered as an important source of prebiotic compounds required for the emergence of life on Earth. The results from Stardust and Deep Impact have identified a suite of primitive organic matter in comets. Some of the amino acids and amines were similar with those in carbonaceous chondrites and interplanetary dust particles （IDPs）. We suggest systematically building the biomarkers to guide the search for extraterrestrial life in future, developing the technology and instruments suitable for organic detection, studying the biology of extremophiles and expanding the diversity of life. As a large nation in the world, China should carry out searching for life in future missions, which could promote our space exploration capability and exploit the new space frontier.