生命是否只存在于地球?

Does life exist only on the Earth?

虽然长期的努力尚未发现地球之外存在生命,但越来越多的证据表明,地球不可能是宇宙中唯一孕育生命的星球.在太阳系中,火星仍是发现地外生命最有希望的天体.火星探测和火星陨石研究都证明,它在早期历史上有过一个湿润的环境,其表面曾经有过河流、湖泊、甚至海洋,完全可以孕育和支撑生命活动.直至现在,火星的次表层等局部区域仍有地下水的活动.木卫二和其他外行星的卫星很可能存在冰下海洋,也是未来探测地外生命的重要目标.开普勒太空望远镜发现了大量的系外行星,其中一部分落在宜居带,并具有岩石表面,验证了宜居行星在宇宙中存在的普遍性.原始的球粒陨石、碳质小行星、彗星、星际尘埃等存在有大量复杂的有机分子,为构建生命体提供了关键的物质基础,是联系无机-有机-生命演化链条的重要环节.生命起源与地外生命的存在与否,不仅是最基本的自然科学问题,也是深空探测的重大科学目标.

Does life exist only on the Earth？ Are there extraterrestrial living things in the universe？ This is a long-standing question. Although no extraterrestrial living things have been found yet, the space explorations and study of meteorites do reveal the existence of habitable environments on other planets or their satellites today or in the past. In the solar system, Mars is a planet most favoring life other than the Earth. According to the numerous explorations and studies of Martian meteorites, Mars had a wet environment in its early history, with water flowing in rivers, lakes and probably oceans. Life could survive and/or emerge under such conditions, ln-situ analysis of water contents and H isotopes of Martian meteorites indicated the presence of underground water -180 Ma ago, via melting subsurface glacier by magma. Moreover, high special resolution topographic images revealed recent activities of underground water on Mars. Other important targets for future exploration missions are Europa and other satellites of the giant planets, they likely have oceans under the ice crusts. On the other hand, a large number of exoplanets have been found, especially by Kepler space telescope. A part of the exoplanets probably have rocky surface and locate within the habitability zones. These discoveries further confirm the common presence of habitable planets in the universe. Organic compounds are the building blocks of living things, and they have been found common in various primitive chondrites, carbonaceous asteroids, comets, and interstellar dust particles. The extraterrestrial organic matter is a bridge connecting inorganic matter and the origin of life. The extraterrestrial organic matter has experienced cosmic-ray and UV radiation, thermal and hydrothermal metamorphism in the solar nebula and/or in the parent bodies, and the evolution processes can be constrained by isotopic ratios of C and other elements. Organic compounds have also been found in Martian meteorites and detected in situ on Mars. However, their origins are still controversial, including adding by impact of carbon-rich asteroids or comets, Fisher-Tropsch-type reactions and bioenic activity. Organic carbon has been found in the recently fell Tissint Martian meteorite, with light C isotopes typical for biogenic origins. The origin of life and the presence of extraterrestrial living things are the basic questions of science. Exploration for extraterrestrial life has the top priority in the future missions. Besides studies of paleoclimate, search for living things or possible fossils on Mars or satellites of the giant planets is the ultimate goal.