Outline of astrobiology

Abstract

Are we alone? Is our Earth, the tiny blue planet in this infinite Universe, the only harbor for life? Astrobiology concerns the study of the origin, evolution and destiny of life in the Universe and touches almost all the fundamental questions asked by mankind. The emergence of this science is the natural outcome of the great advances in astronomy, biology, and geology. The development of molecular biology has revealed the unitary origin and DNA-coding of all life on Earth in despite of the great diversity after a long Darwinian evolution. Geologists dated the origin of life back to almost 3.8 billion years ago. Planetary explorations in our solar system have indicated abundant life materials being stored in the planetesimal zone and beyond. Furthermore, the detection of exoplanets has practically extended life exploration into the deep Universe. The astrobiological studies can be classified into cosmic, planetary, ecosystematic scales on a space dimension and stellar and interstellar evolution, chemical evolution of prelife on Earth, and modern ecosystem on a time dimension. Stellar nucleosynthesis and the evolution of galactic chemistry tell us about the formation of life-essential elements, such as H-C-O-N-S-P and Fe-peaked transition metals. The breakthrough in exoplanet detection is a great step in searching a second life and scientists began to develop techniques to examine the atmosphere composition of Earth-like exoplanets. Based on our understanding of life origin on Earth, a body of liquid water, enough carbon and a silicate crust on a planet with a right distance to its sun are sufficient premises to make a planet habitable. When those conditions are met, life might start in hundred million years or even shorter time period. It is understood now that life deeply changes the surface spheres of Earth and our current atmosphere is pretty much the result of biological respiration. It is also wondered that the great events happened on Earth, such as the appearance of free oxygen, the starting of plate tectonics, the appearance of ice ages, planet -wide formation of banded iron may have either stimulated the evolution of life, or on the contrary, resulted in mass extinction. Now mankind is on a fast lane going forward but has a quite unpredictable future. How long can our Gaia last? Can our offspring find another oasis in the Universe? Astrobiology can not answer these questions now but they remain for exploration. 天体生物学研究生命在宇宙中的起源、演化、分布和未来。天体生物学在20 世纪90 年代初正式形成，是人类对地球上生命起源、演化的追问，对人类文明未来的思考，对环境变迁的忧虑，以及对空间和宇宙探索的结果。天体生物学整合了天文学、行星科学、地球科学、生命科学和空间探索技术等领域，并且一出现就成为这些学科的前沿。在天文学的时间和空间尺度上，天体生物学探讨对 生命至关重要的非金属元素氢-碳-氮-氧-磷-硫，以及金属元素例如镁、铁等在宇宙创生、星系和恒星演化过程的核合成和分布，以及这些过程对宜居住行星在恒星系中的密度和分布的制约。在行星系统尺度上，天体生物学比较太阳系各行星的物理、化学和地质特征，并试图据此建立太阳系和其他行星系统的宜居住带的模型。地球上生态系统的起源和演化是天体生物学最重要的研究内容。 地球从一个炙热的无生命世界逐渐演变成一个适宜生命产生和演化的宜居住行星，是了解宇宙中生命的产生和演化的唯一例子。天体生物学比较现在地球上的极端环境、极端环境中的生命，并结合对地球历史上地质和生物事件的重建，试图建立宇宙中生命在类似地球的行星上起源和演化的普适模型。寻找地球以外的生命世界是人类空间探索的终极使命。天体生物学根据对地球上生命的认 识，确定了液态水、水-岩石相互作用化学是微生物生命存在的基本条件和证据，并据此确定了最近行星探索技术的发展方向。