There has been considerable attention on how to detect life on other worlds by searching for biomolecules. However, there has been much less clarity as to when it becomes warranted to focus a mission on the search for life on another world. At a minimum, a life-detection mission should follow convincing evidence of (1) Liquid water of suitable salinity, past or present; (2) Carbon in the water; (3) Biologically available N in the water; (4) Biologically useful energy in the water; (5) Organic material that can possibly be of biological origin and a plausible strategy for sampling this material. Based on these prerequisites, the most promising targets for a life search are currently the plume of Enceladus and the subsurface of Mars-in equatorial lake bed sediments and in polar ice-cemented ground. Neither the surface of Europa nor the clouds of Venus meet the criteria listed here but may with further exploration.

中文翻译：

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什么是生命，以及我们何时在其他世界寻找生命。

如何寻找生物分子来检测其他世界的生命已引起人们的极大关注。但是，关于何时应将任务集中在寻找另一个世界的生命上的问题，目前还没有那么清楚。至少，生命探测任务应遵循令人信服的证据：（1）过去或现在的适当盐度的液态水；（2）水中碳；（3）水中的生物可用氮；（4）水中的生物有用能量；（5）可能是生物来源的有机材料，以及对该材料进行采样的可行策略。基于这些先决条件，目前寻找生命的最有希望的目标是土卫二的羽状流和赤道湖底沉积物和极地冰水泥地面中火星的地下。