地外有机化合物

球粒陨石中的有机化合物起源于星际介质，是构成太阳星云的初始组分，并与其他物质一起吸积形成小行星和行星。在小行星内，有机质经历了不同程度的水蚀变和热变质作用。球粒陨石中的有机化合物尽管是非生命成因，但组成极为复杂，主要是类似于干酪根的大分子物质，以及少量可溶性有机物。大部分可溶有机分子也发现于地球生物圈，但前者可具有完全不同的H、C、N等同位素组成，这也是它们来源于地球之外的重要证据。星云中宇宙线和紫外线（UV）的辐射、小行星的热变质和水蚀变，是地外有机质演化的主要过程。球粒陨石中的有机质是地球生命起源的物质基础，是生命起源不可或缺的重要环节。同样重要的是，大量的火星探测表明，火星历史上有过满足生命存在的基本条件，而在火星陨石中还发现了一些生物活动相关的线索。未来很可能首先在火星上发现地外生命存在的证据。

Extraterrestrial organic compounds

The organic compounds in chondrites were originated from the interstellar media, and they were precursor components of the solar nebula. The organic compounds incorporated with other material into asteroids and planets by accretion processes. In the parent bodies of meteorites, organic compounds underwent various degrees of aqueous alteration and thermal metamorphism. Regardless of their abiogenic origin, extraterrestrial organic matter is very complex, consisting mainly of kerogen like insoluble macromolecular materials with less abundant soluble organic material (SOM). Most of the SOM have identical counterparts in the terrestrial biosphere, but the former has distinct isotopic compositions of C, H and N, the critical evidence for their extraterrestrial origins. The irradiation of cosmic rays and UV light in the nebula and the secondary processes (e.g., aqueous alteration and thermal metamorphism) in meteorite parent bodies are the main contributions to the evolution of the organic matter. The extraterrestrial organic compounds have probably played a critical role in the origins of life. Meanwhile, Mars exploration has showed that at least the paleoclimate on Mars was favorable for the existence of microbes. In addition, several hints for past biotic processes were found in Martian meteorites. The first evidence of extraterrestrial life will likely be found on Mars in the future.