Russell Michael J, Ponce Adrian
Dipartimento di Chimica, Università degli Studi di Torino, via P. Giuria 7, 10125 Turin, Italy.
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
Life (Basel). 2020 Nov 19;10(11):291. doi: 10.3390/life10110291.
Life cannot emerge on a planet or moon without the appropriate electrochemical disequilibria and the minerals that mediate energy-dissipative processes. Here, it is argued that four minerals, olivine ([Mg>Fe]SiO), bridgmanite ([Mg,Fe]SiO), serpentine ([Mg,Fe,]SiO[OH)]), and pyrrhotite (FeS), are an essential requirement in planetary bodies to produce such disequilibria and, thereby, life. Yet only two minerals, fougerite ([FeFeOH]·[(CO)·3HO]) and mackinawite (Fe[Ni]S), are vital-comprising precipitate membranes-as initial "free energy" conductors and converters of such disequilibria, i.e., as the initiators of a CO-reducing metabolism. The fact that wet and rocky bodies in the solar system much smaller than Earth or Venus do not reach the internal pressure (≥23 GPa) requirements in their mantles sufficient for producing bridgmanite and, therefore, are too reduced to stabilize and emit CO-the staple of life-may explain the apparent absence or negligible concentrations of that gas on these bodies, and thereby serves as a constraint in the search for extraterrestrial life. The astrobiological challenge then is to search for worlds that (i) are large enough to generate internal pressures such as to produce bridgmanite or (ii) boast electron acceptors, including imported CO, from extraterrestrial sources in their hydrospheres.
如果没有适当的电化学不平衡以及介导能量耗散过程的矿物质，生命就无法在行星或卫星上出现。本文认为，橄榄石（[Mg>Fe]SiO）、布里奇曼石（[Mg,Fe]SiO）、蛇纹石（[Mg,Fe,]SiO[OH)]）和磁黄铁矿（FeS）这四种矿物质是行星体产生这种不平衡从而孕育生命的必要条件。然而，只有两种矿物质，纤铁矿（[FeFeOH]·[(CO)·3HO]）和马基诺矿（Fe[Ni]S），作为最初的“自由能量”导体和这种不平衡的转换器，即作为CO还原代谢的启动者，是至关重要的——它们构成沉淀膜。太阳系中比地球或金星小得多的潮湿岩石天体，其地幔无法达到产生布里奇曼石所需的内部压力（≥23 GPa），因此还原性过强，无法稳定和释放生命的主要物质CO，这或许可以解释这些天体上明显没有或只有可忽略不计浓度的这种气体的现象，从而在寻找外星生命时构成一种限制。那么，天体生物学面临的挑战就是寻找这样的世界：（i）足够大以产生能形成布里奇曼石的内部压力，或者（ii）其水圈中有包括从外星来源输入的CO在内的电子受体。
