Chevrier Vincent, Poulet Francois, Bibring Jean-Pierre
W. M. Keck Laboratory for Space Simulation, Arkansas Center for Space and Planetary Sciences, MUSE 202, University of Arkansas, Fayetteville, Arkansas 72701, USA.
Nature. 2007 Jul 5;448(7149):60-3. doi: 10.1038/nature05961.
Images of geomorphological features that seem to have been produced by the action of liquid water have been considered evidence for wet surface conditions on early Mars. Moreover, the recent identification of large deposits of phyllosilicates, associated with the ancient Noachian terrains suggests long-timescale weathering of the primary basaltic crust by liquid water. It has been proposed that a greenhouse effect resulting from a carbon-dioxide-rich atmosphere sustained the temperate climate required to maintain liquid water on the martian surface during the Noachian. The apparent absence of carbonates and the low escape rates of carbon dioxide, however, are indicative of an early martian atmosphere with low levels of carbon dioxide. Here we investigate the geochemical conditions prevailing on the surface of Mars during the Noachian period using calculations of the aqueous equilibria of phyllosilicates. Our results show that Fe3+-rich phyllosilicates probably precipitated under weakly acidic to alkaline pH, an environment different from that of the following period, which was dominated by strongly acid weathering that led to the sulphate deposits identified on Mars. Thermodynamic calculations demonstrate that the oxidation state of the martian surface was already high, supporting early escape of hydrogen. Finally, equilibrium with carbonates implies that phyllosilicate precipitation occurs preferentially at a very low partial pressure of carbon dioxide. We suggest that the possible absence of Noachian carbonates more probably resulted from low levels of atmospheric carbon dioxide, rather than primary acidic conditions. Other greenhouse gases may therefore have played a part in sustaining a warm and wet climate on the early Mars.
那些看似由液态水作用形成的地貌特征图像，被视作早期火星表面存在湿润状况的证据。此外，近期在古老的诺亚纪地形中发现了大量叶硅酸盐矿床，这表明原始玄武岩地壳曾长期受到液态水的风化作用。有人提出，富含二氧化碳的大气所产生的温室效应维持了诺亚纪时期火星表面保持液态水所需的温和气候。然而，明显缺乏碳酸盐以及二氧化碳的低逃逸率，表明早期火星大气中的二氧化碳含量较低。在此，我们通过计算叶硅酸盐的水相平衡，来研究诺亚纪时期火星表面普遍存在的地球化学条件。我们的结果表明，富含Fe3+的叶硅酸盐可能在弱酸性至碱性的pH条件下沉淀，这一环境与随后时期不同，后者以导致火星上发现硫酸盐矿床的强酸性风化作用为主导。热力学计算表明，火星表面的氧化态已经很高，这支持了氢气的早期逃逸。最后，与碳酸盐的平衡表明，叶硅酸盐沉淀优先在极低的二氧化碳分压下发生。我们认为，诺亚纪碳酸盐可能不存在，更有可能是由于大气中二氧化碳含量低，而非原始酸性条件所致。因此，其他温室气体可能在维持早期火星温暖湿润的气候方面起到了作用。
