Knauth L Paul, Burt Donald M, Wohletz Kenneth H
Department of Geological Sciences, Arizona State University, Box 871404, Tempe, Arizona 85287-1404, USA.
Nature. 2005 Dec 22;438(7071):1123-8. doi: 10.1038/nature04383.
Mars Exploration Rover Opportunity discovered sediments with layered structures thought to be unique to aqueous deposition and with minerals attributed to evaporation of an acidic salty sea. Remarkable iron-rich spherules were ascribed to later groundwater alteration, and the inferred abundance of water reinforced optimism that Mars was once habitable. The layered structures, however, are not unique to water deposition, and the scenario encounters difficulties in accounting for highly soluble salts admixed with less soluble salts, the lack of clay minerals from acid-rock reactions, high sphericity and near-uniform sizes of the spherules and the absence of a basin boundary. Here we present a simple alternative explanation involving deposition from a ground-hugging turbulent flow of rock fragments, salts, sulphides, brines and ice produced by meteorite impact. Subsequent weathering by intergranular water films can account for all of the features observed without invoking shallow seas, lakes or near-surface aquifers. Layered sequences observed elsewhere on heavily cratered Mars and attributed to wind, water or volcanism may well have formed similarly. If so, the search for past life on Mars should be reassessed accordingly.
火星探测漫游车“机遇号”发现了具有层状结构的沉积物，这些结构被认为是水成沉积所特有的，并且含有归因于酸性咸海蒸发的矿物质。引人注目的富铁小球被归因于后来的地下水蚀变，并且推断出的丰富水量增强了人们对火星曾经适宜居住的乐观态度。然而，这些层状结构并非水成沉积所特有，而且这种设想在解释高溶解性盐与低溶解性盐混合、酸岩反应产生的粘土矿物缺失、小球的高球形度和近乎均匀的尺寸以及盆地边界缺失等问题时遇到了困难。在此，我们提出一种简单的替代解释，涉及由陨石撞击产生的岩石碎片、盐、硫化物、盐水和冰的紧贴地面的湍流沉积。随后由粒间水膜进行的风化作用可以解释所观察到的所有特征，而无需引入浅海、湖泊或近地表含水层。在陨石坑密布的火星其他地方观察到的归因于风、水或火山作用的层状序列很可能也是以类似方式形成的。如果是这样，那么对火星上过去生命的探寻就应该相应地重新评估。
