Salt-induced rock fragmentation on Mars: The role of salt in the weathering of Martian rocks

Abstract

Large well rounded boulders and angular rock fragments characterizes the Martian landscape as seen on the recent excellent quality photos (except: Meridiani landing side, which formed by different processes, and is therefore not considered in this paper). Analyzing the different rock-shapes indicates a time sequence of emplacement, fragmentation and transport of different rocks on Mars, which might give interesting insight into transport and weathering processes. Larger commonly well rounded boulders were emplaced onto gravel plains. After emplacement, these rocks were fragmented and disassembled. Nests of angular rock fragments are marking the locations of preexisting larger rocks. Frequently, it is possible to reconstruct larger rounded rocks from smaller angular fragments. In other cases, transport after fragmentation obscured the relationship of the fragments. However, a strewn field of fragments is still reminiscent of the preexisting rock. Mechanical salt weathering could be a plausible explanation for the in situ fragmentation of larger rounded blocks into angular fragments. Impact or secondary air fall induced fragmentation produces very different patterns. Salt weathering of rocks is a common process in terrestrial environments. Salt crystallization in capillaries causes fragmentation of rocks, irrespective of the process of salt transportation and concentration. On Earth significant salt weathering can be observed in different climatic environments: in the transition zone of alluvial aprons and salt playas in deserts and in dry valleys of Antarctica. In terrestrial semi-arid areas the salt is transported by salt solution, which is progressively concentrated by evaporation. In Antarctic dry valleys freeze–thaw cycles causes salt transportation and crystallization resulting in rock fragmentation. This salt induced process can lead to complete destruction of rocks and converts rocks to fine sand. The efficient breakdown of rocks is dominating the landscape in some dry valleys of the Earth but possibly also on Mars [Malin, Salt weathering on Mars. JGR 79,26, pp. 3888–3894, 1974.]. However, irrespectively of the climatic environment a liquid brine is a necessity for salt induced fragmentation of rocks. If salt weathering is responsible for the fragmented rocks on the Martian surface it implies a temporary presence of liquid H 2O. However, due to the present dry atmosphere on Mars, brines can only be present in restricted places without being in equilibrium with the atmosphere.