Post by Dr. James Wray, Cornell University

Recent missions to Mars have pursued a theme of “following the water,” with orbital and surface observations revealing locations where groundwater processes have affected rocks exposed at the modern surface.  Rock fractures or joints can act as conduits for subsurface fluids, which may precipitate fracture-filling minerals or alter pre-existing rocks along the fracture walls.  Both outcomes are evident in orbital images of sulfate-bearing layered rocks in Candor Chasma, part of the vast Valles Marineris canyon system near the Martian equator.  Joints in these layered rocks are surrounded by bright “halos” attributed to chemical bleaching by paleo-fluids, as observed in sedimentary rocks on Earth (Image 1).  Some joints in Candor Chasma also exhibit positive relief  (Okubo and McEwen, 2007), suggesting that fluids cemented the fracture walls and increased their resistance to subsequent erosion.  These ridged fractures would therefore represent another example of inverted topography on Mars.

Image 1: Rocks bleached by fracture fluids on Mars and Earth. (a) Portion of HiRISE image TRA_000836_1740 in Candor Chasma, from Okubo and McEwen (2007). (b) Jurassic Entrada Sandstone, Salt Wash graben, southeast Utah (credit C. Okubo).

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