Geophys. Res. Lett. http://dx.doi.org/10.1002/grl.50106 (2013)

Credit: © NASA / JPL-CALTECH / MSSS

Alteration of the early martian crust by subsurface hydrologic processes has been proposed to explain the presence of clay minerals exposed on the surface of Mars. High-resolution images of topographic ridges on the floors of martian impact craters suggest that the ridges represent fossilized conduits through which subsurface fluids once flowed.

Lee Saper and John Mustard at Brown University, USA, used orbital spacecraft imagery to map the distribution, orientations and characteristics of linear ridges in Nilosyrtis and Nili Fossae, two heavily eroded regions of Mars. They find that the ridges are associated with clay-bearing basement rocks. They exclude a magmatic origin and instead interpret the ridges as a complex of fractures and other impact-related structural features through which hydrothermal fluids percolated. Cemented by precipitated minerals, the filled fractures would have been relatively resistant to erosion compared to the surrounding rocks, leading to the ridges observed today.

The hydrologically active conduits would have facilitated widespread alteration of the early martian crust. The evidence for regional hydrothermal activity in the martian subsurface suggests that these regions could have once contained a viable and persistent habitable zone.