Narrow ridges found in Martian impact craters in the Nilosyrtis highlands and the Nili Fossae region are the fossilized remnants of underground cracks through which water once flowed, Brown University scientists say.
This 3D image of an impact crater in the Nilosyrtis area on the Red Planet shows long pipe-like ridges, fossilized evidence of ancient subsurface water flow (NASA Mars Reconnaissance Orbiter)
The study, published online in the journal Geophysical Research Letters, supports the idea that the Martian subsurface environment once had an active hydrology and could be a good place to search for evidence of past life.
The ridges had been noted in previous research, but how they had formed was not known. The scientists thought they might once have been faults and fractures that formed underground when impact events rattled the planet’s crust. Water, if present in the subsurface, would have circulated through the cracks, slowly filling them in with mineral deposits, which would have been harder than the surrounding rocks. As those surrounding rocks eroded away over millions of years, the seams of mineral-hardened material would remain in place, forming the ridges seen today.
To test their hypothesis, they mapped over 4,000 ridges in two crater-pocked regions on Mars – Nili Fossae and Nilosyrtis. Using high-resolution images from NASA’s Mars Reconnaissance Orbiter, the researchers noted the orientations of the ridges and composition of the surrounding rocks.
At Nili Fossae, the orientations are similar to the alignments of large faults related to a mega-scale impact. At Nilosyrtis, where the impact events were smaller in scale, the ridge orientations are associated with each of the small craters in which they were found.
“This suggests that fracture formation resulted from the energy of localized impact events and are not associated with regional-scale volcanism,” said study lead author Lee Saper.
This Mars Reconnaissance Orbiter image shows ridges formed by fossilized subsurface water flow. Orientation of the ridges, mapped by researchers, is consistent with fractures formed by impact events (NASA / Mustard Lab / Brown University)
Importantly, Saper and his supervisor Prof Jack Mustard found that the ridges exist exclusively in areas where the surrounding rock is rich in iron-magnesium clay, a mineral considered to be a telltale sign that water had once been present in the rocks.
“The association with these hydrated materials suggests there was a water source available,” Saper said. “That water would have flowed along the path of least resistance, which in this case would have been these fracture conduits.”
As that water flowed, dissolved minerals would have been slowly deposited in the conduits, in much the same way mineral deposits can build up and eventually clog drain pipes. That mineralized material would have been more resistant to erosion than the surrounding rock. And indeed, the scientists found that these ridges were only found in areas that were heavily eroded, consistent with the notion that these are ancient structures revealed as the weaker surrounding rocks were slowly peeled away by wind.
Taken together, the results suggest the ancient Martian subsurface had flowing water and may have been a habitable environment.
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Bibliographic information: Lee Saper, John F. Mustard. Extensive linear ridge networks in Nili Fossae and Nilosyrtis, Mars: implications for fluid flow in the ancient crust. Geophysical Research Letters, published online ahead of print January 30, 2013; doi: 10.1002/grl.50106
