Ridges in impact craters on Mars appear to be the fossilized remnants of underground cracks through which water once flowed.
The analysis, by researchers from Brown University, supports the idea that there was once liquid water beneath the planet's surface, and that this could be a good place to search for evidence of life in the past.
The ridges, many of them hundreds of meters in length and a few meters wide, were first spotted some time ago, but how they had formed was not known. It now seems, though, that they represent faults and fractures that formed underground when impact events rattled the planet's crust.
Water would have circulated through the cracks, slowly filling them in with mineral deposits, harder than the surrounding rocks. As those surrounding rocks eroded away over millions of years, the seams of mineral-hardened material remained in place, forming the ridges seen today.
The team mapped over 4,000 ridges in two crater-pocked regions on Mars, Nili Fossae and Nilosyrtis, and concluded that the ridges started out as fractures formed by impact events, rather than volcanic activity.
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," says Brown graduate Lee Saper.
Importantly, the scientists found that the ridges only exist in areas where the surrounding rock is rich in iron-magnesium clay, a mineral considered to be a telltale sign of water.
"The association with these hydrated materials suggests there was a water source available," says Saper. "That water would have flowed along the path of least resistance, which in this case would have been these fracture conduits."
The team found that the ridges only occurred 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.
"This gives us a point of observation to say there was enough fracturing and fluid flow in the crust to sustain at least a regionally viable subsurface hydrology," says Saper.
"The overarching theme of NASA's planetary exploration has been to follow the water. So if in fact these fractures that turned into these ridges were flowing with hydrothermal fluid, they could have been a viable biosphere."
There's a chance that the Curiosity rover may be able to shed more light on these structures.
"In the site at Gale Crater, there are thought to be mineralized fractures that the rover will go up and touch," says Saper. "These are very small and may not be exactly the same kind of feature we studied, but we'll have the opportunity to crush them up and do chemical analysis on them. That could either bolster our hypothesis or tell us we need to explore other possibilities."