Inca City
During the Mariner 9 mission, in 1972, when Martian features were being mapped close-up for the first time, imaging team members found a very strange feature at the high southern latitude of 82° S and longitude 66° W. It seemed to be a rectilinear grid of straight walls and squarish depressions, typically 3 km on a side. It was a natural formation, but its origin was difficult to judge. Just as Viking team members later named the Face on Mars with whimsical affection, Mariner 9 team members referred to this formation informally as Inca City. It vaguely resembled an ancient, abandoned urban ruin, complete with walls and plazas. If you wanted to pretend you had an alien artifact on a distant world, Inca City would have been a better candidate than the Face on Mars, but mercifully no one made a cottage industry of ballyhooing Inca City in the tabloids.
What processes could possibly have formed this pattern? Early speculation fell into two camps. One said that the winds created some sort of giant dune complex. Wind does weird things, and the intersecting ridges might be the product of two different prevailing wind patterns. However, the feature was too big to be convincingly explained in this manner. The other possibility was some sort of fracture or fault pattern. Tectonic forces can often produce a set of parallel fractures, and it is not uncommon to have two sets of forces that produce intersecting fractures. Once the fractures exist, magma may ascend into them, filling them with molten rock, which then cools. At high latitudes such as 82° S, where Inca City lies, we know that there are polar sedimentary layers, and they are likely to be weaker, more crumbly rock than freshly solidified igneous rock. Thus, as erosion and wind strip away the host-rock sediments, they could leave the igneous rock standing in ridges that marked the original fractures. Resistant rock formations standing in wall-like slabs are well known on Earth; they are called dikes because they resemble ragged walls built to hold back water. In short, the second theory was that Inca City was a set of intersecting dikes exposed by erosion. But in that case, why such a localized, striking pattern?
A regional MGS image, under just the right lighting, showed that Inca City was an isolated part of a larger circular pattern, 86 km (53 miles) in diameter. From the images, it appears that most of the circle is obscured by a broad, flat sediment layer, typical of the margins of the south polar cap. Modern Mars reveals a lot of exhumation of ancient structures, and the circular structure may be an ancient impact crater whose roots are just being revealed. Possibly, the underground circular and radial fractures associated with the impact were filled with ascending magma, forming resistant dikes. Probably the crater was then eroded down to its originally buried roots, then covered over, then partly revealed by exhumation.