New vivid images show why this is dubbed Mars' 'Inca City'

A European spacecraft orbiting Mars has acquired some of the most detailed images yet of a region at the Red Planet's south pole known as "Inca City."

How this mysterious place earned its nickname is pretty obvious: From space, the natural grid-like pattern of pin-straight ridges, right angles, and polygons looks like the ruins of Machu Picchu in Peru. Though NASA's Mariner 9 spacecraft discovered this area — more formally referred to as Angustus Labyrinthus — 50 years ago, planetary scientists are still unsure what natural phenomenon drove its formation. 

"It could be that sand dunes have turned to stone over time," according to the European Space Agency, which runs the Mars Express orbiter. "Perhaps material such as magma or sand is seeping through fractured sheets of Martian rock."

Or, in another possible theory, the ridges could be winding structures related to glaciers, the agency said. The German Aerospace Centre, whose High Resolution Stereo Camera shot the photos, believes the most compelling explanation is that the narrow ridges are solidified lava.

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The features that resemble walls appear to trace part of a 53-mile-wide circle. Scientists think perhaps the "city" sits in a large crater from an ancient asteroid collision. The impact may have caused faults to spread through the surrounding plain, filled with magma bubbling up. 

"Later, the softer material surrounding the polar plains was eroded, leaving behind ridges of the harder components of magmatic rock," the German Aerospace Centre said. 

Mars Express has taught scientists a lot about the Red Planet over the past 20 years. The spacecraft has been observing the Martian surface, mapping its minerals, revealing its composition and other aspects of its environment.

An area at Mars' south pole region has been compared to these ruins at Machu Picchu in Peru. Credit: joSon / Getty Images

The new photos also found hints of spidery formations scattered over the polar region — features that are better understood by researchers than Inca City. Dark splotches seen in the image below are buried "spiders," or so-called araneiform terrain.

The spiders tend to emerge when spring sunlight shines on layers of carbon dioxide deposited over the dark winter. The sun causes the dry ice trapped below the ice cap to turn into gas, which eventually breaks through the ice. 

The grid-like ridges of the Angustus Labyrinthus region on Mars along with dark-splotched "spiders." Credit: ESA / DLR / FU Berlin (CC BY-SA 3.0 IGO)

If those don't look like spiders to you, take a look at the image below, taken by another European spacecraft known as the ExoMars Trace Gas Orbiter. These ice spiders are lying just outside the region captured in the new Mars Express images. NASA's Mars Reconnaissance Orbiter has also repeatedly photographed the spiders.

Another European spacecraft known as the ExoMars Trace Gas Orbiter has taken detailed pictures of the ice spiders as well. Credit: ESA / TGO / CaSSIS

"The emerging gas, laden with dark dust, shoots up through cracks in the ice in the form of tall fountains or geysers, before falling back down and settling on the surface," according to ESA.

The geysers make blotches about 50 yards to a half-mile wide. These patterns — when seen beneath the ice, as photographed recently by Mars Express — are a sign that "spiders'' will eventually pop out. Scientists think this process, which doesn't happen on Earth, is a key mechanism for carbon dioxide exchange between the polar ice cap and the Martian atmosphere.