In the vast, cold stretches of Mars’ southern polar region, a mysterious and visually striking phenomenon occurs each spring. Known colloquially as the ‘Inca City,’ this area becomes the stage for an extraordinary display of what appear to be hundreds of black ‘spiders’ sprawling across the Martian surface. These formations, however, are not living creatures but the result of seasonal geysers of carbon dioxide gas that push through the planet’s icy exterior.
The term ‘Inca City’ refers to a part of Mars characterized by its structured, ridge-like patterns, which resemble the geometric precision of ancient ruins. Officially named Angustus Labyrinthus, this region was once believed to be the remnants of petrified sand dunes or perhaps the leftover walls of glaciers that have long since receded. A discovery in 2002 by the Mars Orbiter, however, suggested a more tumultuous origin. The area is part of a circular formation approximately 53 miles wide, possibly an ancient impact crater formed by a celestial collision. The resulting heat and pressure may have caused magma to seep through Mars’ crust, forming the ridge patterns visible today, much like ruins being slowly uncovered by eroding sediment.
The phenomenon of the black ‘spiders’ begins with the arrival of spring in Mars’ southern hemisphere. As temperatures rise, buried layers of carbon dioxide ice start to melt—or more accurately, sublimate—directly turning from solid ice into gas. This sublimation occurs first at the bottom layers of ice, closest to the warming ground. As the gas forms, it builds up pressure until it forcefully erupts through the ice above, often penetrating ice layers up to 3.3 feet thick.
The force of these eruptions expels dark Martian dust that was once beneath the ice, creating plumes that rise and then settle upon the surface. This deposited dust outlines the paths of the escaping gas, giving them an eerie, spider-like appearance. These channels can stretch between 45 meters to almost a kilometer in width. The European Space Agency (ESA) has been able to capture stunning images of this process through the Mars Express orbiter and the ExoMars Trace Gas Orbiter, offering a closer look at these dusty gas geysers.
This springtime Martian spectacle offers not only a deeper understanding of the dynamic processes at work on Mars but also a glimpse into the planet’s colder, more volatile past. It reminds us that even now, Mars is a world of active geological change, driven by forces that can sculpt its surface in dramatic and beautiful ways.
