The dark spots in the images taken by the Trace Gas Orbiter probe of the ExoMars mission, launched in 2016 by the European Space Agency and the Russian space agency Roscosmos, look like hundreds of little black spiders swarming on the reddish surface of Mars. What gives this unusual appearance to the Martian soil are explosions that can measure up to a kilometer in diameter and which fracture the frozen ground at the arrival of spring and milder temperatures. The photos, released by ESA, come from the so-called Inca City, a strange formation which owes its name to its resemblance to ancient ruins seen from above.
Having an axis inclined in a similar way to Earth’s, Mars also has seasons, although very different from those we are used to on Earth. In winter, for example, temperatures drop to 120 degrees below zero, also freezing surface deposits of carbon dioxide, which thus form a thick layer of dry ice. When the winter season begins to give way to the spring season, the dry ice does not melt as normal ice would, but sublimates, passing directly from the solid to the gaseous state.
The phenomenon increases the pressure inside these deposits until an explosion occurs which makes the Martian surface appear to be boiling: cracks appear in the ice and the dark dust coming from the underlying layers is ‘shot’ out through small geysers . This process is responsible for the spider-shaped spots, whose ‘legs’ are actually fracture lines that lie beneath the surface ice layer.
