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Gaseous geysers create ‘spiders’ on Mars




left image- This rectangular image shows part of the martian surface as if the viewer is looking down and across the landscape, with the irregular, mottled ground appearing in swirled tones of brown and tan. right image- A slice of the martian surface is shown here. A rounded segment of an eroded crater basin is visible to the right. The key features seen across the image are dark spots with tendrils that are eerily reminiscent of spiders. These are visible in large numbers to the left, and scattered irregularly across the rest of the image.

It’s ‘spider season’ on the Red Planet. There are no real spiders on Mars – that we know of – but spider-like black spots dot some parts of our celestial neighbor every spring.

[Related: Mars’s mascara-like streaks may be caused by slush and landslides.]

The European Space Agency (ESA) has released new images of these seasonal eruptions in a formation called Inca City in the southern polar region of Mars.

How do ‘spiders’ form on Mars?

Mars has four different seasons, similar to Earth. Each Martian season lasts about twice as long as a season here. According to the ESA, these spider-like spots appear in Martian spring, when sunlight hits layers of carbon dioxide deposited during Mars’ dark winter. The sunlight causes carbon dioxide ice in the bottom layer to turn into gas. The gas builds up and eventually breaks through the plates that lie on the ice around Mars’ poles. When they erupt, the dark material is dragged to the surface along the way, shattering layers of ice that can be up to a meter thick.

Spider-like features are created when the spring sun hits layers of carbon dioxide deposited during the dark winter months. CREDIT: ESA/TGO/CaSSIS.

The emerging gas is full of dark dust and shoots up through cracks in the ice, similar to a fountain or geyser. The gas then travels back down and settles on the surface. The settling gas causes dark spots range from 0.3 to 0.6 miles wide. This same process creates the spider-shaped patterns that are etched beneath the ice.

The image was captured by the CaSSIS instrument on board the ESA ExoMars Trace Gas Orbiter (TGO). CaSSIS stands for Color and stereo surface imaging system and it was built at the University of Bern in Germany. It creates high-resolution images designed to complement the data collected on Mars. It consists of a telescope and a slit plane system mounted on a rotation mechanism three electronics units that send images back to the ESA.

The mysterious Inca city of Mars

Most of the spots in this new image can be seen at the edge of Angustus Labyrinthus–better known as Inca City. NASA’s Mariner 9 probe first discovered Inca City in 1972, and its geometric-looking network of ridges reminded astronomers of Inca ruins.

They are scientists still don’t know exactly how Inca City came into existence. They could be sand dunes that have turned to stone over millennia. Materials such as magma or sand can also seep through cracked slabs of Martian rock. The ridges can also be serpentine structures associated with glaciers called eskers.

This rectangular image shows part of Mars' surface as if the viewer is looking down and across the landscape, with the irregular, mottled ground appearing in vertebrate shades of brown and tan.
This oblique perspective overlooks a part of Mars nicknamed the Inca City (formally called Angustus Labyrinthus). The reason for this is no mystery; the linear network of ridges is reminiscent of Inca ruins. Traces of features known as ‘spiders’ can be seen; these small, dark features form when carbon dioxide gas heats up in sunlight and breaks through plates of overlying ice. CREDIT: ESA/DLR/FU Berlin

Inca City turns out to be the same part of a large circle-about 53 miles in diameter. Scientists believe the formation is inside a large crater that may have formed when a boulder from space crashed onto the surface of Mars. The impact probably caused fractures in the surrounding plain. The faults were then filled with rising lava and eroded away over time.

Towards the center portion of the image the landscape changes slightly, with large round and oval swirls creating an effect reminiscent of marble. This effect is believed to occur when layered deposits wear away over time.

[Related: Scientists brought ‘Mars spiders’ to Earth—here’s how.]

A few prominent steep, flat-topped hills and hills standing nearly 5,000 feet above the surrounding terrain. These mounds form when softer material is eroded by wind, water or ice. The harder material left behind forms these mounds. Some signs of the ‘spiders’ are scattered across the dust-covered plateaus, lurking between various canyons and troughs.

The data for these images was captured on October 4, 2020 during Mars’ most recent spring. The Red Planet does currently in the fall and the next vernal equinox will occur on November 12, 2024.