This image shows features known as “spiders” near the south pole of Mars, as seen by the CaSSIS (Colour and Stereo Surface Imaging System) instrument on board ESA’s ExoMars Gas Orbiter. The data for this image was captured by CaSSIS on October 4, 2020. The image is centered at approximately 323°E/75°S. Credit: ESA/TGO/CaSSIS
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This image shows features known as “spiders” near the south pole of Mars, as seen by the CaSSIS (Colour and Stereo Surface Imaging System) instrument on board ESA’s ExoMars Gas Orbiter. The data for this image was captured by CaSSIS on October 4, 2020. The image is centered at approximately 323°E/75°S. Credit: ESA/TGO/CaSSIS
ESA’s Mars Express has picked up tracks of “spiders” scattered across Mars’ south polar region.
Rather than being true spiders, these small, dark features form when the spring sun beats down on layers of carbon dioxide deposited during the dark winter months. Sunlight causes the carbon dioxide ice at the bottom of the layer to turn into a gas, which then accumulates and bursts through the overlying ice sheets. The gas bursts free in the Martian spring, pulling the dark material up to the surface as it goes and shattering layers of ice up to a meter thick.
The escaping gas, laden with dark dust, erupts through cracks in the ice in the form of towering fountains or geysers, before falling back down and settling on the surface. This creates dark spots from 45 m to 1 km. The same process creates characteristic “spider-like” patterns etched into the ice – and so these dark spots are a telltale sign that spiders may be lurking below.
Another of ESA’s Mars explorers, the ExoMars Trace Gas Orbiter (TGO), has imaged the tendril-like spider patterns particularly clearly (see image above). The spiders captured by TGO lie near, but outside, the region shown in this new Mars Express image. The Mars Express view shows dark spots on the surface formed by escaping gas and material, while the TGO perspective also captures the spidery, web-like channels that are carved into the ice below.
The aforementioned dark spots can be seen throughout the Mars Express image, creeping across high hills and vast plains. However, most can be seen as small specks in the dark region to the left, which is just on the outskirts of a part of Mars nicknamed Inca City. The reason for the name is no mystery, with the linear, almost geometric grid of ridges reminiscent of Inca ruins. Officially known as Angustus Labyrinthus, Inca City was discovered in 1972 by NASA’s Mariner 9 probe.
A rectangular slice of Mars is shown here in brown and tan tones. The terrain is darker on the left, and becomes smoother and lighter on the right. In the middle part of the image, swirling spots of various material deposits can be seen. To the left, two main features can be seen: an elevated network of ridges and linear, web-like walls known as the City of the Incas, and a scattering of dark spots indicating the presence of features known as ‘spiders’ beneath ice. Credit: ESA/DLR/FU Berlin
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A rectangular slice of Mars is shown here in brown and tan tones. The terrain is darker on the left, and becomes smoother and lighter on the right. In the middle part of the image, swirling spots of various material deposits can be seen. To the left, two main features can be seen: an elevated network of ridges and linear, web-like walls known as the City of the Incas, and a scattering of dark spots indicating the presence of features known as ‘spiders’ beneath ice. Credit: ESA/DLR/FU Berlin
This new view of the Inca city and its hidden arachnid inhabitants was captured by Mars Express’ high-resolution stereo camera.
A mysterious origin
We are still not sure how the Inca City was formed. It may be that the sand dunes have turned to stone over time. Perhaps materials such as magma or sand are seeping through fractured sheets of Martian rock. Or, the ridges may be “eskers,” winding structures associated with glaciers.
The “walls” of the Inca city appear to trace part of a great circle, 86 km in diameter. Therefore, scientists suspect that the “city” is located inside a large crater that formed as space rock crashed into the planet’s surface. This impact likely caused fissures to flow through the surrounding field, which were then filled with rising lava and have since worn away over time.
Towards the middle of the image, the landscape changes somewhat, with large round and oval swirls creating an effect reminiscent of marble. This effect is thought to occur when layered deposits wear down over time.
This rectangular image shows a portion of the Martian surface as if the viewer is looking down and across the landscape, with the irregular and mottled ground appearing in swirling tones of brown and brown. Credit: ESA/DLR/FU Berlin
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This rectangular image shows a portion of the Martian surface as if the viewer is looking down and across the landscape, with the irregular and mottled ground appearing in swirling tones of brown and brown. Credit: ESA/DLR/FU Berlin
In the right center of the frame lie several prominent steep-sided, flat-topped hills and hills that rise more than 1,500 m above the surrounding terrain. These forms as the softer material is eroded over time by wind, water or ice streams, leaving behind the harder material that forms these hills.
The ground to the right (to the north) is increasingly covered with smooth, light-colored dust. Several signs of spiders can be seen scattered across the plateaus here, hidden among the various canyons and troughs.
Exploring Mars
Mars Express has discovered a lot about Mars in the past two decades and counting. The orbiter continues to image the surface of Mars, map its minerals, explore the composition and circulation of its atmosphere, probe beneath its crust, and study the Martian environment.
The spacecraft’s HRSC has shown us everything from wind-sculpted ridges and pits to craters on the flanks of colossal volcanoes to impact craters, tectonic faults, river channels and ancient lava basins. The mission has been extremely productive during its lifetime, creating a far more complete and accurate understanding of our planetary neighbor than ever before.