The spider-like shapes near Mars’s south pole are not animals or fossils. They are branching channels carved into the ground by seasonal carbon-dioxide ice. When sunlight warms the base of the dry-ice sheet in spring, pressurized gas can escape through vents, carrying dust and leaving dark fans at the surface. Scientists call the channel systems araneiform terrain, from a word meaning spider-shaped.
What the spacecraft actually photographed
NASA’s Mars Reconnaissance Orbiter photographed bright, branching troughs with darker fans nearby. The narrow lines are low channels in the ground—not raised spider legs. The dark patches are deposits of dust carried and dropped by escaping gas.
How spring sunshine makes spider-shaped channels
First, the Martian south polar winter deposits a seasonal layer of frozen carbon dioxide. In spring, sunlight can pass into this translucent ice and warm the darker material underneath. The ice at its base sublimates: it changes directly from solid to gas rather than melting into a puddle.
Gas can become trapped beneath the overlying ice. As pressure builds, it finds a route outward and can burst through a weak point or opening. Gas flowing below the ice entrains loose material, eroding the ground into branching paths leading toward vents. Once the gas reaches open air, transported dust settles as a fan whose direction can reflect winds during the eruption. This is a physical model supported by orbital patterns and seasonal observations, not a video of every step in real time.
Why they show up near the south pole
Mars’s polar regions experience pronounced seasonal cycles of carbon-dioxide frost. The relevant combination is a seasonal ice cover, sunlight warming from below, and loose ground that gas can scour. That is why classic araneiform terrain is associated particularly with the south polar region, rather than appearing uniformly across all of Mars.
What the picture cannot prove by itself
An image of branching lines does not tell scientists how old each channel is or whether every branch formed in one spring. They compare images taken in different seasons, observe dark fan changes and test ice-and-gas physics. Nor do dark fans establish present-day liquid water: dust transported by carbon-dioxide gas provides a separate explanation.
How to recognize a Mars spider photo
Look for radiating or branching troughs that converge near a central point, often accompanied by narrow dark fans. Check the photo’s scale and season: in this NASA image, the map scale is approximately 25 centimeters per pixel, so the features are geological rather than insect-sized. Different lighting and processing can make the grooves stand out more or less.
Frequently asked questions
Can an astronaut see these “spiders” from the ground?
Some channels extend across areas large enough to be noticeable in a surface landscape, but the broad branching pattern is easier to understand from overhead, much like a river delta.
Are the dark fans lava?
No. For the photographed south polar terrain, NASA identifies the fans as dust carried and deposited by pressurized carbon-dioxide gas.
Do the same spiders form on Earth?
Earth has frost, gas vents and erosional features, but the particular Martian cycle of seasonal dry-ice slabs and trapped basal gas is unusual. A familiar Earth spiderweb is only a visual analogy.
Explore another Martian polar mystery
Mars has other cold-climate patterns, too. Explore why south-polar ice sometimes looks like Swiss cheese and see the NASA HiRISE comparison of spider development stages.
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