Dark shapes pictured on the surface of Mars resembling spiders. Image credits: ESA/TGO/CaSSIS.
For years, satellites around Mars kept finding mysterious, spider-like geological structures. Mars spiders, scientifically known as araneiform terrain, are unique geological formations predominantly found in the southern hemisphere of Mars. They appear as branching, spider-like channels etched into the Martian surface, sometimes spanning over a kilometer in length.
In 2024, NASA-led lab experiments pinned down the likeliest way these “spiders” form: seasonal CO₂ ice. Now, a 2026 follow-up showed that the soil beneath the ice can make or break the process, and that these structures could hold clues to Mars’ wet past.
Spiders on Mars
From the get-go, scientists had a strong suspicion that these “spiders” are linked to carbon ice.
Mars may be a frozen, barren world, but its surface is far from static. When temperatures drop during the Martian winter, a significant portion of the planet’s carbon dioxide atmosphere freezes, coating the surface with frost. As spring arrives, this ice sublimates (turns from solid to gas), giving rise to a variety of unusual and mysterious features that have no equivalents on Earth.
These include dark spots, spider-like structures, and oriented fans, collectively known as the “Kieffer zoo.” These features have made scientists curious for decades, especially as there has been no direct, in-situ observation on Mars to investigate them up close.
These spider-shaped features were spotted in 2009 by the Mars Reconnaissance Orbiter over an area of over 1 km. Image credits: NASA.
“The spiders are strange, beautiful geologic features in their own right,” said Lauren McKeown of NASA’s Jet Propulsion Laboratory in Southern California, when the results were first announced in 2024. “These experiments will help tune our models for how they form.”
In the 2024 study, researchers set out to reproduce all three main stages of the proposed process in a single laboratory experiment: CO₂ condensation, sublimation and plume formation, and the resulting changes to the simulated Martian surface.
The first model, proposed roughly two decades ago, holds that seasonal CO₂ ice and its springtime sublimation drive much of this strange polar activity. Sunlight penetrates the translucent ice, heating the underlying regolith (a layer of loose material on the surface), which leads to sublimation beneath the ice slab. The trapped gas eventually builds up pressure, causing the ice to crack, releasing high-velocity jets of gas and dust. These jets carve out unique formations such as “spiders” and create the spots and fans visible on Mars’ surface.
Some examples of the “zoo” of features proposed to be formed by seasonal CO2 sublimation dynamics on Mars. Credit: Planetary Science Journal.
Recreating Mars on Earth
The hardest part about these experiments was recreating the conditions on Mars. First, you need extremely low temperatures of minus 301 degrees Fahrenheit (minus 185 degrees Celsius). Then, you need low pressures, as Mars has a very thin atmosphere. McKeown and colleagues managed this by using a liquid-nitrogen-cooled test chamber at JPL, the Dirty Under-vacuum Simulation Testbed for Icy Environments, or DUSTIE.
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The team used a substance that simulated Martian soil and submerged it in freezing liquid nitrogen. They then placed it inside DUSTIE and pumped carbon dioxide gas into the chamber, which condensed over the course of 3-5 hours.
The spider-like patterns didn’t appear every time. The researchers had to find a narrow range of conditions in which a sufficiently thick, translucent CO₂ layer could develop. Their results supported the broad model in the laboratory, while also exposing processes that the simple version of the model did not predict.
Here’s a look inside of JPL’s DUSTIE, a wine barrel-size chamber used to simulate the temperatures and air pressure of other planets. In this case, the carbon dioxide ice found on Mars’ south pole. Experiments conducted in the chamber confirmed how Martian formations known as “spiders” are created. Image credits: NASA/JPL-Caltech
These features form when sunlight penetrates transparent layers of carbon dioxide ice that accumulate during the Martian winter. As the underlying dark soil absorbs the sunlight, it warms up and causes the bottom layer of the ice to sublimate. The trapped gas builds up pressure beneath the ice until it cracks the surface, releasing plumes of gas and dust. This process can carve or disturb the substrate beneath the ice and is thought to be responsible for the family of seasonal features associated with araneiform terrain.
But the laboratory spiders came with a surprise. Rather than all of their branching forms being excavated by fast-moving gas scouring the surface, some crack networks arose as CO₂ that had condensed inside pores in the simulated soil sublimated and generated thermal stresses. The authors therefore proposed cracking as an additional mechanism for some “cracked” spider morphologies, not a replacement for gas-driven erosion in every araneiform.
Illustration: ZME Science.
Also, these features don’t always look like spider legs.
Each feature depended on subtle variations in gas release and ice thickness. Dark spots form where CO₂ gas breaks through the ice layer and ejects dust. Bright halos can also appear around such spots.
The 2026 follow-up makes the recipe even more specific. When the researchers repeated CO₂ condensation and sublimation experiments over different substrates, finer simulated Martian soil allowed CO₂ to penetrate more deeply and over a wider area. Those fine-grained samples produced more vigorous, longer-lasting plumes and were more likely than coarser material to develop the cracked spider morphologies. Adding water ice to the pore spaces, meanwhile, encouraged a thicker surface layer of CO₂ ice but substantially hampered its sublimation.
“Cracked” spider morphologies observed when heaters were stopped after plume activity began.
There are still some questions we don’t have an answer to yet. Most importantly, despite years of high-resolution monitoring, researchers have not directly detected growth in well-developed araneiforms. Smaller related dendritic troughs have been observed appearing and evolving under today’s Martian climate, showing that CO₂-driven processes can alter the substrate, but mature spiders may grow too slowly to detect, may currently be dormant, or may partly record conditions from an earlier climate.
If the latter is the case, the Mars spiders could provide a window into the planet’s geological and climatic past.
The results have been published in The Planetary Science Journal.
This article originally appeared in September 2024 and was updated with new information. A correction has been added to the first image caption.