NASA finally figures out what's up with those "Mars spiders"

Dark shapes pictured on the surface of Mars resembling spiders. Image credits: ESA/TGO/CaSSIS.

For years, satellites around Mars have been 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.

Now, NASA researchers have managed to recreate these structures in a lab for the first time, demonstrating how they form in the process.

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 spider-shaped features were spotted in 2009 by the Mars Reconnaissance Orbiter over an area of over 1 km. Image credits: NASA.

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.

Now, NASA researchers have decided to see whether they can recreate the structures in experiments here on Earth.

“The spiders are strange, beautiful geologic features in their own right,” said Lauren McKeown of NASA’s Jet Propulsion Laboratory in Southern California. “These experiments will help tune our models for how they form.”

The first model for these structures proposed two decades ago, hypothesizes that seasonal CO₂ ice sublimation is the driving force behind Mars’ springtime surface 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.

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.

As it turns out, these spider-like features don’t always form — they need just the right conditions. But after several iterations, the researchers found the underlying mechanism and confirmed the model.

Inside JPLs DUSTIE — 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 carves out the intricate, spindly channels that resemble spider legs, making them a distinctive aspect of Martian geology linked to the planet’s seasonal carbon dioxide cycles.

The gas release is what creates the distinct surface features most of the time. However, in the lab, the team found that some of these cracks appeared to form due to thermal stresses rather than gas erosion. This suggests that multiple processes may contribute to spider formation on Mars, and they may even record past climate changes in Martian history.

“Cracked” spider morphologies observed when heaters were stopped after plume activity began.

Also, these features don’t always look like spider legs.

Each feature depended on subtle variations in gas release and ice thickness. For example, dark spots formed where CO₂ gas broke through the ice layer and ejected dust. Meanwhile bright halos surrounded these spots, likely caused by frost deposition from the gas plumes.

There are still some questions we don’t have an answer to, yet. For instance, why don’t these shapes seem to get bigger in size, and why don’t they show up everywhere? The answers could be significant: they could be a leftover relic from a long time ago when the Mars climate looked very different. If this is the case, the Mars spiders could provide a window into the planet’s geological 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.