Martian Spiders: How Carbon Dioxide Geysers Carve Alien Terrain

Known formally as araneiform terrain, these "Martian spiders" are the scars of a violent and alien process—a seasonal explosion of carbon dioxide gas that rips through the polar ice caps, carving the ground with the force of a thousand geysers. This is the story of how the sun, the ice, and the thin Martian atmosphere conspire to create one of the solar system's most bizarre landscapes.

1. The Discovery of the Abyss

The mystery began in 2003, when cameras aboard NASA’s Mars Global Surveyor beamed back high-resolution images of the Martian South Pole. The images revealed a terrain unlike anything planetary geologists had ever seen. Across the vast, frozen expanses of the South Polar Layered Deposits (SPLD), the white ice was interrupted by strange, dark dendritic patterns.

The Scale of the Spiders

These are not small features. A single "spider" can span anywhere from 45 meters to 1 kilometer (0.6 miles) across. If you were standing in the center of one, the "legs" would be deep troughs, potentially knee-to-waist deep, stretching out to the horizon. They appear in swarms, often clustered together in thousands, creating a wrinkled, etched texture on the planet's surface that looks like wrinkled skin.

2. The Engine of Creation: The Kieffer Model

To understand how a spider forms, we must first understand the alien nature of the Martian polar climate. On Earth, our polar caps are made of water ice. On Mars, while water ice exists, the seasonal caps are composed almost entirely of dry ice (frozen carbon dioxide).

In the dead of the Martian winter, temperatures at the south pole drop to -130°C (-200°F). The atmosphere itself freezes, depositing a slab of transparent carbon dioxide ice onto the surface. This slab can be several meters thick.

The formation of the spiders is explained by a theory known as the Kieffer Model, named after geophysicist Hugh Kieffer who proposed it in 2006. It describes a process that is physically impossible in nature on Earth: the Solid-State Greenhouse Effect.

Step 1: The Glass Roof

As spring rises over the South Pole, the sun returns. However, the slab of CO2 ice that formed during the winter is unusual—it is translucent to visible light but opaque to infrared radiation (heat). Sunlight passes through the ice sheet like it’s a glass window, striking the dark Martian soil (regolith) underneath.

Step 2: The Sub-Surface Pressure Cooker

The dark soil absorbs the sunlight and heats up. This heat radiates back upward but is trapped by the ice slab, unable to escape. The result is a rapid rise in temperature at the base of the ice sheet.

Because Mars has very low atmospheric pressure, carbon dioxide does not melt into a liquid; it sublimates—turning directly from a solid into a gas. The ice at the bottom of the slab turns into CO2 gas, but it is trapped by the heavy weight of the ice slab above.

Step 3: The Geyser Eruption

Pressure builds. And builds. Pockets of high-pressure gas form beneath the ice, lifting the slab slightly off the ground. The gas begins to flow toward weak points in the ice sheet, scouring the ground as it rushes by. This flowing gas acts like a river, carving channels into the loose soil. These channels form the "legs" of the spider.

Eventually, the pressure becomes too great. The ice slab cracks at a weak point.

A high-velocity jet of carbon dioxide gas bursts through the crack, shooting up into the Martian sky at speeds of up to 100 miles per hour. This is a CO2 geyser.

Step 4: The Black Spiders

The geyser doesn't just spew gas; it carries the dark dust and sand carved from the channels below. This dark material shoots hundreds of feet into the air. As it falls back down, the Martian winds blow it into long, dark streaks across the top of the white ice. From orbit, these look like "fans."

When the summer comes and the ice slab completely sublimates away, the "legs" carved into the ground are revealed. These permanent scars in the terrain are the araneiforms—the Martian Spiders.

3. Confirmation: The 2024 Laboratory Breakthrough

For nearly 20 years, the Kieffer Model was the leading theory, but it remained just that—a theory. No lander had ever visited the South Pole to witness a geyser eruption. The conditions were too extreme to replicate easily on Earth.

That changed in late 2024, when a team at NASA’s Jet Propulsion Laboratory (JPL) successfully recreated a Martian spider in a lab for the first time.

The DUSTIE Experiment

Researchers used a specialized cryo-chamber named DUSTIE (Dirty Under-vacuum Simulation Testbed for Icy Environments). The team, led by planetary scientist Lauren Mc Keown, recreated the exact conditions of the Martian South Pole:

1. They cooled a Martian soil simulant to extremely low temperatures.
2. They condensed carbon dioxide gas into a thick slab of ice over the soil.
3. They simulated the Martian sun using heaters from below.

The results were spectacular. The team watched as the CO2 ice cracked and plumes of gas erupted from the regolith, blasting holes in the soil. When they cleared away the ice, they found the signature dendritic, spider-like troughs carved into the dirt.

4. Geography of the Spiders: Inca City and the Cryptic Terrain

Martian spiders are not found everywhere. They are endemic to specific regions of the South Pole, creating a unique geography of alien erosion.

Inca City (Angustus Labyrinthus)

One of the most famous homes of the spiders is a region informally known as "Inca City." Discovered by Mariner 9 in 1972, this area features a geometric network of rectilinear ridges that look like the ruins of an ancient civilization (hence the name).

While the "ruins" are likely ancient sand dunes that turned to stone or volcanic dikes, the spiders here are a modern addition. ESA’s Mars Express orbiter has captured breathtaking images of "Inca City" dusted with thousands of black spiders, their dark legs contrasting sharply against the rectilinear walls of the labyrinth.

The Cryptic Terrain

Many spiders are found in a zone called the "Cryptic Region." This area is peculiar because, in thermal imaging, it remains cold (indicating CO2 ice is present) but appears dark to the eye. This paradox is explained by the spider activity: the geysers are so active here that they cover the top of the ice with a thick layer of dark dust, hiding the bright ice below.

5. Why Not on Earth? (The Uniqueness of Araneiforms)

Why don’t we see spiders in Antarctica? The answer lies in the atmospheric chemistry.

• Earth: Our polar caps are water ice. Water melts into a liquid. Liquid water flows downhill, carving river valleys and gullies, not radial spiders. Furthermore, Earth’s atmosphere is too thick to allow significant CO2 deposition, and our temperatures are too warm for dry ice to form natural slabs thick enough to pressurize.
• Mars: Mars is the only planet in the solar system with the "Goldilocks" conditions for spiders:

1. A surface rich in loose, erodible sediment.

2. Temperatures low enough to freeze the atmosphere (CO2).

3. A transparent ice type (CO2 slab ice) that allows the solid-state greenhouse effect.

While we see "plumes" on other worlds—like the water-vapor geysers of Enceladus (Saturn's moon) or the nitrogen geysers of Triton (Neptune's moon)—the specific ground-carving mechanism of the Martian spiders is unique to the Red Planet.

6. The "Life" of a Spider: A Seasonal Cycle

The spiders are not static. They breathe with the seasons of Mars.

• Autumn: The atmosphere cools. CO2 frost begins to settle. The spiders are covered in darkness and ice.
• Winter: The ice slab thickens to roughly 1 meter. The spiders are entombed under a transparent, heavy sheet.
• Early Spring: The sun rises. Sublimation begins at the base. The first geysers erupt. Dark fans appear on the surface, oriented by the prevailing wind. If the wind blows north, the fans streak north.
• Late Spring: The activity peaks. Thousands of geysers are erupting daily. The surface becomes a speckled mess of black spots and streaks.
• Summer: The ice slab completely vanishes (sublimates away). The dark fans blow away in the wind. All that remains are the deep scars—the araneiform troughs—etched into the ground, waiting for the cycle to begin again.

Over thousands of years, this annual scouring deepens the channels. Some scientists believe the larger spiders are actually "fossilized" versions from a time when Mars’ climate was more active, while the smaller ones are currently forming.

7. The Biological Confusion

It is crucial to clarify: There is no biology involved. The radial branches look organic—like veins, roots, or mycelium—because nature tends to use fractal patterns to distribute energy efficiently. Just as a river creates branches to drain water, the pressurized gas creates branches to drain pressure toward a central vent. They are a perfect example of convergent evolution in physics: different forces creating similar shapes.

8. Future Exploration: Into the Spider’s Nest