New laboratory experiments reveal that the enigmatic, freshly carved gullies seen on Martian sand dunes are likely the work of frozen carbon dioxide blocks that slide and burrow through the sand. For years, the origin of these winding channels has puzzled planetary scientists, with some theories pointing to flows of liquid water. However, research led by Earth scientist Dr. Lonneke Roelofs of Utrecht University demonstrates a powerful, waterless mechanism driven by the explosive sublimation of dry ice, a process that forcefully sculpts the Red Planet’s surface in its frigid, thin atmosphere.
The findings provide a compelling explanation for features that appear actively formed, resolving the discrepancy of how such channels could be created on a planet now considered too cold and arid to support the necessary liquid flows. By recreating Martian conditions in a specialized chamber, researchers were able to watch in real time as slabs of CO₂ ice carved deep, sinuous trenches flanked by raised ridges, mirroring observations from orbiting spacecraft. This violent, gas-driven process explains not only the shape of the gullies but also their appearance during the Martian spring, when seasonal ice begins to retreat.
A Long-Standing Martian Puzzle
Planetary scientists have long debated the formation of thousands of gullies observed on the slopes of craters and sand dunes on Mars. These features, often resembling channels carved by flowing water on Earth, presented a tantalizing possibility that liquid water might still exist, or have existed recently, on the Martian surface. This hypothesis carried significant implications for the potential habitability of the planet. Yet, the conditions on modern Mars—with its extremely low atmospheric pressure and frigid temperatures—are not conducive to stable liquid water on the surface.
High-resolution monitoring from orbiters, such as images captured by NASA’s HiRISE camera, showed that these specific dune gullies were active in the late winter and spring. This timing coincided perfectly with the presence and subsequent disappearance of seasonal CO₂ frost, casting doubt on the liquid water theory and suggesting a process linked to frozen carbon dioxide. While researchers had previously theorized that CO₂ ice might play a role, the exact physical mechanism that could dig such distinct and often erratic channels remained unproven until now.
Recreating Mars in a Laboratory
Simulating Martian Conditions
To test the hypothesis, Dr. Roelofs and her team utilized a specialized low-pressure chamber at The Open University in the U.K., designed to mimic the atmospheric conditions on Mars. Inside this “Mars chamber,” the atmospheric pressure was reduced to levels comparable to the Red Planet’s thin air. The team created slopes of sand to simulate the face of a Martian dune, which could be adjusted to different angles of steepness. This controlled environment was critical for studying physical processes that cannot occur naturally on Earth.
Observing the Carving Process
The researchers placed blocks of solid carbon dioxide, or dry ice, onto the sand slopes within the chamber and carefully documented the results with high-speed cameras. As the experiment began, they observed a remarkable phenomenon. The blocks of CO₂ ice began to move downslope, but not by simply sliding. Instead, they actively dug into the sand, plowing forward and creating deep troughs. Roelofs noted the process was surprisingly vigorous, comparing the burrowing motion to that of a mole or the fictional sandworms from the novel Dune. The experiments successfully reproduced straight and winding gullies with pit-like endings, closely matching the morphology of gullies observed in areas like the Matara and Russell craters on Mars.
The Physics of Explosive Sublimation
Seasonal Formation of Ice
The process begins in the harsh Martian winter, particularly in the planet’s southern hemisphere, when temperatures plummet to minus 120 degrees Celsius. During this period, carbon dioxide from the atmosphere freezes and deposits onto the surface, forming a seasonal ice cap. On dune fields, this frost can accumulate into thick layers, sometimes reaching up to 70 centimeters. When spring arrives, the sunlight warms the ground, causing this thick layer of CO₂ ice to become unstable. Eventually, large slabs, some up to a meter long, fracture and break away from the tops of the dunes, beginning their journey downslope.
Gas-Driven Burrowing Motion
As an ice block slides over the warmer sand, its bottom surface rapidly heats. This triggers sublimation, the direct transformation of the solid CO₂ into a gas. Because a kilogram of CO₂ gas occupies much more volume than the equivalent amount of ice, this rapid phase change generates immense pressure beneath the block. The trapped gas builds until it vents in explosive bursts, blasting sand away in all directions. This repeated, violent release of gas doesn’t just lift the block; it fluidizes the sand beneath it and propels the block forward and downward, forcing it to burrow into the slope. The displaced sand is thrown aside, forming the characteristic small ridges, or levees, seen along the gully edges in orbital images. The block continues this self-propelled digging until it reaches the bottom of the slope or completely sublimates, leaving a hollow pit at its final resting place.
Connecting Simulation to Planetary Observation
The laboratory results provide a strong physical model that aligns remarkably well with data gathered from Mars. The distinctive features created in the Mars chamber—the deep, narrow troughs, the flanking levees, and the terminal pits—are all consistent with high-resolution images of linear dune gullies on the Red Planet. The researchers also modeled the trajectory of the sand grains ejected during the sublimation bursts, finding that their flight paths in Martian gravity would produce levees of the same width and channels of the same depth as those measured in places like the Russell crater megadune.
Furthermore, the model explains the often twisting and turning paths of the gullies. In the simulation, the team observed that even tiny bumps or wind-formed ripples on the sand’s surface could nudge a burrowing block, deflecting its path and causing it to change direction. This accounts for the sinuous, almost organic-looking patterns that were difficult to explain with simple debris flows. The success of these experiments provides compelling evidence that this unique, gas-driven process is actively shaping the Martian landscape today.
