# Carbon Dioxide Ice, Not Water, Sculpts Mars' Active Gullies
Researchers have solved a decade-long puzzle about Mars' changing landscape. The gullies that appear and shift across the Martian surface are not carved by water, but by seasonal cycles of carbon dioxide frost, according to work led by Apolline Leclef at the Institut d'Astrophysique Spatiale at Université Paris-Saclay.
When NASA's Mars Reconnaissance Orbiter first transmitted high-resolution images of gullies on Mars in the early 2000s, scientists recognized their morphology immediately. These channels and ravines mirrored erosional patterns carved by water on Earth. The shock came when repeat imaging showed the gullies were actively changing. Fresh deposits appeared. Channels deepened. Features vanished. The transformation implied an active geomorphological process happening on a planet that today cannot sustain liquid water on its surface.
The Martian atmosphere holds only about 0.6 percent the pressure of Earth's. Surface temperatures plunge far below water's freezing point, typically ranging from minus 80 to minus 5 degrees Celsius depending on season and latitude. Liquid water cannot exist under these conditions. Yet something was carving gullies with remarkable efficiency, particularly in the southern hemisphere during spring and early summer.
Leclef's research points to dry ice sublimation as the culprit. Carbon dioxide comprises 95 percent of Mars' atmosphere. At high latitudes, during winter, CO2 condenses directly from gas into solid frost on the ground, accumulating in layers several centimeters thick. When spring arrives and sunlight intensifies, the frost does not melt into liquid. Instead, it converts directly to gas in a process called sublimation, skipping the liquid phase entirely.
This phase transition releases energy and creates pressure. Where frost has accumulated in depressions and along slopes, sublimating CO2 gas expands forcefully. The expanding gas destabilizes overlying regolith and loosely-bound rocks. Particles get mobilized and transported downslope. Over multiple seasonal cycles, this mechanism progressively deepens channels and reshapes valleys.
The evidence supports this mechanism thoroughly. Gully activity clusters at high southern latitudes where seasonal CO2 frost accumulates most heavily. Changes occur predictably with Martian seasons. Thermal imaging from orbiting instruments confirms temperature fluctuations align with observed gully modifications. The process operates year-round in some locations, but reaches maximum intensity during the spring transition when sublimation rates peak.
This discovery reframes how scientists assess Mars' geomorphology. For decades, the search for signs of ancient water dominated rover and orbiter missions. Gullies resembling water-carved features fueled speculation about recent liquid water activity. Now researchers understand that Mars can sculpt dramatic erosional landforms through purely mechanical, icy processes operating under its current harsh conditions.
The finding carries implications for lunar and icy body exploration. Worlds without significant atmospheres experience different erosion patterns than wet planets. Understanding how sublimation reshapes terrain helps predict surface stability for future landing sites and bases. It also constrains models of where to search for subsurface water ice reservoirs that might support human exploration.