Mars gullies active during frigid winter months are carved by frozen carbon dioxide, or dry ice, rather than liquid water, according to a study led by Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center using NASA’s Mars Reconnaissance Orbiter.
The Shift from Liquid Water to Sublimating Dry Ice
For years, planetary scientists debated whether the dramatic channels crisscrossing the Martian landscape were carved by contemporary flows of liquid water. That working hypothesis is undergoing a major structural rewrite. As recently as five years ago, researchers strongly suspected liquid water drove modern gully activity. But fresh data captured by the High Resolution Imaging Science Experiment (HiRISE) camera onboard NASA’s Mars Reconnaissance Orbiter has completely flipped that script.
By tracking 356 sites across the Red Planet since 2006, Dundas and his team identified 38 locations showing active gully formation. The critical diagnostic metric wasn’t spatial distribution; it was temporal timing. These geological alterations occurred exclusively during deep winter conditions—ambient thermal environments far too cold for liquid water to maintain a fluid state.
The culprit? Carbon dioxide frost. Every winter, atmospheric conditions on Mars trigger the formation of a seasonal polar cap composed of dry ice. When that solid carbon dioxide warms, it undergoes sublimation, transitioning directly from a solid phase into a gas without ever passing through a liquid intermediary state.
Mechanics of Martian Dry Ice Avalanches
Sublimation acts as a high-energy mechanical trigger on Martian slopes. As trapped gas escapes from beneath the warming dry ice, it destabilizes loose surface regolith, touching off localized avalanches. Furthermore, the sublimation process initiates what researchers designate as “frosted granular flow.” In this dynamic, the evaporating frost effectively acts as a low-friction lubricant between individual mineral particles. This reduction in inter-granular friction lets sand and debris cascade down steep gradients much more easily than dry terrestrial materials normally would.
According to Dundas, multiple mechanisms are likely operating in tandem across the Martian surface. The data reveals that gullies situated on active sand dunes experience the highest frequency of change. Dune material is inherently loose, weak, and structurally uncompromised by cementation, making it exceptionally vulnerable to dry-ice-driven erosion.
Individual channel networks likely require millions of years to achieve full morphological maturity. Yet, the active mass movements sculpting them occur on a much tighter timescale. The typical interval between active events within a given gully is measured on the order of centuries, though certain localized systems exhibit significantly higher activity.
What This Means for the Search for Extraterrestrial Life
This discovery pours cold water on hopes that contemporary liquid water routinely flows across the active Martian surface today, echoing ancient hydrological epochs. Without running water to shape these dynamic channels, astrobiologists must refine their search parameters for modern Martian habitability.
However, the door isn’t entirely shut on liquid solutions. Other geological markers, specifically “recurring slope lineae”—narrow, dark streaks creeping down Martian slopes—exhibit distinct behavioral patterns. These features appear strictly during summer months and concentrate exclusively on warmer, equator-facing inclinations. Those elevated thermal profiles leave open the distinct possibility that water or hyper-saline briny solutions could still play a role in modern planetary surface evolution, keeping the search for present-day liquid water alive on Mars.
