Unraveling Mars's Mysterious Gulleys: The Dry Ice Secret

For decades, the enigmatic Martian landscape has captivated humanity, none more so than the puzzling "wormlike gulleys" etching their way across its rust-colored surface. These intricate, dendritic channels, observed primarily on steep slopes and crater walls, presented a profound mystery, initially sparking hopes and theories centered around liquid water—a substance vital for life and terrestrial geological activity.

Early observations, particularly from orbiters like Mars Global Surveyor, revealed these distinct erosional features, which bear a striking resemblance to water-carved channels on Earth.

This led scientists to hypothesize that occasional meltwater flows, perhaps from subsurface ice thawing during warmer periods, were responsible for sculpting these intricate patterns. The idea was compelling, aligning with the romantic notion of a Mars that might once have harbored abundant liquid water, potentially even sustaining life.

However, as our understanding of the Red Planet's harsh environmental realities deepened, this hypothesis began to face significant challenges.

The primary hurdle for the liquid water theory lay in Mars's current climate. With an atmospheric pressure barely 1% of Earth's and average temperatures plummeting far below freezing, liquid water simply cannot persist on the surface for any meaningful duration.

It would either instantly freeze or rapidly boil away into the thin atmosphere, a process known as sublimation. This stark reality forced planetary scientists to reconsider, pushing them to seek alternative explanations for the dynamic gulleys that clearly demonstrated ongoing erosional processes.

Enter the intriguing and now widely accepted theory: the gulleys are not carved by water, but by seasonal flows of liquid carbon dioxide.

Yes, the very gas that makes up the bulk of Mars's atmosphere, in its solid and liquid forms, appears to be the primary geological sculptor. This hypothesis suggests that during the Martian winter, vast amounts of carbon dioxide ice accumulate, particularly in colder, shadowed regions. As spring arrives, and temperatures begin to rise, this dry ice doesn't just sublimate directly into a gas; some of it might transition into a temporary liquid state under specific pressure and temperature conditions, or more commonly, the rapid sublimation of trapped CO2 ice causes eruptions that mobilize surface material.

Imagine pockets of dry ice buried beneath a thin layer of regolith.

As sunlight warms the surface, the solid CO2 beneath can rapidly turn into gas, building immense pressure. When this pressure exceeds the strength of the overlying material, it can burst forth, entraining loose soil, dust, and rocks, creating a slurry-like flow that carves out these distinctive channels.

This process is akin to a "dry avalanche" or a "liquid CO2 flow," where the rapidly expanding gas acts as a lubricant and propellant, mobilizing vast quantities of material downslope.

Compelling evidence supporting the carbon dioxide hypothesis has emerged from continued orbital observations. High-resolution images from missions like the Mars Reconnaissance Orbiter (MRO) have captured gully activity specifically correlating with seasonal changes in CO2 frost distribution, rather than with periods where water ice might melt.

Furthermore, these activities are observed in regions and at temperatures far too cold for liquid water to exist, but perfectly suited for the dynamics of carbon dioxide ice and gas. The features themselves, with their distinctive bulbous fronts and intricate networks, are consistent with the rapid, volatile flows expected from CO2 processes.

This paradigm shift in understanding the Martian gulleys not only resolves a long-standing planetary mystery but also paints a more dynamic and intriguing picture of Mars than previously imagined.

It reveals a planet actively being reshaped by its own atmosphere, where the ebb and flow of carbon dioxide, rather than water, dictates significant geological changes. While the dream of extensive liquid water on present-day Mars remains elusive, the discovery of carbon dioxide as a primary erosional agent reminds us that the Red Planet continues to hold profound secrets, offering endless opportunities for scientific exploration and discovery, continually challenging our Earth-centric perspectives on planetary evolution.