Mars Gullies Formed by Sand and CO2, Not Water

When NASA’s Mars Global Surveyor first spotted strange, narrow channels carved into the slopes of craters on the Red Planet in the late 1990s, many scientists thought they might be evidence of something remarkable: liquid water flowing on Mars today.


On Earth, similar gullies are classic signs of water erosion. But after more than two decades of observation and research, the consensus has shifted dramatically. Instead of liquid water, scientists now believe that sand and seasonal carbon dioxide (CO₂) ice are the real sculptors of many Martian gullies, a revelation with deep implications for how we understand Mars’ geology and its potential for past habitability.

Early Theories and the Water Hypothesis

Initially, the discovery of gullies, features consisting of an alcove at the top, a narrow channel, and a depositional apron at the bottom, raised hopes that fresh water once shaped parts of Mars’ surface. In a 2003 analysis of images from the Mars Odyssey and Mars Global Surveyor spacecraft, planetary scientist Philip Christensen of Arizona State University proposed that melting snow could have carved these channels, sheltering liquid water beneath insulating snowpacks long enough to flow downslope.

That idea was compelling because liquid water is a prerequisite for life as we know it. The possibility that water carved gullies on Mars in the recent geological past was widely discussed in scientific circles and popular media alike.

Clues from Rocks: Water or No Water?

But as new data from NASA’s Mars Reconnaissance Orbiter (MRO) began pouring in in 2006, researchers gained more tools to test that hypothesis. The orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) can identify minerals and surface compositions, while the High Resolution Imaging Science Experiment (HiRISE) camera provides razor-sharp views of surface features.

In a 2016 study published in Geophysical Research Letters, researchers led by Jorge Núñez of the Johns Hopkins University Applied Physics Laboratory reported that CRISM showed no mineralogical evidence of abundant liquid water or hydrated minerals in or around most of the gullies examined. Instead, gully channels cut through unaltered basaltic rock, and the minerals present were consistent with dry processes. “On Earth and on Mars, we know that the presence of phyllosilicates, clays, or other hydrated minerals indicates formation in liquid water,” Núñez said. “In our study, we found no evidence of clays or other hydrated minerals in most of the gullies we studied.”

Those results didn’t entirely rule out water in other contexts on Mars, such as the “recurring slope lineae” features that may involve briny flows, but they strongly suggested that classic gullies were not carved by flowing liquid water under current Martian conditions.

Carbon Dioxide Takes the Lead

So, if not liquid water, what could explain the shape and activity of Martian gullies? Enter the planet’s most seasonal ingredient: carbon dioxide ice.

Mars’ thin atmosphere is mostly CO₂, and during winter, surface temperatures drop so low that CO₂ condenses directly into frost and ice. In the spring, sunlight heats slopes, causing this dry ice to sublimate, change directly from solid to gas. According to new laboratory experiments and climate modeling reported in Communications Earth & Environment, this sublimation process can fluidize and mobilize sand and dust under Martian conditions, carving channels that closely resemble the gullies seen from orbit.

In these experiments, researchers simulated Martian atmospheric pressure in a specialized chamber and mixed sand with CO₂ ice. As the CO₂ sublimated, gas formed beneath the ice, creating flows capable of moving sediment and forming channel morphologies similar to those on Mars. The models also matched where and when gullies are active on modern Mars, primarily in mid-latitude regions where CO₂ frost accumulates and later disappears with seasonal warming.

New Lab Evidence of Dry Ice “Burrowing”

Complementary work by planetary scientists from Utrecht University, Le Mans Université, and other institutions has demonstrated a vivid process by which blocks of CO₂ ice can literally dig into dune slopes and slide downhill under Martian climate conditions. In laboratory setups that mimic Mars’ low atmospheric pressure and temperature swings, researchers observed CO₂ ice blocks burrowing through sand like tiny bulldozers, propelled by gas released during sublimation. This dynamic action carved sinuous, gully-like features with raised levees and pits that mirror what orbiters see on Mars’ dune fields.

According to lead researcher Lonneke Roelofs and colleagues, the combination of sliding and explosive gas bursts from sublimating CO₂ not only transports sediment but also builds gully features that, until recently, were assumed to require liquid erosion.

Why This Matters for Mars’ Water Story

The shift from a water-based explanation to a CO₂-driven mechanism doesn’t erase the possibility that liquid water existed on Mars billions of years ago; ancient river valleys and lakebeds remain strong evidence for that era. But it does reshape how scientists interpret recent and active features.

As planetary scientist Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center said in a 2014 NASA report, repeated imaging by MRO “indicates the gullies on Mars’ surface are primarily formed by the seasonal freezing of carbon dioxide, not liquid water.”

That matters because many astrobiological predictions hinge on liquid water’s presence. If current gully formation is dominated by CO₂ processes, it suggests that Mars today is drier and less hospitable near the surface than some had hoped, at least in terms of the availability of liquid water.

Peek Into a Quirky, Living Planet

In the end, the story of Martian gullies is a reminder that planetary geology can be counterintuitive. Channels that look like ancient riverbeds may instead be the signatures of a living, breathing, seasonal world shaped by dry-ice dynamics and gravity, not by water flows.

As scientists continue to monitor Mars from orbit and prepare future landers and rovers, understanding these processes helps refine the search for environments that might once have held liquid water, and maybe even life.