Since detecting gullies on Mars, scientists really hoped they meant our neighboring planet once hosted liquid water. Then, in 2014, NASA put the brakes on that theory; the ravines were much younger than we thought. Instead of water, the seasonal freezing of carbon dioxide (dry ice) was responsible for these familiar plots. But now the mystery changed: how exactly did frozen carbon dioxide form these gullies? In a new paper, a French team of astrophysicists explores this question in great detail based on decades of observational data from Mars Express and the Mars Reconnaissance Orbiter. For their analysis, the team specifically studied Sisyphus’s Cave, a region near the Martian south pole with a network of crisscrossing ravines. The researchers found two things. Firstly, it is very unlikely that liquid water was involved in the creation of the ravines. Second, and the most surprising result, CO₂ geysers, a popular alternative, also do not appear to be responsible for the ravines. “Instead, the data favor CO₂ ice-driven fluidization or avalanche processes that may operate during the later stages of seasonal ice sublimation,” the team wrote in the paper, which has not yet been peer-reviewed and is currently available as a preprint on arXiv. Ravines, by definition, are trenches typically dug into the earth by running water. Emphasis on “water.” Needless to say, when NASA’s Mars Global Surveyor sent back images of multiple gullies on Mars, scientists and space enthusiasts were excited. This must indicate the presence of water (the supposed irrefutable proof of extraterrestrial life) on our neighboring planet. There are many, many articles explaining liquid water on Mars. But not all astronomers agreed with this idea. For example, Australian geologist Nick Hoffman proposed in 2002 that gullies formed from frozen carbon dioxide. For that blasphemy, Hoffman was supposedly met with a lot of skepticism from his colleagues, but of course, as of 2026, we now know that he wasn’t necessarily on the wrong side of science history. The universe is very funny in that sense. Going further Once dry ice escaped the territory of the “fringe theory,” astronomers proposed a number of alternatives, such as CO₂ geysers, aquifers, and snow sheets of ice and dust. But first, for the new work, the team made sure to check that the most recent data also reliably ruled water out of the equation. According to the paper, the researchers could not find any spectral signals associated with water during the Martian spring, when gullies form. Images of dark spots and dark flows on the Martian surface, taken by the Mars Reconnaissance Orbiter. © Leclef et al., 2026 The team then compared the timing of ravine formation to the appearance of CO₂ geysers. However, there was a “clear temporal mismatch” between the two; The dark spots formed by geyser activity appeared long before the ravines. It is possible that there is some connection between geysers and ravines, but time and physical limitations make the correlation quite weak, the team explained in the study. Martian Equifinality While the researchers didn’t go into too much detail about the alternatives, they did mention that one possible explanation is ice-driven sublimation with CO₂. In this scenario, turbulent bubbles of carbon dioxide gas serve as a lubricant that allows dry ice and dust to levitate, somewhat like an air hockey puck suspended in the air thanks to a thin cushion of air beneath. This amalgam of Martian ice and regolith then flows across the surface, forming a ravine. The resulting terrain is like a ravine that we see on Earth, but formed by different means. The team added that ravines at lower altitudes may or may not share this process, and again, it is too early to completely ignore geysers. Importantly, ravines are far from the only thing scientists can count on to find water on Mars.
