Some gullies on Mars show activity during winter and early spring, when carbon-dioxide frost coats the slopes. That timing matters: a channel or fresh-looking deposit does not automatically mean liquid water flowed there. Seasonal dry ice and movement of loose sediment offer an important explanation for many modern Martian gully changes.
How to spot the frost in a real Mars photograph
In the image below, bright white patches cover the colder parts of the slope, while exposed reddish ridges catch more sunlight. The gullies run down between these ridges. NASA reports that the frost in the scene is mostly carbon dioxide, with smaller amounts of water ice.
Why the coldest slopes stay white longer
Slopes that face away from the Sun receive less direct light. They tend to stay colder, so seasonal frost can persist while adjacent slopes or ridge tops have already lost their coating. On Mars, the contrast can be especially striking because much of the seasonal frost is frozen carbon dioxide, which sublimates as conditions warm.
Notice what that means when you interpret a photo: a single hillside can contain both frosted and exposed areas. The difference is not necessarily a different mineral or permanent snowfield. Direction, local topography and the time of year can explain much of the pattern.
How dry ice may help move sand and debris
As carbon-dioxide frost or ice sublimates, escaping gas can reduce the resistance between particles and encourage loose material to flow. Changes in seasonal frost loads or local avalanches can contribute as well. Researchers examine repeat orbital images for new deposits and changes to gully alcoves and channels rather than assuming all gullies share one mechanism.
Importantly, the NASA image shown here does not show an obvious new gully event occurring. It is evidence of an environment where frost persists on gullied slopes, not a freeze-frame of an avalanche. NASA cautions that frost may trigger activity in some years but not most years.
Are all Mars gullies carved by dry ice?
No. Different gullies formed at different times and locations, and their geological histories can differ. Scientists have debated potential roles of water, ice, dry-ice-driven sediment motion and slope failure. The evidence that some present-day gully activity coincides with the frost season narrows the explanation for those observed changes, but it does not settle the origin of every gully on Mars.
A three-step way to read Mars gully photographs
Start with the season: Is the scene from winter, spring or summer? Check the slope orientation: White frost should persist longer in shade. Compare repeat images: Only a before-and-after pair can establish that the ground changed between observations. An impressive channel in one image can be ancient.
Frequently asked questions
Is Martian frost the same as snow on Earth?
Not exactly. Mars can have water-ice frost, but seasonal high-latitude frost is often dominated by carbon-dioxide ice, which behaves differently from ordinary water snow.
Does a white patch in the picture prove there is water?
No. White is a visual clue to frost or reflective material. NASA identifies the frost in this image primarily as carbon dioxide; color alone cannot determine the chemistry.
Why do gullies appear in shadow?
Gullies can exist on slopes with many orientations. In this particular late-winter scene, the shadowed, less sunlit slopes also retain more frost, making their channels stand out.
Related reading
For a different kind of south-polar feature, see how dry ice creates branching spider-shaped patterns on Mars. The NASA frosted gullies observation from 2019 offers another seasonal comparison.
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