Something Is Carving Fresh Gullies Near Mars' South Pole. Spectra Say It Is Not Water. It Is Dry Ice Floating on Its Own Gas.

When orbiters first sent back pictures of gullies on Mars, they looked so much like the water-cut channels on Earth's hillsides that planetary scientists had to suppress the obvious conclusion. Mars today is too cold and its atmosphere too thin for liquid water to survive on the surface. Then the gullies were seen changing between orbits, which meant something is actively carving them right now. A new study led by Apolline Leclef of the Institut d'Astrophysique Spatiale at Université Paris-Saclay argues the culprit is carbon dioxide frost that briefly turns into a fluid and slides downhill.

Leclef and her colleagues focused on Sisyphi Cavi, a system of deep pits near the planet's south polar cap. The site is unusual: it has active gullies but sits well outside the mid-latitudes where most Martian gullies form, which makes it a clean test case. The team pulled years of data from two spectrometers, CRISM on NASA's Mars Reconnaissance Orbiter and OMEGA on Europe's Mars Express, and examined what the surface was made of season by season.

The first job was to kill the water hypothesis properly. Liquid water and water ice absorb light strongly at a wavelength of 1.5 micrometers. In spring, when the winter blanket of CO2 ice disappears from the pits, that absorption band was completely absent. The team did detect salts and clays, minerals that usually need water to form, but only on the high plateaus and crater rims, nowhere near the gullies themselves. Whatever is moving material through the channels is doing it without a drop of liquid water.

The second suspect was geysers. Mars has a well-documented mechanism, named for planetary scientist Hugh Kieffer, in which sunlight passes through a translucent slab of winter dry ice, warms the dark soil beneath it, and sublimates CO2 at the base until the pressure ruptures the slab and a jet of gas blasts sand and dust skyward. Those eruptions leave the dark spots and fans that MRO and Mars Express photograph every southern spring. But the timing does not fit. Geyser spots appear in late winter and peak around the spring equinox. Gully activity at Sisyphi Cavi does not begin until mid to late spring, by which time the geysers have gone quiet.

That leaves a third mechanism, which the authors call CO2 frost fluidization. Instead of gas venting through cracks, the sublimating carbon dioxide gets trapped beneath a slab of dry ice and sediment and lifts it, the way the air jets on an air-hockey table lift a puck. A mass that would otherwise sit still on a slope suddenly has almost no friction and starts to move, accelerating into a fast, granular flow that can gouge trenches and dump material into the alluvial-style aprons seen at the mouths of the gullies. The end result is a landform that looks exactly like a water-carved channel and was built by the freeze-thaw cycle of the atmosphere itself.

The paper, posted to the arXiv preprint server ahead of peer review, is not the news astrobiologists were hoping for. Every active gully that turns out to be dry is one fewer place to look for present-day habitable conditions. But it is a reminder that Earth-like landscapes do not require Earth-like physics, and it gives geologists a mechanism they can test directly: if the frost-fluidization model is right, gully activity should track the seasonal disappearance of CO2 ice, not the geyser calendar, everywhere on the planet where gullies are still moving.