What's carving active gullies on Mars? It's not water‎

Scientists were shocked when they received the first images of Martian gullies. They were even more shocked as those gullies appeared to change over time. Their similarities to gullies seen on Earth were uncanny, but all of Earth's gullies are formed by water runoff, and Mars is too cold and has too sparse an atmosphere to have liquid water on its surface. Whatever has been causing those changing gullies couldn't have been water, so what was it? A new paper from Apolline Leclef of the Institut d'Astrophysique Spatiale at Université Paris-Saclay and her colleagues, available on the arXiv preprint server, shows how they are likely caused by CO2 frost turning into a fluid.