WHAT MARTIAN GULLIES MEAN FOR WATER ON MARS
By EMILY LAKDAWALLA , Sky & Telescope.
Excerpt: Martian gullies have been the center of a debate about whether Mars ever has flowing water. Now, a comprehensive study examines the question. Lots of Mars’s hillslopes have gullies, steep ravines that grow when we’re not looking. They look so much like Earth’s own gullies, formed when water and debris carve into steep slopes, that it’s easy to think that water must be involved on Mars, too. But physics says water shouldn’t ever be liquid anywhere on the Martian surface today. Many scientists therefore think gully formation must be triggered by some kind of dry process, involving ice (either water ice or carbon dioxide ice) that sublimates directly from solid to gas. In a new study, published in the August 2024 issue of Icarus, Axel Noblet (University of Western Ontario, Canada) and colleagues amass data on nearly 8,000 gullied slopes and come up with an answer to the “wet or dry” question: It’s both, and it depends. ...The correlation with some ground ice strongly suggests that gullies have formed in places at the edges of ice deposits. That suggests they formed during a transition from a wetter period, favoring the formation of ground ice, to the present-day, dry period. ...But what about the gullies that have been seen to expand on Mars today,.... And what about the gullies near the poles? ...A plausible answer to these questions involves carbon dioxide. While dry ice doesn’t expand as it freezes, like water does, condensation of dry ice can fill all the pores in the ground and glue particles together. If that ice suddenly goes away — say, when sunrise on a spring day lights up the ground for the first time in months — the sudden withdrawal of support can collapse the ground, and the rush of expanding gas can “fluidizing” the soil, reducing friction and encouraging flow even though no liquid is present....