Well … when one Martian gully loves another Martian gully very much …
OK, actually the question is really whether or not these things are formed by flowing water. As Phil Plait explains, there are pretty compelling arguments both for and against that idea—the primary alternative to water being sand and dust rolling downhill. How's that work? A recent paper provides a possibility:
The authors did a clever experiment. They assumed that it was dry ice — frozen carbon dioxide — that was behind the gullies, and not water. As the dry ice turns into a gas in warmer weather, they supposed, it blows out of the ground and gets in between the sand particles, causing them to run downslope like a fluid. That's a fair assumption, given how common dry ice is on Mars. They then set up a tub filled with Mars-like sand, piled it into a slope, and used an air pump to force air under the sloping pile. Sure enough, the sand flowed down, making gullies that seem very much like what's seen on Mars!
Bad Astronomy: Are Martian gullies formed from water or not
