From a geology point of view, many of these features all over Mars are pretty clearly fluvial in origin. They have very specific characteristics, different from

From a geology point of view, many of these features all over Mars are pretty clearly fluvial in origin. They have very specific characteristics, different from aeolian (wind) landforms such as dunes, which are also present. There's really a good deal known about Martian geology. For surface processes, it is (or was) more like Earth than different. Definitely caused by presence of a liquid, almost certainly water. Considering Mars absolutely has a great deal of water, it's the only reasonable explanation. There's also glaciers on Mars currently, as well as glacial landforms such as eskers and moraines from much more extensive previous glaciation, which only could have occured with a more vigorous hydrologic cycle. It's pretty hard to believe all of these landforms weren't formed by hydrological processes. There's also not much methane on Mars. It's way too warm to be a solid or liquid. There's CO2, but it's either gas or solid. Water is really the only known liquidy thing there.