I remember reading a possible explanation of 'water erosion' features when the first evidence of them popped up. Turns out that as frozen CO2 sublimates on a slope, bits can break off and, contacting the warmer ground below, form a cushion of gaseous CO2 that facilitates the slide further down the slope, contacting yet more warm ground etc (by warm I mean hotter than the sublimation point of CO2, around -78deg C).
So it could have been chunks of solid cascading down the slope, buoyed by cushions of gas, which in the aggregatel can be a reasonably 'liquid' phenomenon.
The problem with liquid water flows on the surface any time in the (geologically) recent past is the temperature and pressure measurements we have seem to make it very unlikely. Still its a long ways away and there is much more to learn about this remote, forbidding planet
