Rivulets
 

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 :)

I to have found a picture proving life on Mars.

This I believe to be the first sighting of a mouse on mars.

I suddenly have to have some chocolate ice-cream.

On Mars, anything is possible, but in this case, there are *lots* of these sorts of features. Statistically, it is far more likely that we are seeing some flow of inherent water rather than from a single extra-martian source like a comet.

My opinion. I'm not an astrogeologist, and I don't play one on TV, either.

Maybe it was a, what's the word, um... a big (yeah, that's the word) comet that was smashed into little bitty pieces before it landed, kinda like the fireworks on Jupiter not so long ago. Perhaps the happinings are all along a straight line, or perhaps the comet landed in a scatter pattern rather than single file like Shoemaker-Levy 9. Just hypothanesthetising.

"Would you like to swing on a star, or would you rather be a pig?"

The most likely explanation
 

In brief: snow forms in these craters in the winter months, then later melts - from the rim of the crater down - and the water flows under the still frozen snow further down the slope, which protects it from evaporation in the tenuous atmosphere. If it were a subterranian (subarean?) water source, its unlikely to occur so close to the raised crater rims. This explanation is the most consistet yet.
http://science.nasa.gov/headlines/y2...htm?list922696