Nick, you've got a very vivid imagination. I can clearly see the reflection of the Sun on the convex aritfact where these two items in question biforcate. It concures with the direction the rest of the sunlight hitting the planet in the photograph. And shadowing is visable where one would expect to see if these artifacts are transparant tubular structures. It would appear these are large drain channels. I looked intensly at these pics and find it difficult to imagine these as dunes as you stated below.

"You see similar chains also at the foot of slope of valley walls, and more widespread sand dunes of identical colour on flat crater floors and on flat plains."

I would be very interested in veiwing other examples of dunes which you see as similar, such as the slopes of valley walls as you mentioned.

Thanks,
Stan