Strange "Tubes" on Mars

Martian "Tubes"

These tube-like formations on Mars are actually fairly common. I have not seen any good explanation about how they might have formed by natural processes. One must wonder if they could be artificial or perhaps even organic. I can't help imagining the spice-worms on Dune when I see these pictures.

My first guess is that they are the result of some geologic process, but they certainly do stir the imagination.

Follow this link to see some more pictures. On this website the "tubes" are described as linear dune patterns filling in large surface cracks. They don't look like dunes to me. Of course dunes are common on Mars, but it should be simple to determine if the "tubes" are dunes- they won't look the same if they are photographed again... unless you want to claim they are ancient dunes that have been cemented and preserved. I think some other explanation is needed.