Glass tubes on Mars?

Did Mars Global Surveyor discover what look like glass tubes on Mars?

Seen in the NASA image are what appear to be some kind of tube system.

Equally spaced arches or ribs about 180 metres in diameter run on the surface and appear to go underground.

Glass tubes
Glass tubes (click for larger image)
Glass tubes - Artificially coloured
Glass tubes – Artificially coloured

Original NASA source:

Are these sand dunes as NASA claims or are these in actual fact artificially constructed structures?