These sulfates were wet once. Sulfates can contain lots of water, which expands the crystal structure.

Now these sulfates are dry. At least, a lot drier than they were. Shrinkage has occured. With shrinkage comes polygonal cracking. This polygonal jointing is observed on outcrop scale, and, being a big fan of fractal geometry in natural processes, I think that if these regional fractures were mapped, they'd be polygonal too.

Other pit chains on Mars are believed to be younger, aeolean sediments dropping into normal faults ( a result of extension, or regional shrinkage). Here on Earth, you can see pits like this in the Grabens area of Canyonlands National Park that are 50 feet deep.

This is an easy table-top experiment. Get some epsom salt, grind it up fine, wet it into a paste, let it slowly dry. Record your results.

What you'll get are two generations of extensional fractures. The intial phase will be more orthagonal. The second order structures are polygonal. Try it. Cornstarch works too.