Seasonally mobile ground with frost heave of gravel vs sand or dust gives these sort of features throughout permafrost regions on Earth.

From the scale of the features you can calculate the depth of the seasonally active layer (about half the size of the polygons).

from the original MSSS page we see that this is at 54 degrees north, where Mars Odyssey has just told us there is H2O permafrost. The whole image is ~2.43 km wide, so the polygons, which are 1/10 to 1/40 of the scene width are 60 to 240m across, implying an active layer some 30 to 120m deep/thick.

This is too deep for seasonal temperature effects to penetrate, and suggests that longer-term cycles are responsible, such as multi-year ebb and flow of permafrost, or millenial-scale orbital fluctuations.

Note that these types of features do not neccesarily imply melting of the ground ice. Very similar features can occur by solid-state thermal contraction and expansion of ice-saturated ground that never thaws - thermal cracking.

Over millions of years, the complex rounded and polygonal patterns we see here can evolve from simple angular cracks.

Also, as Ed points out, these could be desiccation cracks, but due to slow sublimation of ground ice. (although there would have to be times when the ice was returned to allow for many cycles to take place for such a mature pattern as this one)