[doodad]

I'm with you wondering if the Martian gravity had some effect in all this. As I noted earlier, the spheres appear to be in the top of each layer. A gas would certainly behave that way, in fact I have collected samples at Sheepeater's cliff in Yellowstone that each discrete flow of basalt was marked by the froth at the top. I am not sure that lithophysae would be of such a different density that the remainder of the rock. The obsidian flows I have seen were just as hard across the devitrified sections as the remaining rock.

[quantim]

"I'm with you wondering if the Martian gravity had some effect in all this."

It is everything about gravity.  And temperature.  My 2 cents is from a giant impact (this crater obviously) as molten rock is blasted into the vacuum (or atmosphere) it will coalesce into a sphere, harden possibly (smaller ones) and fall back to the surface and begin the surface erosion processes.  Subsequent seismic activity can account, along with liquid water, for almost any bizarre landscape geology over millennia. 

If there was significant moisture or even a body of water then many of these various spheres would crack just as a hot marble will dropped in a glass of water.