Re: Soil Structures of Mars?
On Earth, after an uppermost few cm of loosely bound "crunchy" aggregate, permafrost is iron-hard and shatters pickaxes. (The extreme cold helps here by making most iron rather brittle).
The best places to look for data on soil structure on Mars is the Viking project reports, well summarised in the volume "Mars" eds H.H. Kieffer et al., U. Arizona Press. The Viking missions actually dug into the soil of Mars, but in places where it is not frozen.