Digging out

Opportunity's present view at Meridiani.

The rover Opportunity has been stuck for weeks now in some very strange fine powdery material, but after a lot of analysis and testing on Earth, its wheels started moving again on Friday and it made about an inch of progress in getting out of the low dune-like ripple where all six of its wheels had gotten stuck axle-deep or more.
Here is a nice animation of the wheel finally making its first successful rotation since getting stuck.

But what really intrigues me is the way the soil behaves. From earlier images, it's clearly very fine and powdery and sticks together in the wheel treads and so on, but the way things move on the surface as the wheel rotates looks more like a solid frozen mass. Is this really wet, frozen ground?