The idea of sinking into dust "lakes" has long been disproved on both the Moon and Mars. Sure, some areas may have thicker dust, but the dust doesn't "flow" in the way needed to sink into it.

However, look at the airbag print marks and the way stones have been pushed into the soil. This suggests to me that the soil is very porous and friable and can literally be compressed by a weight.

This is similar to the way the top cm or so of the moon was excellent for making bootprints. It behaved like damp mud, but wasn't wet at all.

Similarly, the uppermost layer of "soil" on Mars appears to behave like a well-tilled fine loam on Earth - it it flexible, compessible, and mouldable, yet it is also dry and powdery!

Wierd.

(one possible explanmation is that the seasonal [or annual or millenual] formation and loss of ground ice tends to open up the "pores" in the soil. Frozen ground on earth is often very loose and "crunchy" after it's thawed. This is especially common in desert areas where the soil is either frozen in winter or dry in summer - go visit the painted desert in Az. one springtime and have fun making footprints.

As for subsurface sampling, bopmbs are not popular when you're building a multi-million dollar spacecraft. There's already too much stuff that can burn violently like pyrotechnic devices and the fuel.

Far simpler to go find a fresh meteorite impact crater - they work just like small bombs. Once the rover gets rolling, it'll probably find a few small to medium-sized craters in fairly fresh condition.