Fresh material, that not good right?
Nah, it's specifically what they're looking for - inner material that hasn't been eroded by solar wind etc, that is material that is "still fresh".
Basically they want the blue material in this:
The equatorial region is equivalent to the bulge visible in earlier pictures, probably pushed out and cleaned somewhat of older regolith through its rotation, flinging material there off the surface. The surface itself across the asteroid is "uniformly diversified" - i.e. it contains very diverse material, but in the same mix everywhere.
They don't have any drilling mechanisms to go deep.
For the third touchdown a shaped charge EFP warhead will be fired at the site first to crater it. Doesn't dig all that deep either but
craters pretty well (note: turn down volume).