[Djarum]

My speculation:

It looks like the rover drove over the pile of dirt it excavated, leaving a white track of the substance that was stuck to the wheel.

How did they manage to end the dig at nearly the precise point where the white substance starts?

[Djarum]

How did they manage to end the dig at nearly the precise point where the white substance starts?

I take that back. Looking closer, it appears a fine layer of dust fell into the trench, possibly obscuring any visible stratum.