As I understand, the samples collected by Perseverance and returned by Mars Sample Return will be taken at surface level. So they will not be able to tell that

As I understand, the samples collected by Perseverance and returned by Mars Sample Return will be taken at surface level. So they will not be able to tell that much about the sub-surface. Also of course that is just one small region that Percy is taking samples from, it can not provide proof about conditions of other regions of the planet.