Even if the probe was not contaminated the planets already are. We are likely to find that there is life on Mars (underground) and it looks just like life on Earth.

One interesting theory is that life originated outside the Earth (say on Mars) and was transported to Earth at a later time. This theory would explain why LUCA [0] is so complex and why life appears almost as soon the Earth had liquid water [1].

0. https://en.wikipedia.org/wiki/Last_universal_common_ancestor

1. https://en.wikipedia.org/wiki/Earliest_known_life_forms

Yes, but you can't test that hypothesis if the probe is contaminated. Science isn't about assuming likely things to be true.

>This theory would explain why LUCA [0] is so complex and why life appears almost as soon the Earth had liquid water [1].

from the wikipedia article:

>LUCA should not be assumed to be the first living organism on Earth

On the other hand it should not be assume that it is not. The complexity of LUCA is a problem since LUCA is not much less complex than a modern bacteria which is very, very complex.