Terrestrial sanitation.....

[Brian37]

Cross-contamination is always a thing to be avoided in scientific studies.

Unless, of course, you are studying cross-contamination.

I am all for avoiding false positives.

But considering the distance of space, it would seem more important to prove that our life could survive on other bodies. 

I can see for example a tardigrade surviving on the poles of Mars or Pluto. 

Considering the trillions of galaxies, which would mean googles of planets I have no doubt that other life exists in the universe. So the only thing to me, left to figure out is if there is other life out our level, and that also should not surprise anyone. I would however argue, because of distance any life that can think like humans is probably so remote it is stuck in the same position we are. 

So short term, I think to prove life exists elsewhere would be to prove it can exist locally outside our planet.

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Here is my question, WHY is this important?

If you went to Mars and found cockroaches, you want to be sure that the cockroaches didn't come with you.

Um no, if our cockroaches could survive on Mars, that would prove to me life is possible elsewhere.