On Fri, 29 Jun 2018 22:27:13 -0700 (PDT), RichA
wrote:

On Friday, 29 June 2018 20:23:30 UTC-4, Chris L Peterson wrote:
On Fri, 29 Jun 2018 17:14:01 -0700 (PDT), RichA
wrote:

So they send a mission to Enceladus. They find no life. "Ah well, maybe the equipment isn't good enough yet?" 10 years pass, another mission is sent. This time they find bacteria and viruses. Now, was this missed the first time around, or was it evolved contamination from the first ship?

https://www.bbc.com/news/science-environment-44630121

All life on Earth evolved from a single common ancestor, and shares a
huge amount of common genetic coding. Anything we leave behind will be
readily identifiable as originating on Earth. Even if alien life uses
the same genetic chemistry as Earth life, there's no chance it's going
to code for the same genes (and especially for all the inactive
segments).

Unless panspermia is right and it came from the same comet, comet swarm, etc.

Sure. But if panspermia is right (which it almost certainly is not, at
least in terms of delivering complex life) then it doesn't really
matter, since all life would be the same. "Earth life" wouldn't have
so much meaning.