In article .com,
Smith wrote:
How is it possible to know that we have not just deposited some "earth
DNA" on Titan, via a single (or multiple) viral or bacterial hitchhiker?

Well, it's not, because Huygens wasn't sterilized. There *have* been
sterilized spacecraft -- notably, the two Viking landers -- but it's a
difficult thing to do and it runs up the cost badly. Heat sterilization
is about the only fully effective method, and it's hard on electronics.

Modern practice is to sterilize only if (a) there are life-detection
experiments on board (which obviously might be confused by hitchhikers),
or (b) the destination environment is a place where Earth life might have
some chance of growing and spreading.

Almost certainly there *were* bacteria on Huygens at launch, and some of
them may even have still been technically viable on arrival.

Titan, interesting though it is, is *way way* too cold for Earth life.
(Temperatures not much colder than that are used for long-term pristine
*storage* of biological samples.) There is no realistic prospect of
hitchhiking organisms proliferating on Titan.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |