Genuine question: How do we know the organic compounds seen by Rosetta do not come from Earth and were not brought by Rosetta itself?
Rosetta was built in a clean room according to COSPAR
rules, but "sterilisation [was] generally not crucial since
comets are usually regarded as objects where you can find
prebiotic molecules, that is, molecules that are precursors
of life, but not living microorganisms", according to
Gerhard Schwehm, Rosetta's project scientist.
https://en.wikipedia.org/wiki/Planetary_protection#Target_ca...