[mdup]

Genuine question: How do we know the organic compounds seen by Rosetta do not come from Earth and were not brought by Rosetta itself?

[okket]

From https://en.wikipedia.org/wiki/Rosetta_(spacecraft)#Design_an...

  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.

See also the Committee on Space Research (COSPAR) Target Categories:

https://en.wikipedia.org/wiki/Planetary_protection#Target_ca...

[giomasce]

Usually spacecrafts like that are built in clean rooms and care is taken so they are sterilized when sent to space. I have no specific answer, though.

[ygra]

This will be especially true for instruments that are designed to find such compounds. It would be fairly embarrassing to send the probe into space, only to notice that apparently interplanetary space is full of organic compounds no one else has ever seen ...

[JohnDoe365]

That blurry something on the inner side of the lens turned out to be an ... ant?