What is the scientific foundation for the hypothesis that human space exploration accidentally could spread Earth life to e.g. Mars and cause contamination which is indistinguishable from any indigenous Martian life?
The argument I know goes that if we later detect life on Mars, we wouldn't know if it is our own contamination or if it is indigenous Martian life which happens to be indistinguishable from Earth life. But that is obviously not a valid argument! Proteins are chains of hundreds to tens of thousands of 20 different amino acids. For any protein length, there are at between 20^100 to 20^30000 possible variants. So we know for sure that two similar proteins never occur independently anywhere in the visible universe.
Note that "convergent evolution" doesn't even come in to play here as a potential trend to uniformity, since variation and selection only works on actually existing proteins, not any of the possible but never existing proteins. Indigenous Martian life will very very easily be distinguishable from Earth life.
Are there any other arguments?