I personally doubt that this contamination is much of a problem. The planets of the solar system have been dusted in earth rocks containing bacterial spores thrown up by large asteroid strikes for billions of years.
However tiny the spacecraft's contamination is going to be right next to the sensors making false positives easy. Longer term they will likely all die out, but they could still contaminate samples taken over huge areas for the next decade.
One interesting theory is that life originated outside the Earth (say on Mars) and was transported to Earth at a later time. This theory would explain why LUCA [0] is so complex and why life appears almost as soon the Earth had liquid water [1].
0. https://en.wikipedia.org/wiki/Last_universal_common_ancestor
from the wikipedia article:
>LUCA should not be assumed to be the first living organism on Earth
An extremely expensive although effective approach would be to "seed" the spacecraft with isotopically enriched and documented solvents and food sources such that all the oxygen or carbon or both on the spacecraft, if any contamination DOES exist, would be a peculiar and documented isotope ratio that is different than Mars ratios.
I think we know the natural ratios for Mars in bulk material, so anything immediately found to be growing that matched the documented "salted" isotope ratios obviously was a spacecraft contaminant whereas anything mars ratio was local. Of course bacteria can reproduce fast, so Earth bacteria eating Mars material would rapidly revert to Mars isotope ratios. Still for the first hour on the ground this would likely be usable for life detection experiments. Although it would probably be unimaginably expensive.
So there's potential there I would imagine in regards to testing that hypothesis. However what mainly bothers me with it is that it doesn't explain the origin of life and why Earth couldn't have original life begin on it.
Unless we can rule out that early Earth conditions would have been too inhospitable for life to start but was sufficient to sustain it I see no reason to think an extraterrestrial origin to Earth life is more likely.
Actually, information is stored as RNA, with DNA as long-term storage (mnemonic: RAM nucleic acid vs disk nucleic acid) and then amino acids are assembled into proteins (hardware) based on that information. Proteins only function as information storage in pathological cases like prions.
But yes, if 'alien' life used the same 20 amino acids, and especially if it has the same mapping from 64 nucleic acid triplets to 20 amino acids, then that's confirmation that it's actually just a earth microbe that escaped or was left behind.
Therefore, yes, life would have diverged sufficiently that scientists should be able to tell the difference.
Why do you assume so? It's possible a sattelite in the Solar System was contaminated by Earthling bacterias via asteroids and somehow miracalously adapted that environment. If this is true, scientists would find a group of bacteria branched off of bacteria here on Earth a billion year ago or so.
What is a cleanroom's stated goal? Implying the cleanroom's only purpose is for reducing the risk of forward contamination.
Spacecraft will be manufactured in a cleanroom regardless of effectiveness at stopping microbial forward contamination. Foreign Object Debris, payload and bus contamination during manufacture are the primary concerns. For example, dust or condensate on an optical payload would ruin a Discovery class mission.
The stated goal of a clean room is for quality control, reducing microorganism forward contamination is a secondary.
It seems intuitive to me that if you're maintaining such a clean room the only bacteria that are going to live there are ones hardy enough to survive in those extreme conditions, you're just going to be running an accidental breeding program for bacteria likely to survive on a spacecraft.
As opposed to just assembling all this stuff in a room open to the elements. It would then be completely inundated with bacteria, but none of them would have any selective advantage in being hardy. You could even feed them on purpose by spraying food for them everywhere.
Then when you launch the spacecraft those fat comfortable bacteria would all instantly die because vacuum isn't an environment that's anything like what they've had to deal with, unlike the hardy ones in the clean room, and the few survivors would have no time to develop hardiness. They'd all die within hours from launch.