What would need to be discovered to prove there is extraterrestrial life?
Curiosity is on the Martian surface and is equipped with a slew of laboratory equipment.


But not, incidentally, equipment to detect life.


  What would Curiosity need to discover to prove there is or has been life on Mars? Would it have to find DNA …?


No. In fact, DNA would be lousy evidence of life on Mars, it would almost certainly be a contamination from Earth.

Lacking equipment, I’m not even sure how Curiosity would go about finding traces of DNA but assuming that it did (through some sort of chemical assay), it has no way of telling the origin of said DNA.

And even if the genetic material could be sequenced there are a number of scenarios. It could be very similar to what is found in extremophiles on Earth, in which case we wouldn’t necessarily know the origin (but due to the aforementioned contamination risk, the prior probability of it being Earth life would be very high). If it were completely different, that might be compelling enough to consider it extraterrestrial.

But maybe not: estimates for the number of existing species on Earth vary drastically. One thing is certain, though: we’ve only scratched the surface of prokaryotic life in particular. So if we find something new, that wouldn’t even be very surprising.

More interesting would be an alternative genetic material, neither DNA nor RNA. So far, we haven’t discovered anything like this on Earth (despite earlier hopes) so it would be more likely of extraterrestrial origin. But again, as the refuted study shows, we cannot entirely exclude the possibility of such life being found on Earth either.

All this is just to show that proving microbial life to be extraterrestrial is quite hard. It would be much easier if we were to find non-microbial life on Mars (i.e. something multicellular). However, I don’t know of any serious scientist who seriously entertains this possibility.

But the reason to prefer such a discovery is simple: the bigger the life form, the less likely that we haven’t found it yet on Earth, and the smaller its viable niche. So if we find multicellular life adapted to the surface of Mars there aren’t many places on Earth where it could have originated from, never mind the low chance of having survived the ride aboard the probe.