There's a dazzling array of possibilities for each option.
If substantially-DNA-based life is found on Mars, then either it shares a common source with life on Earth (likely) or it did not (unlikely). The former case is interesting in a predictable sort of way, whereas the latter case would be extremely fascinating (as it implies that Earth-like life is uniquely probable in this universe).
If something like RNA-based life is found on Mars, then the same two possibilities exist, but the former explanation (common source) becomes slightly less likely and the latter explanation (independent origin) becomes slightly more likely. Either case is quite interesting and would give us a better idea of where to look for an abiogenesis vector.
tomandlu wrote:I'd hope for the first, but purely because it seems to offer the best odds for life being common in general. Also, it's one hell of a kick in the teeth for creationists.
Nah. They'd just say something like, "God created Earth life first... and then got bored and filled the rest of the cosmos with life as well. We're still super-special, though. Really."
Indeed. Like any conspiracy theory or pseudoscience, they will readily adapt their imaginations just enough to squeeze in contradictory evidence without ever admitting that it is contradictory. In fact, they'd probably claim something about how the Bible really "actually predicts this sort of thing". In particular, if it was DNA-based life, then they'd be screeching about "common design" in mere seconds.
tomandlu wrote:I was more thinking that it would be odd for God to use a completely different set of tools, since that contradicts the notion that he used the same tool-box on Earth to create different organisms. i.e. the creationist answer to "why does all life on earth appear to be related?" is "because god is the common creator, so all life shares a common technique".
A completely different form of life or an RNA-based life would be immediately embarrassing, but after the initial shock they'd bounce back with something like, "Every form of life that God created was designed to be uniquely suited to its environment, and so we could expect God to use unique biochemistry if he chose to populate alien worlds. This is obviously evidence of God's intelligence and purposefulness in design! It couldn't be anything else, because ARGUMENT FROM INCREDULITY LA LA LA LA"