(those darn keyboards...)
can become when the same "message" remains while the conditions change.
Let's drop the "Galaxy conquest thing" which is far too farfetched and won't involve us anyway.
Mars is a different matter.
I've read lots of science-fiction and they always involve humans (even 2010, while I thought (before ACC wrote it) a follow up to 2001 had to explain or describe Bowman's transformation, which ACC really didn't do-that's why 2001 should not have had a sequel).
Many of those SF books were written 20 years before Sputnik,while it was thought going up there was no more a big deal than crossing the pond in a 4 propeller Constellation.
Mars could involve humans (homo-sapiens) but I wonder. The speed of the biotech industry, of genetic manipulation, of computer development (AI, cybernetics,), of nanotech promises, compared to the difficulty of dealing with space transportation (CATS) should deeply transform our notion of space colonization. We thought people would always be interested in a manned space program, and Mars Pathfinder completely obliterated Columbia's flight at the time, same for the NEAR soft landing on Eros while an EVA with TVcams on helmets took place. Clearly, robots fascinate the public much more than we thought possible (even 10 years ago). But, as EBNelson's post shows, it's still very hard to fathom a universe without us, homo sapiens.
But Homo sapiens are very recent, and extremely localized, in the history of the Universe. If we want to leave a legacy (sometimes I wonder why; after all, we don't last long, do we?), we're going to have to let other sentient "things" do it for us. There is the huge responsability issue: Suppose, for theological reasons, we send AI out there to spread our culture, with the best intentions. We cannot be there to correct what that AI would do, strictly applying our "gospel" and spreading a most damaging message to encoutered life forms out there. What kind of legacy would that be?
I'm convinced the best kind of legacy is to explain what and who we were and not bother at all to convince other life forms into doing the same things we did. They might choose to, but that would be their own decision.

Raoul