As the title says:
Are there any PnP RPGs out there that are based on the unforgettable works of Ray Bradbury (R.I.P.)? (On Martian Chronicles, for example?)
As far as I know there isn't anything based directly on that source material. Gurps is a system that is very moldable (name literally means Generic universal roleplaying system) and is great for that, but can also be overwheming at times. Gamma World has a post-apocalyptic setting but the tone is definitely off I think for something like the Martian Chronicles. Traveller also could be a fit though its always reminded me most strongly of Asimov's foundation series.